Date   
Re: Uncooperative 7834

 

On Fri, Mar 1, 2019 at 07:12 PM, David C. Partridge wrote:


Apart from scattergun replacing every bead tantalum I can see, does anyone
have any thoughts on how to pin down the fault?
Did you check / disconnect the HV Multiplier ?

/Håkan

Re: Help with 7A18 problem

Bill Carns
 

Now think I will, when I get time today, go out and study some more, reseat those attenuator modules, then try and jigger those attenuator covers back on and try some careful testing - now that I know more about what I am looking for.

Then I will read up in the manual and make a decision on what I am going to try and do. You guys have all been really helpful. Nice group.

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 5:48 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Bill,

The attenuator modules are the little rectangular things a bit like Chiclets - there are four of them. They will have something like "307-10xx-0x" (the part number) written on them as well as "2X", "4X", "10X" or "100X", i.e. the attenuation value. As I said, they just pull out using your fingernails; there shouldn't be anything stopping that, once the thin ally cover is off. I'm not sure what you are calling the "scaling shaft", but I don't see anything like a shaft on mine. EDIT: I think I have worked it out - is this shaft going to a pot at the rear of the plug-in and is for the "POSITION" and "IDENTIFY" controls? If that is so, then yes, it will need to be removed to get the cover off, probably. My 7A18 has its "POSITION" and "IDENTIFY" controls mounted on the front and the four-way ribbon cables with the ferrite rings connect them to the circuit-board.

No! don't get Deoxit anywhere near that board. It is made from polyphenylene oxide and it has "excellent electrical characteristics" as stated in the manual. It also says, regarding cleaning it "use only water-soluble detergents, ethyl, methyl or isopropyl alcohol". One assumes that any water-soluble detergents used would need to be thoroughly removed, too (probably with deionized water).

I sort of wish I hadn't poked around in mine, because that cover over the module is a swine to get back on properly. To make sure that the "springy fingers" all go back to the right place, the top of the frame pretty much has to come off, unless you have really tiny fingers with lights on the ends. As I said, that entails taking all of the knobs off the front panel and popping it off. A small screwdriver is useful to prise at top and bottom. I neglected to say that there are also two countersunk screws which fix brackets for a circuit-board which have to be removed, plus another countersunk screw which fastens the earth-strap and an earth tag. These are easily removed, but really fiddly to put back together again (I still haven't got that fixed yet). Why did Tek make some things so easy to maintain and others such a pain?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 11:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Quick comment. No smoking at all here. I have had that scope and plug ins for over 30 years. No Diesel engines and very very clean environment (Go to K0CXX.com) so not sure what caused this issue to rear its head. I am now wondering if just a good reseat will work - with perhaps just a very carefully applied tiny bit of Deoxit on the pins. What do you think of that?

I will read more but I take it the scaling shaft will have to come out before that attenuator module is pulled and reseated. All material and plugins are in shop a couple hundred feet away and it is middle of night here. :-)

B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:40 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Albert,
I think you are right about the 5mV/DIV setting, thinking about it. However, I believe I remember something about detecting which attenuator module(s) are the culprit(s) by finding which VOLT/DIV settings show problems, referring to the info in the manual. It seems that they are all bad, as Bill hasn't indicated that they differ very much. This seem pretty improbable to me, especially since some channels are fine and some not. I wonder if this scope has been used near to a badly-maintained diesel engine or by a chain-smoker?
Anyway, the problem might be getting solved by having a good 7A18 from Jim.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 02 March 2019 21:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:


---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert

Re: Tek 577 high collector voltage interlock

DW
 

Thanks

577 whine with variable collector

DW
 

With the variable collector at 0 or near 0 there is a noticeable whine as well as a interference pattern affecting the trace on the display. This is with the collector supply under no load and witch each voltage selection. My assumption is feedback or ringing in the collector supply circuit, has anyone experienced these symptoms, thanks

Re: Uncooperative 7834

 

Disconnecting the supply to the HT circuit (i.e. no EHT, no 130V), changes the nature of the ticking, but doesn't stop it.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of zenith5106
Sent: 03 March 2019 14:47
To: TekScopes@groups.io
Subject: Re: [TekScopes] Uncooperative 7834

Did you check / disconnect the HV Multiplier ?
/Håkan

Re: Help with 7A18 problem

Bill Carns
 

Good news all. I went out and first inspected and learned more, then I removed (one at a time) the attenuator modules and reseated them. That is the short version of a longer story. The pins are very thin and bend easily if the modules are not removed straight up. That was a learning lesson that I leave in the thread here. The PC board holes have almost no taper and you have to have those pins really straight to get them back in.

In stating a little about my background before, I did not mention that I worked for almost half my career in the semiconductor industry, first in process modeling and device development, then running a RF Power design group, then running a manufacturing and business operation. From that perspective, I now have another observation. Three of the modules showed good gold plating on the pins, The PCB female sockets are tin plated. This was not the best thing TEK could have done. One of the modules showed very little remaining gold on the pins. This is typical of a Gold tin contact system after many years. Gold migrates easily and eventually forms a eutectic with tin. The result can be depleting of the Gold on the pins and worsening contacts that are much more prone to oxidation.. I am guessing that is what happened here.

Finally, in retrospect, my recommendation would be that those attenuator modules not be pulled all the way out to "reseat" them.

The reseating fixed the channel I worked on. I had not removed the cover of the other channel. I think I am going to save this plugin that had two bad channels (now just one) for parts and I am ready to attempt to fix (hopefully just reseat) the one better plugin that had just one bad channel.

Boy am I glad I apparently do not have to tilt with that contact cleaning windmill.

