Date   
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







Re: 475 with no Display and no HV

SuddenLink
 

UPDATE:

I replaced C1317 and CR1317 with no change. You still had to goose the base of Q1318 with that 1K resistor to the +5 volt rail to get the Oscillator waveform at TP1318 up to 30 volts p-p.

Next I started to check the DC voltages on the primary side of the HV oscillator circuit and compare them to what the manual shows. Some voltages are closer to what the manual says that they should be but are still pretty far off. What is really interesting is when I got over to transistors Q1306 and Q1308, touching the test probe of my HP 3466A DMM to the base of Q1308 caused the oscillator waveform at TP1318 to drop back to 2.2 volts p-p and the trace on the CRT went away. I had to tap the base of Q1318 with that 1K resistor to bring it back up again. The same thing would happen when I touched the test probe to the base of Q1306. Touch the base of the Q1306 with my DMM test lead and the trace on the CRT goes away and the peak to peak voltage back at Test Point 1318 is back down to around 2 volts.

So I switched to my HP 3478A DMM with a higher input impedance and this time I could read the DC voltage at the base of Q1308 but cant remember what it was because when I went to the base of Q1306 the waveform dropped back down to about 1.4 volts p-p, CRT trace disappeared, and now it wont come back. Taping on Q1318 with that 1K resistor with the other end connected to +5 volts only brings the oscillator waveform at TP1318 up to only about 5 volts p-p but it drops back down again when I disconnect.

Hmmm. Well that is enough for now. Time to step away for a while.

Ripley

Sent from Mail for Windows 10

From: SuddenLink
Sent: Sunday, March 3, 2019 3:01 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 475 with no Display and no HV

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: 475 with no Display and no HV

Reed Dickinson
 

I just re-read your trouble note and I suspect you have identified the problem when you said the trace is only 2/3 of the normal width.  This points immediately to a defective U1321 voltage tripler.  Unsolder the lead from T1320, pin 12, tripler and power on, look for normal waveforms around Q1318.  If your tripler is defective a replacement 3X can be had in an NTE539 or ECG539 (an NTE538 or ECG538 would work too).
By momentarily connecting the base of Q1318 to +5 through a 1K resistor you have bypassed all the regulation that you get from Q1306 through Q1316 and you have proved that the transformer and associated circuit are working.  The shortened trace you saw was probably due to the unregulated HV giving a shorter trace or a defective voltage tripler U1321.  I would still look for problems around C1304.  When you power up the time constant of R1304 and C1304 is about 680ms.  This means that you have under a second for the HV to stabilize since the base of Q1306 is being pulled toward +110V until it is nailed at +0.6 by the conduction of CR1304.  After about 1 second the base of Q1306 should go to about +0.6V and oscillations should occur via Q1318.
To troubleshoot the regulator circuit pull F1318 out and disconnect P1313, the transistor Q1318 is now out of the circuit.  Connect a 25K pot between +15 and - 5  and connect the wiper of the pot through a 10K resistor to the Q1306 base node.  Connect a sensitive voltmeter to the - side of C1316 and power on your scope.  Connect another voltmeter to the Q1306 base node and vary the pot.  When the base of Q1306 is around +0,6V you should see some variation at the - side of C1316 (a 4.7K temporary resistor to ground will be necessary to give a little current through Q1316),  If no variation is seen when rotating the pot then you have a problem in the regulator circuit, troubleshoot it and you will find the problem.  If a variation is seen then the regulator is working and you need to fix problems on the secondaries of the transformer.  Before using the following shotgun replacement procedure unsolder the lead from T1320 pin 12 going to the 3X voltage multiplier.  If the multiplier is bad it would put a load on the oscillator circuit and kill the oscillations.  Next, use the shotgun approach and replace C1320, CR1320, CR1321, C1322, C1323, C1328 and CR1329.  I recommend you use HV caps rated at least 5KV as the surge of HV during the first second is high.
Good luck:
Reed Dickinson

On Sunday, March 3, 2019, 12:01:12 PM PST, SuddenLink <bob.ripley@...> wrote:

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

Bill Carns
 

What ?? Now confused. Was there something wrong with your first reply?

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Phillip Potter
Sent: Sunday, March 03, 2019 3:46 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

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








Re: Uncooperative 7834

fiftythreebuick
 

Definitely good ideas, Harvey! I particularly like the idea of pulsing a current limited supply into the rail and checking around along that rail for the lowest amplitude pulse. That should get you close to the offending component! Have to remember that one....



"Harvey White wrote:

There's another way, that's to take a current limited supply and pulse
it on and off. For a supply of X amps, I'd limit it to x/10 amps.
Now, if you have an HP current probe, the one that's made for shorted
nodes, you can see where the bright lights go, and that shows you the
track where the short is.

You might be able to make a probe that does that.