Thanks and I will come back with the final chapter after I clean up the other one (or not).. :-)

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 5:48 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Bill,

The attenuator modules are the little rectangular things a bit like Chiclets - there are four of them. They will have something like "307-10xx-0x" (the part number) written on them as well as "2X", "4X", "10X" or "100X", i.e. the attenuation value. As I said, they just pull out using your fingernails; there shouldn't be anything stopping that, once the thin ally cover is off. I'm not sure what you are calling the "scaling shaft", but I don't see anything like a shaft on mine. EDIT: I think I have worked it out - is this shaft going to a pot at the rear of the plug-in and is for the "POSITION" and "IDENTIFY" controls? If that is so, then yes, it will need to be removed to get the cover off, probably. My 7A18 has its "POSITION" and "IDENTIFY" controls mounted on the front and the four-way ribbon cables with the ferrite rings connect them to the circuit-board.

No! don't get Deoxit anywhere near that board. It is made from polyphenylene oxide and it has "excellent electrical characteristics" as stated in the manual. It also says, regarding cleaning it "use only water-soluble detergents, ethyl, methyl or isopropyl alcohol". One assumes that any water-soluble detergents used would need to be thoroughly removed, too (probably with deionized water).

I sort of wish I hadn't poked around in mine, because that cover over the module is a swine to get back on properly. To make sure that the "springy fingers" all go back to the right place, the top of the frame pretty much has to come off, unless you have really tiny fingers with lights on the ends. As I said, that entails taking all of the knobs off the front panel and popping it off. A small screwdriver is useful to prise at top and bottom. I neglected to say that there are also two countersunk screws which fix brackets for a circuit-board which have to be removed, plus another countersunk screw which fastens the earth-strap and an earth tag. These are easily removed, but really fiddly to put back together again (I still haven't got that fixed yet). Why did Tek make some things so easy to maintain and others such a pain?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 11:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Quick comment. No smoking at all here. I have had that scope and plug ins for over 30 years. No Diesel engines and very very clean environment (Go to K0CXX.com) so not sure what caused this issue to rear its head. I am now wondering if just a good reseat will work - with perhaps just a very carefully applied tiny bit of Deoxit on the pins. What do you think of that?

I will read more but I take it the scaling shaft will have to come out before that attenuator module is pulled and reseated. All material and plugins are in shop a couple hundred feet away and it is middle of night here. :-)

B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:40 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Albert,
I think you are right about the 5mV/DIV setting, thinking about it. However, I believe I remember something about detecting which attenuator module(s) are the culprit(s) by finding which VOLT/DIV settings show problems, referring to the info in the manual. It seems that they are all bad, as Bill hasn't indicated that they differ very much. This seem pretty improbable to me, especially since some channels are fine and some not. I wonder if this scope has been used near to a badly-maintained diesel engine or by a chain-smoker?
Anyway, the problem might be getting solved by having a good 7A18 from Jim.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 02 March 2019 21:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:


---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert

Re: Help with 7A18 problem

Colin Herbert
 

That is good news, Bill, well done. Now that you have achieved that, why not sort the other channel, then you will have a working 7A18?

I wasn't aware that the attenuator modules had gold-plated contact pins, but it doesn't surprise me. What *does* surprise me is that the pcb sockets are tin-plated. I would have thought that they would be gold-plated, as is most of that board. Considering that the board is located, circuit-wise, right at the front of the scope, one would expect that the greatest care would have been made to ensure the best possible contacts. There is a lot of care there, what with the complex and fiddly-to-put-back-together screening and Tek could have done better.

I think I did warn you about the care needed in re-seating the attenuator modules, didn't I?

The pictures that Peter linked to on TekWiki were excellent info. I was wondering how to get some pictures, but completely overlooked that TekWiki might have just the ticket!

Something that occurred to me today, after what Albert pointed out about the 5mV/DIV setting being possibly compromised by dirty switch contacts, was that it is possible to see if the attenuator assembly has any problems by putting an appropriate signal into the input and a scope probe on to the output, which is the junction of R130, C210 and R210 or the equivalent components on the other channel. This of course requires either an 7000-series extender or some other way to access the 7A18 plug-in with a probe, plus another fully-functioning scope. Turning the VOLTS/DIV switch will allow examination of the signal-path, to some extent. This could be achieved without an extender, with one scope of the 7000-series having two vertical amplifiers, by removing both the side-panel of the DUT amplifier and the left side-panel of the mainframe. If the DUT amplifier plug-in is in the left bay, then I think it would be possible to connect the scope probe from the working vertical amplifier plug-in to the DUT one? You wouldn't need to be accepting the signal from the DUT amplifier, though this may be of use. Is there any sense in my suggestion, or have I got it completely wrong?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 17:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Good news all. I went out and first inspected and learned more, then I removed (one at a time) the attenuator modules and reseated them. That is the short version of a longer story. The pins are very thin and bend easily if the modules are not removed straight up. That was a learning lesson that I leave in the thread here. The PC board holes have almost no taper and you have to have those pins really straight to get them back in.

In stating a little about my background before, I did not mention that I worked for almost half my career in the semiconductor industry, first in process modeling and device development, then running a RF Power design group, then running a manufacturing and business operation. From that perspective, I now have another observation. Three of the modules showed good gold plating on the pins, The PCB female sockets are tin plated. This was not the best thing TEK could have done. One of the modules showed very little remaining gold on the pins. This is typical of a Gold tin contact system after many years. Gold migrates easily and eventually forms a eutectic with tin. The result can be depleting of the Gold on the pins and worsening contacts that are much more prone to oxidation.. I am guessing that is what happened here.

Finally, in retrospect, my recommendation would be that those attenuator modules not be pulled all the way out to "reseat" them.

The reseating fixed the channel I worked on. I had not removed the cover of the other channel. I think I am going to save this plugin that had two bad channels (now just one) for parts and I am ready to attempt to fix (hopefully just reseat) the one better plugin that had just one bad channel.

Boy am I glad I apparently do not have to tilt with that contact cleaning windmill.

Thanks and I will come back with the final chapter after I clean up the other one (or not).. :-)

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 5:48 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Bill,

The attenuator modules are the little rectangular things a bit like Chiclets - there are four of them. They will have something like "307-10xx-0x" (the part number) written on them as well as "2X", "4X", "10X" or "100X", i.e. the attenuation value. As I said, they just pull out using your fingernails; there shouldn't be anything stopping that, once the thin ally cover is off. I'm not sure what you are calling the "scaling shaft", but I don't see anything like a shaft on mine. EDIT: I think I have worked it out - is this shaft going to a pot at the rear of the plug-in and is for the "POSITION" and "IDENTIFY" controls? If that is so, then yes, it will need to be removed to get the cover off, probably. My 7A18 has its "POSITION" and "IDENTIFY" controls mounted on the front and the four-way ribbon cables with the ferrite rings connect them to the circuit-board.

No! don't get Deoxit anywhere near that board. It is made from polyphenylene oxide and it has "excellent electrical characteristics" as stated in the manual. It also says, regarding cleaning it "use only water-soluble detergents, ethyl, methyl or isopropyl alcohol". One assumes that any water-soluble detergents used would need to be thoroughly removed, too (probably with deionized water).

I sort of wish I hadn't poked around in mine, because that cover over the module is a swine to get back on properly. To make sure that the "springy fingers" all go back to the right place, the top of the frame pretty much has to come off, unless you have really tiny fingers with lights on the ends. As I said, that entails taking all of the knobs off the front panel and popping it off. A small screwdriver is useful to prise at top and bottom. I neglected to say that there are also two countersunk screws which fix brackets for a circuit-board which have to be removed, plus another countersunk screw which fastens the earth-strap and an earth tag. These are easily removed, but really fiddly to put back together again (I still haven't got that fixed yet). Why did Tek make some things so easy to maintain and others such a pain?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 11:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Quick comment. No smoking at all here. I have had that scope and plug ins for over 30 years. No Diesel engines and very very clean environment (Go to K0CXX.com) so not sure what caused this issue to rear its head. I am now wondering if just a good reseat will work - with perhaps just a very carefully applied tiny bit of Deoxit on the pins. What do you think of that?

I will read more but I take it the scaling shaft will have to come out before that attenuator module is pulled and reseated. All material and plugins are in shop a couple hundred feet away and it is middle of night here. :-)

B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:40 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Albert,
I think you are right about the 5mV/DIV setting, thinking about it. However, I believe I remember something about detecting which attenuator module(s) are the culprit(s) by finding which VOLT/DIV settings show problems, referring to the info in the manual. It seems that they are all bad, as Bill hasn't indicated that they differ very much. This seem pretty improbable to me, especially since some channels are fine and some not. I wonder if this scope has been used near to a badly-maintained diesel engine or by a chain-smoker?
Anyway, the problem might be getting solved by having a good 7A18 from Jim.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 02 March 2019 21:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:


---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert

2465-erratic cursors

@DC912
 

The Delta Ref and Delta pots on my 2465 are very erratic (both cursors jump around when the pots are turned whether in independent or tracking mode), but all of the other pots on the front panel seem to work fine. Any troubleshooting suggestions would be appreciated. Thanks. Dave

Re: 475 with no Display and no HV

SuddenLink
 

I missed identified C1304. Instead it was C1468 with a broken lead that I pulled from the circuit board. A perfect example of a mistake made when one works on a problem to long without stepping away for a while.

C1436 on the -8 volt rail was replaced with a 33uf capacitor.

Reed,

I checked C1304 and it is fine, measures 2.7uf on the dot. CR1304 was replaced with a IN4007 per your recommendation and I tried your test to the base of Q1318.

The first time I taped the base of Q1318 with a 1K resistor in series with the +5 volt rail the waveform at TP1318 more than doubled in amplitude from 760mv p-p to 2.2 volts p-p and has stayed there. The frequency however, is now down to 14.4Khz

Now when I touch the lead of the 1K resistor from the +5 volt rail to the base of Q1318 the Oscillator waveform at TP1318 jumps up to 7.2 volts p-p still at 14.4 Khz. When I take the 1K resistor away the waveform drops back to 2.2 volts.

BREAKING NEWS:

While typing this report up I turned back to the scope, taped the 1K resistor from the +5 volt rail to the base of Q1318 again. This time the waveform at TP1318 jumped up to 30.8 volts p-p and now I have a trace on the CRT. Take the 1K resistor away and the waveform at TP1318 keeps running at 30.8 volts p-p and there is still a trace on the CRT. Note: The trace is only covering about a two-thirds of the screen and the horizontal position is behaving strange. Instead of sliding the whole trace left or right. It erases part of trace moving towards the middle of the screen if the Horizontal position is turn left or right. I suspect that is because the HV oscillator is only running at 13.8Khz.

I shut the scope off then turn it back on. The waveform at TP1318 is down to 2.2 volts p-p. Tap the base of Q1318 with the lead from the 1k resistor going to +5 volt rail and the oscillator waveform is goosed to 30.8 volts p-p again. The trace on the CRT is present as before and stays there until I shut the scope off and turn it back on.

How weird is that!

So now what? Is Q1318 flaky? I kind of doubt that. C1316 was replaced with a new 1uf tantalum capacitor. C1317 was pulled and checked. The LCR meter says it’s a 10uf capacitor. I don’t remember what the ESR was but I don’t remember it raising an eyebrow with me. CR1317 was check with the diode function on my HP 3466 DMM but it could be leaky…maybe?

What the Heck?

Ripley

Sent from Mail for Windows 10

From: Steph L
Sent: Saturday, March 2, 2019 5:43 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 475 with no Display and no HV

Page 3-35 of manual has a good functional description including point that ambient current supplied to base (of Q1318) determines frequency (ie not LC arrangement) and as such there isn’t a specific cap component for resonate frequency (a function oscillator still relies upon however to function).

You said “The rest of the voltage measurement compared to the manual are no where near correct” so perhaps a closer look at the whole setup noting Q1308/10 push-pull, gain and phase reversal Q1306 and bias arrangement behind Q1310 (ie the Q1312 and associated components).

I have an old Tek544 where the HT falls dramatically when ambient room temp too hot and was amazed in the frequency shift involved.

It is a different circuit but 475 still has resonance involved and the output voltage is regulated by driving current harder into the base.

Just a thought. Please excuse me/any error as nubie here. Interesting fault.