With this same setup, you can also take a probe and with a high
sensitivity range on the scope, look for the lowest amplitude of
pulse. You'd like to measure across the component itself, if at all
possible.

This is a different implementation of the second mode.

Harvey"

Re: Uncooperative 7834

Majdi S. Abbas
 

On Fri, Mar 01, 2019 at 06:11:50PM -0000, David C. Partridge wrote:
Specifed Measured
+130V 6.6k 6.8k
+50V 1.8k 1.75k
+15V 70R 52R
+5V 9R 8.4R
-15V 100R 60R
The -15V worries me a bit -- I've seen a shorted tantalum
around 40 ohms on this rail.

The character generator board has 1 uF tantalums directly
across the -15V and -50V rails; try disconnecting it and see
what happens with your -15V measurement.

--msa

Re: 2247A PSU Troubleshooting

Nicholas Keller
 

I finally got a chance to take a look at this 2247a. At the test point you
mentioned (c2202, before the startup and pre-regulator, marked 14A on the
schematic), I’m only getting 77VDC, but got about the same 77V when I
tested for AC. I measured 120-122VAC up to the diode ring. Does this
suggest that C2202 is bad? Or something else?

Thanks!

Nick





On Thu, Oct 25, 2018 at 12:55 PM @Nielsentelecom <
@Nielsentelecom> wrote:

Nick,

I have a 2246A that I had to overhaul, which most likely has the
identical power supply for your 2247A. I would do a search for the 2246A
power supply also here. So I will refer to the component number in your
schematic if there is a difference. I wouldn't be concerned with the diodes
and caps mentioned in Alex's post until you verify the following.

I also blew up U2201 when I accidentally shorted my scope probe shield
against a heatsink while probing. That caused a no power on the main 44VDC
bus measured at C2203. even if that is good, I would get a spare
replacement. they are not expensive and still available as of a couple
years ago.

Get the service manual, refer to page 97, or 3-59, the power supply block
diagram, and verify the voltage indicated prior to the preregulator and
startup circuit at c2202. that is the main DC bus that feeds the Switching
power supply pre-regulator. If you are plugged into 120VAC, it should be at
about 160-170VDC. then switch meter to AC, and see if there is any ripple
there. That's 120 hz so any meter will work. If that is good then check the
previous paragraph for the 44VDC check.

I also was able to test my power supply out of the cabinet. It was risky,
but necessary. I used a 10KV AC rated linemans glove and placed the CRT
lead inside of it. I also have the resistor values that were close loads
for the power supply to operate at a typical load.

NielsenTelecom



Re: Uncooperative 7834

 

On Mon, Mar 4, 2019 at 12:04 AM, fiftythreebuick wrote:


There's another way, that's to take a current limited supply and pulse
it on and off. For a supply of X amps, I'd limit it to x/10 amps.
Now, if you have an HP current probe, the one that's made for shorted
nodes, you can see where the bright lights go, and that shows you the
track where the short is.
Decoupling caps and tants and conducting PN junctions and regular power supply current would (partially) short out your pulse, thereby hiding the current info you're looking for, *unless* you use a low level, very low frequency, low slew rate signal.
A few months ago, I successfully used a low-level sinewave (about 0.4 Vpp) signal of about 10 Hz across the power supply pins with an HP 547A current tracer on a spectrum analyzer board, containing rows and rows of digital logic, 0.1" apart with dozens of interspersed decoupling caps. To my own amazement, it took about five minutes to find a shorted cap!

Raymond

Re: Uncooperative 7834

 

On Mon, Mar 4, 2019 at 12:27 AM, Raymond Domp Frank wrote:


low slew rate
Correcting myself: That snippet was left over editing away the erroneous mentioning of using a square wave (pulse). Please ignore.

Apparently, you don't have a full short. Still, traces of lower impedance readily come up by probing. Don't forget that the HP 547A is direction sensitive, so rotating it around an axis perpendicular to the trace indicates the direction of the current (modulo180 degrees)!

Raymond

Re: 475 with no Display and no HV

SuddenLink
 

Thank you so much Reed for the highly detailed roadmap to follow at this point and forward. I am going to read through your post a few more times to get a clear picture before I proceed.

One thing that still troubles me is why did the waveform at TP1318 go back down to less than 2 volts p-p and thus the HV disappear when I touched the test probe of my DMM to the Base of Q1306? Now I can’t tickle the HV back to life by touching the 1K resistor with the +5v rail at the other end to the base of Q1318 like I was able to do before.

One last data point: When I did have the shortened trace on the CRT I took my Simpson 260 with it’s 10KV high voltage probe to measure the voltage at the HV test point. It looks like it’s about 100 volts high (-2550).

Once again thank you for your help.

Ripley

The words are mine but this iPad does what it will with them.

On