Re: Help with 7A18 problem

 

On Sun, Mar 3, 2019 at 06:31 PM, Bill Carns wrote:


The reseating fixed the channel I worked on.
I'm a bit surprised that a well known problem with the removable attenuator modules has not been mentioned here (unless I missed it somewhere). The problem has been mentioned in this group several times and is much more common than bad contact between module and socket:
The pins of the attenuator are fixed to the circuit inside (conductive traces on ceramic substrate) by low-temperature soldering. In a great many cases the quality of the connection is bad, over time leading to contact deterioration.
Removing and reseating the module often causes a very slight movement of the pin inside with the rubbing action temporarily restoring contact between pin and conductive trace on the ceramic substrate.
This leads to the incorrect assumption that the problem has been solved by the rubbing action in the socket whereas the problem will reappear after a while.
Often just repositioning (tilting the module slightly) makes the problem disappear and subsequent repositioning (or time) makes it reappear.
It is quite feasibile to repair the module by opening it (click-locked lid) and resoldering but you need good eyes, dexterity and good tools.

Raymond

Re: Help with 7A18 problem

 

Those inserts into which the attenuators plug could be miniserts or related to them. If they are, the outside is tin/lead plate, but inside there's a gold plated spring BeCu contact.

See: <https://groups.io/g/TekScopes/files/Minisert.pdf>

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 03 March 2019 18:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

That is good news, Bill, well done. Now that you have achieved that, why not sort the other channel, then you will have a working 7A18?

I wasn't aware that the attenuator modules had gold-plated contact pins, but it doesn't surprise me. What *does* surprise me is that the pcb sockets are tin-plated. I would have thought that they would be gold-plated, as is most of that board. Considering that the board is located, circuit-wise, right at the front of the scope, one would expect that the greatest care would have been made to ensure the best possible contacts. There is a lot of care there, what with the complex and fiddly-to-put-back-together screening and Tek could have done better.

I think I did warn you about the care needed in re-seating the attenuator modules, didn't I?

The pictures that Peter linked to on TekWiki were excellent info. I was wondering how to get some pictures, but completely overlooked that TekWiki might have just the ticket!

Something that occurred to me today, after what Albert pointed out about the 5mV/DIV setting being possibly compromised by dirty switch contacts, was that it is possible to see if the attenuator assembly has any problems by putting an appropriate signal into the input and a scope probe on to the output, which is the junction of R130, C210 and R210 or the equivalent components on the other channel. This of course requires either an 7000-series extender or some other way to access the 7A18 plug-in with a probe, plus another fully-functioning scope. Turning the VOLTS/DIV switch will allow examination of the signal-path, to some extent. This could be achieved without an extender, with one scope of the 7000-series having two vertical amplifiers, by removing both the side-panel of the DUT amplifier and the left side-panel of the mainframe. If the DUT amplifier plug-in is in the left bay, then I think it would be possible to connect the scope probe from the working vertical amplifier plug-in to the DUT one? You wouldn't need to be accepting the signal from the DUT amplifier, though this may be of use. Is there any sense in my suggestion, or have I got it completely wrong?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 17:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Good news all. I went out and first inspected and learned more, then I removed (one at a time) the attenuator modules and reseated them. That is the short version of a longer story. The pins are very thin and bend easily if the modules are not removed straight up. That was a learning lesson that I leave in the thread here. The PC board holes have almost no taper and you have to have those pins really straight to get them back in.

In stating a little about my background before, I did not mention that I worked for almost half my career in the semiconductor industry, first in process modeling and device development, then running a RF Power design group, then running a manufacturing and business operation. From that perspective, I now have another observation. Three of the modules showed good gold plating on the pins, The PCB female sockets are tin plated. This was not the best thing TEK could have done. One of the modules showed very little remaining gold on the pins. This is typical of a Gold tin contact system after many years. Gold migrates easily and eventually forms a eutectic with tin. The result can be depleting of the Gold on the pins and worsening contacts that are much more prone to oxidation.. I am guessing that is what happened here.

Finally, in retrospect, my recommendation would be that those attenuator modules not be pulled all the way out to "reseat" them.

The reseating fixed the channel I worked on. I had not removed the cover of the other channel. I think I am going to save this plugin that had two bad channels (now just one) for parts and I am ready to attempt to fix (hopefully just reseat) the one better plugin that had just one bad channel.

Boy am I glad I apparently do not have to tilt with that contact cleaning windmill.

Thanks and I will come back with the final chapter after I clean up the other one (or not).. :-)

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 5:48 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Bill,

The attenuator modules are the little rectangular things a bit like Chiclets - there are four of them. They will have something like "307-10xx-0x" (the part number) written on them as well as "2X", "4X", "10X" or "100X", i.e. the attenuation value. As I said, they just pull out using your fingernails; there shouldn't be anything stopping that, once the thin ally cover is off. I'm not sure what you are calling the "scaling shaft", but I don't see anything like a shaft on mine. EDIT: I think I have worked it out - is this shaft going to a pot at the rear of the plug-in and is for the "POSITION" and "IDENTIFY" controls? If that is so, then yes, it will need to be removed to get the cover off, probably. My 7A18 has its "POSITION" and "IDENTIFY" controls mounted on the front and the four-way ribbon cables with the ferrite rings connect them to the circuit-board.

No! don't get Deoxit anywhere near that board. It is made from polyphenylene oxide and it has "excellent electrical characteristics" as stated in the manual. It also says, regarding cleaning it "use only water-soluble detergents, ethyl, methyl or isopropyl alcohol". One assumes that any water-soluble detergents used would need to be thoroughly removed, too (probably with deionized water).

I sort of wish I hadn't poked around in mine, because that cover over the module is a swine to get back on properly. To make sure that the "springy fingers" all go back to the right place, the top of the frame pretty much has to come off, unless you have really tiny fingers with lights on the ends. As I said, that entails taking all of the knobs off the front panel and popping it off. A small screwdriver is useful to prise at top and bottom. I neglected to say that there are also two countersunk screws which fix brackets for a circuit-board which have to be removed, plus another countersunk screw which fastens the earth-strap and an earth tag. These are easily removed, but really fiddly to put back together again (I still haven't got that fixed yet). Why did Tek make some things so easy to maintain and others such a pain?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 11:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Quick comment. No smoking at all here. I have had that scope and plug ins for over 30 years. No Diesel engines and very very clean environment (Go to K0CXX.com) so not sure what caused this issue to rear its head. I am now wondering if just a good reseat will work - with perhaps just a very carefully applied tiny bit of Deoxit on the pins. What do you think of that?

I will read more but I take it the scaling shaft will have to come out before that attenuator module is pulled and reseated. All material and plugins are in shop a couple hundred feet away and it is middle of night here. :-)

B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:40 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Albert,
I think you are right about the 5mV/DIV setting, thinking about it. However, I believe I remember something about detecting which attenuator module(s) are the culprit(s) by finding which VOLT/DIV settings show problems, referring to the info in the manual. It seems that they are all bad, as Bill hasn't indicated that they differ very much. This seem pretty improbable to me, especially since some channels are fine and some not. I wonder if this scope has been used near to a badly-maintained diesel engine or by a chain-smoker?
Anyway, the problem might be getting solved by having a good 7A18 from Jim.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 02 March 2019 21:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:


---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert

Re: Uncooperative 7834

 

On Sun, Mar 3, 2019 at 05:59 PM, David C. Partridge wrote:


Disconnecting the supply to the HT circuit (i.e. no EHT, no 130V), changes the
nature of the ticking, but doesn't stop it.
I guess you mean disconnecting the 25kHz from the power supply.
In my manual there is a note I made many years ago to myself which says:
"If disconnected the power supply will go into burst mode" with arrows pointing at the 25 kHz connectors.
So I'd disconnect the multiplier to start with.

/Håkan

Re: 475 with no Display and no HV

Steph L
 

Hi Ripley,
How about C1305 0.1uF. If open, gain of Q1306 (and loop) will be lower and insufficient to kick off oscillation? (Great suggestion from Don re 1K pull up touch on Q1318 base.)
Steph

Re: Help with 7A18 problem

Albert Otten
 

Colin,

Your suggestion should work I think but only helps when the 7A18 Input Amp itself is out of order or can't be trusted. In that case it would even be simpler to test the 7A18 stand-alone, not in a mainframe. With small signal levels, say less than 0.5 V, there will be no leakage yet via conducting semiconductors (like CR210) and no harm can be done to the FET Q210A. The only load is formed by the probe tip.

Bill,

Very good that you solved the problem and explained what the problem was. With my suggested V/div check I overlooked that the cam activated contacts are not the only possible cause of troubles. I could have known better since I have had similar problems with 7A19 or 7A29. I have a dozen of 7A18/7A26 but never felt he need to look inside a switch compartment. Now I opened a 7A18N which seems easier, missing the long strip PCB over one compartment side. But in the 7A18N the compartment is completely closed. Hence it wasn't immediately clear to me that there was just one PCB inside the compartment and that this pcb has switch contacts at both sides of the PCB, in this case "left" and "right", not "top" and "bottom".

Albert

On Sun, Mar 3, 2019 at 07:53 PM, Colin Herbert wrote:


---
Something that occurred to me today, after what Albert pointed out about the
5mV/DIV setting being possibly compromised by dirty switch contacts, was that
it is possible to see if the attenuator assembly has any problems by putting
an appropriate signal into the input and a scope probe on to the output, which
is the junction of R130, C210 and R210 or the equivalent components on the
other channel. This of course requires either an 7000-series extender or some
other way to access the 7A18 plug-in with a probe, plus another
fully-functioning scope. Turning the VOLTS/DIV switch will allow examination
of the signal-path, to some extent. This could be achieved without an
extender, with one scope of the 7000-series having two vertical amplifiers,
by removing both the side-panel of the DUT amplifier and the left side-panel
of the mainframe. If the DUT amplifier plug-in is in the left bay, then I
think it would be possible to connect the scope probe from the working
vertical amplifier plug-in to the DUT one? You wouldn't need to be accepting
the signal from the DUT amplifier, though this may be of use. Is there any
sense in my suggestion, or have I got it completely wrong?

Colin.

Re: Help with 7A18 problem

Bill Carns
 

Your idea about looking down the chain of attenuators with another scope is a good one I think, I will explore about panel removal if that becomes necessary. Re the "other channel, yes - I am tempted but then I also have that broken pot to reattach and also there is a mechanical slippage issue with the AC/DC/GND switch on one channel that I also discovered while trying the old trick of just running things back and forth some number of times, so I am not sure what I will do. Knowing me, I will probably try and fix it. I have NEVER been one to part things out - in fact the opposite. I have bought what I thought were parts rigs over the years and always wound up restoring them.

Thanks so much for all your help and to the others also. I will keep you all informed about what happens .

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 12:53 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

That is good news, Bill, well done. Now that you have achieved that, why not sort the other channel, then you will have a working 7A18?

I wasn't aware that the attenuator modules had gold-plated contact pins, but it doesn't surprise me. What *does* surprise me is that the pcb sockets are tin-plated. I would have thought that they would be gold-plated, as is most of that board. Considering that the board is located, circuit-wise, right at the front of the scope, one would expect that the greatest care would have been made to ensure the best possible contacts. There is a lot of care there, what with the complex and fiddly-to-put-back-together screening and Tek could have done better.

I think I did warn you about the care needed in re-seating the attenuator modules, didn't I?

The pictures that Peter linked to on TekWiki were excellent info. I was wondering how to get some pictures, but completely overlooked that TekWiki might have just the ticket!

Something that occurred to me today, after what Albert pointed out about the 5mV/DIV setting being possibly compromised by dirty switch contacts, was that it is possible to see if the attenuator assembly has any problems by putting an appropriate signal into the input and a scope probe on to the output, which is the junction of R130, C210 and R210 or the equivalent components on the other channel. This of course requires either an 7000-series extender or some other way to access the 7A18 plug-in with a probe, plus another fully-functioning scope. Turning the VOLTS/DIV switch will allow examination of the signal-path, to some extent. This could be achieved without an extender, with one scope of the 7000-series having two vertical amplifiers, by removing both the side-panel of the DUT amplifier and the left side-panel of the mainframe. If the DUT amplifier plug-in is in the left bay, then I think it would be possible to connect the scope probe from the working vertical amplifier plug-in to the DUT one? You wouldn't need to be accepting the signal from the DUT amplifier, though this may be of use. Is there any sense in my suggestion, or have I got it completely wrong?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 17:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Good news all. I went out and first inspected and learned more, then I removed (one at a time) the attenuator modules and reseated them. That is the short version of a longer story. The pins are very thin and bend easily if the modules are not removed straight up. That was a learning lesson that I leave in the thread here. The PC board holes have almost no taper and you have to have those pins really straight to get them back in.

In stating a little about my background before, I did not mention that I worked for almost half my career in the semiconductor industry, first in process modeling and device development, then running a RF Power design group, then running a manufacturing and business operation. From that perspective, I now have another observation. Three of the modules showed good gold plating on the pins, The PCB female sockets are tin plated. This was not the best thing TEK could have done. One of the modules showed very little remaining gold on the pins. This is typical of a Gold tin contact system after many years. Gold migrates easily and eventually forms a eutectic with tin. The result can be depleting of the Gold on the pins and worsening contacts that are much more prone to oxidation.. I am guessing that is what happened here.

Finally, in retrospect, my recommendation would be that those attenuator modules not be pulled all the way out to "reseat" them.

The reseating fixed the channel I worked on. I had not removed the cover of the other channel. I think I am going to save this plugin that had two bad channels (now just one) for parts and I am ready to attempt to fix (hopefully just reseat) the one better plugin that had just one bad channel.

Boy am I glad I apparently do not have to tilt with that contact cleaning windmill.

Thanks and I will come back with the final chapter after I clean up the other one (or not).. :-)

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 5:48 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Bill,

The attenuator modules are the little rectangular things a bit like Chiclets - there are four of them. They will have something like "307-10xx-0x" (the part number) written on them as well as "2X", "4X", "10X" or "100X", i.e. the attenuation value. As I said, they just pull out using your fingernails; there shouldn't be anything stopping that, once the thin ally cover is off. I'm not sure what you are calling the "scaling shaft", but I don't see anything like a shaft on mine. EDIT: I think I have worked it out - is this shaft going to a pot at the rear of the plug-in and is for the "POSITION" and "IDENTIFY" controls? If that is so, then yes, it will need to be removed to get the cover off, probably. My 7A18 has its "POSITION" and "IDENTIFY" controls mounted on the front and the four-way ribbon cables with the ferrite rings connect them to the circuit-board.

No! don't get Deoxit anywhere near that board. It is made from polyphenylene oxide and it has "excellent electrical characteristics" as stated in the manual. It also says, regarding cleaning it "use only water-soluble detergents, ethyl, methyl or isopropyl alcohol". One assumes that any water-soluble detergents used would need to be thoroughly removed, too (probably with deionized water).

I sort of wish I hadn't poked around in mine, because that cover over the module is a swine to get back on properly. To make sure that the "springy fingers" all go back to the right place, the top of the frame pretty much has to come off, unless you have really tiny fingers with lights on the ends. As I said, that entails taking all of the knobs off the front panel and popping it off. A small screwdriver is useful to prise at top and bottom. I neglected to say that there are also two countersunk screws which fix brackets for a circuit-board which have to be removed, plus another countersunk screw which fastens the earth-strap and an earth tag. These are easily removed, but really fiddly to put back together again (I still haven't got that fixed yet). Why did Tek make some things so easy to maintain and others such a pain?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 11:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Quick comment. No smoking at all here. I have had that scope and plug ins for over 30 years. No Diesel engines and very very clean environment (Go to K0CXX.com) so not sure what caused this issue to rear its head. I am now wondering if just a good reseat will work - with perhaps just a very carefully applied tiny bit of Deoxit on the pins. What do you think of that?

I will read more but I take it the scaling shaft will have to come out before that attenuator module is pulled and reseated. All material and plugins are in shop a couple hundred feet away and it is middle of night here. :-)

B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:40 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Albert,
I think you are right about the 5mV/DIV setting, thinking about it. However, I believe I remember something about detecting which attenuator module(s) are the culprit(s) by finding which VOLT/DIV settings show problems, referring to the info in the manual. It seems that they are all bad, as Bill hasn't indicated that they differ very much. This seem pretty improbable to me, especially since some channels are fine and some not. I wonder if this scope has been used near to a badly-maintained diesel engine or by a chain-smoker?
Anyway, the problem might be getting solved by having a good 7A18 from Jim.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 02 March 2019 21:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:


---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert

Re: Help with 7A18 problem

Bill Carns
 

Good inputs also Raymond. Not good news - that, but good info.

I will keep eyes peeled for this effect.

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Raymond Domp Frank
Sent: Sunday, March 03, 2019 2:06 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sun, Mar 3, 2019 at 06:31 PM, Bill Carns wrote:


The reseating fixed the channel I worked on.
I'm a bit surprised that a well known problem with the removable attenuator modules has not been mentioned here (unless I missed it somewhere). The problem has been mentioned in this group several times and is much more common than bad contact between module and socket:
The pins of the attenuator are fixed to the circuit inside (conductive traces on ceramic substrate) by low-temperature soldering. In a great many cases the quality of the connection is bad, over time leading to contact deterioration.
Removing and reseating the module often causes a very slight movement of the pin inside with the rubbing action temporarily restoring contact between pin and conductive trace on the ceramic substrate.
This leads to the incorrect assumption that the problem has been solved by the rubbing action in the socket whereas the problem will reappear after a while.
Often just repositioning (tilting the module slightly) makes the problem disappear and subsequent repositioning (or time) makes it reappear.
It is quite feasibile to repair the module by opening it (click-locked lid) and resoldering but you need good eyes, dexterity and good tools.

Raymond

Re: Help with 7A18 problem

Bill Carns
 

Interesting. I wonder what is causing the deterioration and obvious browning almost to black in some areas of the gold plated pins. This is something that a Gold to Gold contact should not do unless the pin is also touching the Tin area maybe ??

More questions again.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of David C. Partridge
Sent: Sunday, March 03, 2019 2:26 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Those inserts into which the attenuators plug could be miniserts or related to them. If they are, the outside is tin/lead plate, but inside there's a gold plated spring BeCu contact.

See: <https://groups.io/g/TekScopes/files/Minisert.pdf>

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 03 March 2019 18:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

That is good news, Bill, well done. Now that you have achieved that, why not sort the other channel, then you will have a working 7A18?

I wasn't aware that the attenuator modules had gold-plated contact pins, but it doesn't surprise me. What *does* surprise me is that the pcb sockets are tin-plated. I would have thought that they would be gold-plated, as is most of that board. Considering that the board is located, circuit-wise, right at the front of the scope, one would expect that the greatest care would have been made to ensure the best possible contacts. There is a lot of care there, what with the complex and fiddly-to-put-back-together screening and Tek could have done better.

I think I did warn you about the care needed in re-seating the attenuator modules, didn't I?

The pictures that Peter linked to on TekWiki were excellent info. I was wondering how to get some pictures, but completely overlooked that TekWiki might have just the ticket!

Something that occurred to me today, after what Albert pointed out about the 5mV/DIV setting being possibly compromised by dirty switch contacts, was that it is possible to see if the attenuator assembly has any problems by putting an appropriate signal into the input and a scope probe on to the output, which is the junction of R130, C210 and R210 or the equivalent components on the other channel. This of course requires either an 7000-series extender or some other way to access the 7A18 plug-in with a probe, plus another fully-functioning scope. Turning the VOLTS/DIV switch will allow examination of the signal-path, to some extent. This could be achieved without an extender, with one scope of the 7000-series having two vertical amplifiers, by removing both the side-panel of the DUT amplifier and the left side-panel of the mainframe. If the DUT amplifier plug-in is in the left bay, then I think it would be possible to connect the scope probe from the working vertical amplifier plug-in to the DUT one? You wouldn't need to be accepting the signal from the DUT amplifier, though this may be of use. Is there any sense in my suggestion, or have I got it completely wrong?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 17:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Good news all. I went out and first inspected and learned more, then I removed (one at a time) the attenuator modules and reseated them. That is the short version of a longer story. The pins are very thin and bend easily if the modules are not removed straight up. That was a learning lesson that I leave in the thread here. The PC board holes have almost no taper and you have to have those pins really straight to get them back in.

In stating a little about my background before, I did not mention that I worked for almost half my career in the semiconductor industry, first in process modeling and device development, then running a RF Power design group, then running a manufacturing and business operation. From that perspective, I now have another observation. Three of the modules showed good gold plating on the pins, The PCB female sockets are tin plated. This was not the best thing TEK could have done. One of the modules showed very little remaining gold on the pins. This is typical of a Gold tin contact system after many years. Gold migrates easily and eventually forms a eutectic with tin. The result can be depleting of the Gold on the pins and worsening contacts that are much more prone to oxidation.. I am guessing that is what happened here.

Finally, in retrospect, my recommendation would be that those attenuator modules not be pulled all the way out to "reseat" them.

The reseating fixed the channel I worked on. I had not removed the cover of the other channel. I think I am going to save this plugin that had two bad channels (now just one) for parts and I am ready to attempt to fix (hopefully just reseat) the one better plugin that had just one bad channel.

Boy am I glad I apparently do not have to tilt with that contact cleaning windmill.

Thanks and I will come back with the final chapter after I clean up the other one (or not).. :-)

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 5:48 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Bill,

The attenuator modules are the little rectangular things a bit like Chiclets - there are four of them. They will have something like "307-10xx-0x" (the part number) written on them as well as "2X", "4X", "10X" or "100X", i.e. the attenuation value. As I said, they just pull out using your fingernails; there shouldn't be anything stopping that, once the thin ally cover is off. I'm not sure what you are calling the "scaling shaft", but I don't see anything like a shaft on mine. EDIT: I think I have worked it out - is this shaft going to a pot at the rear of the plug-in and is for the "POSITION" and "IDENTIFY" controls? If that is so, then yes, it will need to be removed to get the cover off, probably. My 7A18 has its "POSITION" and "IDENTIFY" controls mounted on the front and the four-way ribbon cables with the ferrite rings connect them to the circuit-board.

No! don't get Deoxit anywhere near that board. It is made from polyphenylene oxide and it has "excellent electrical characteristics" as stated in the manual. It also says, regarding cleaning it "use only water-soluble detergents, ethyl, methyl or isopropyl alcohol". One assumes that any water-soluble detergents used would need to be thoroughly removed, too (probably with deionized water).

I sort of wish I hadn't poked around in mine, because that cover over the module is a swine to get back on properly. To make sure that the "springy fingers" all go back to the right place, the top of the frame pretty much has to come off, unless you have really tiny fingers with lights on the ends. As I said, that entails taking all of the knobs off the front panel and popping it off. A small screwdriver is useful to prise at top and bottom. I neglected to say that there are also two countersunk screws which fix brackets for a circuit-board which have to be removed, plus another countersunk screw which fastens the earth-strap and an earth tag. These are easily removed, but really fiddly to put back together again (I still haven't got that fixed yet). Why did Tek make some things so easy to maintain and others such a pain?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 11:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Quick comment. No smoking at all here. I have had that scope and plug ins for over 30 years. No Diesel engines and very very clean environment (Go to K0CXX.com) so not sure what caused this issue to rear its head. I am now wondering if just a good reseat will work - with perhaps just a very carefully applied tiny bit of Deoxit on the pins. What do you think of that?

I will read more but I take it the scaling shaft will have to come out before that attenuator module is pulled and reseated. All material and plugins are in shop a couple hundred feet away and it is middle of night here. :-)

B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:40 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Albert,
I think you are right about the 5mV/DIV setting, thinking about it. However, I believe I remember something about detecting which attenuator module(s) are the culprit(s) by finding which VOLT/DIV settings show problems, referring to the info in the manual. It seems that they are all bad, as Bill hasn't indicated that they differ very much. This seem pretty improbable to me, especially since some channels are fine and some not. I wonder if this scope has been used near to a badly-maintained diesel engine or by a chain-smoker?
Anyway, the problem might be getting solved by having a good 7A18 from Jim.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 02 March 2019 21:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:


---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert

Re: Help with 7A18 problem

Bill Carns
 

Well, I hope I solved the problem and my "explanation" may be somewhat suspect if the inner contact is Gold plated BeCu? This is one complex little animal I think.

Regards

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: Sunday, March 03, 2019 3:17 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Colin,

Your suggestion should work I think but only helps when the 7A18 Input Amp itself is out of order or can't be trusted. In that case it would even be simpler to test the 7A18 stand-alone, not in a mainframe. With small signal levels, say less than 0.5 V, there will be no leakage yet via conducting semiconductors (like CR210) and no harm can be done to the FET Q210A. The only load is formed by the probe tip.

Bill,

Very good that you solved the problem and explained what the problem was. With my suggested V/div check I overlooked that the cam activated contacts are not the only possible cause of troubles. I could have known better since I have had similar problems with 7A19 or 7A29. I have a dozen of 7A18/7A26 but never felt he need to look inside a switch compartment. Now I opened a 7A18N which seems easier, missing the long strip PCB over one compartment side. But in the 7A18N the compartment is completely closed. Hence it wasn't immediately clear to me that there was just one PCB inside the compartment and that this pcb has switch contacts at both sides of the PCB, in this case "left" and "right", not "top" and "bottom".

Albert


On Sun, Mar 3, 2019 at 07:53 PM, Colin Herbert wrote:


---
Something that occurred to me today, after what Albert pointed out
about the 5mV/DIV setting being possibly compromised by dirty switch
contacts, was that it is possible to see if the attenuator assembly
has any problems by putting an appropriate signal into the input and a
scope probe on to the output, which is the junction of R130, C210 and
R210 or the equivalent components on the other channel. This of course
requires either an 7000-series extender or some other way to access
the 7A18 plug-in with a probe, plus another fully-functioning scope.
Turning the VOLTS/DIV switch will allow examination of the
signal-path, to some extent. This could be achieved without an
extender, with one scope of the 7000-series having two vertical
amplifiers, by removing both the side-panel of the DUT amplifier and
the left side-panel of the mainframe. If the DUT amplifier plug-in is
in the left bay, then I think it would be possible to connect the
scope probe from the working vertical amplifier plug-in to the DUT
one? You wouldn't need to be accepting the signal from the DUT amplifier, though this may be of use. Is there any sense in my suggestion, or have I got it completely wrong?

Colin.

Re: Help with 7A18 problem

Phillip Potter
 

Hi Bill,

I have the same amplifiers and when I first got them, had the same or at least similar issues.

I tore them down, one at a time, and soaked 24lb bond with IPA, to clean those switches.  Yes, they are a bear to get to!  IIRC, I had to disassemble them down a ways to get into them.  There were shields on the inner sides and on the outer sides, too.  Once I had them broken down, it was a simple thing to get the soaked paper into the switch fingers and carefully pull it through, in the same way that a turntable moves a record under the stylus... never backward!

I recommend doing this somewhere where you can leave it laying, disassembled, so that you can see how you got it apart, making it easier to reassemble it.  I took pictures on my phone at each step of the way.  It was a "trick" of a deal getting it broken down to the point where it could be cleaned, however.  Pictures helped me to stay on track and get it all back together!  Tektronix did it's homework... they were made to be repaired... gotta love 'em!

I hope I've inspired you and not the opposite.  I did have to work up my nerve to tear into them, but the result was two operable amplifiers, I am glad that I took my time and worked carefully.

Phil

On 3/2/2019 2:26 PM, Bill Carns wrote:
I'll have to go put the unit back in the mainframe and investigate but my memory says that most were bad.

I have investigated more and remove the upper capacitor cover and, boy are those things hard to get at. Gonna be real hard to get cleaning paper in there on all the switches. Some are not bad, others really bad to get to. One set has an additional little cover over them for some reason.

Working on this thing is not for sissies.


B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: Saturday, March 02, 2019 3:53 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:

---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert





Re: Help with 7A18 problem

Phillip Potter
 

Please disregard!!!

Sorry,

Phil

On 3/3/2019 1:38 PM, Phillip Potter wrote:
Hi Bill,

I have the same amplifiers and when I first got them, had the same or at least similar issues.

I tore them down, one at a time, and soaked 24lb bond with IPA, to clean those switches.  Yes, they are a bear to get to!  IIRC, I had to disassemble them down a ways to get into them.  There were shields on the inner sides and on the outer sides, too.  Once I had them broken down, it was a simple thing to get the soaked paper into the switch fingers and carefully pull it through, in the same way that a turntable moves a record under the stylus... never backward!

I recommend doing this somewhere where you can leave it laying, disassembled, so that you can see how you got it apart, making it easier to reassemble it.  I took pictures on my phone at each step of the way.  It was a "trick" of a deal getting it broken down to the point where it could be cleaned, however.  Pictures helped me to stay on track and get it all back together!  Tektronix did it's homework... they were made to be repaired... gotta love 'em!

I hope I've inspired you and not the opposite.  I did have to work up my nerve to tear into them, but the result was two operable amplifiers, I am glad that I took my time and worked carefully.

Phil

On 3/2/2019 2:26 PM, Bill Carns wrote:
I'll have to go put the unit back in the mainframe and investigate but my memory says that most were bad.

I have investigated more and remove the upper capacitor cover and, boy are those things hard to get at.  Gonna be real hard to get cleaning paper in there on all the switches.  Some are not bad, others really bad to get to.  One set has an additional little cover over them for some reason.

Working on this thing is not for sissies.


B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: Saturday, March 02, 2019 3:53 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar  2, 2019 at 07:58 PM, Colin Herbert wrote:

---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert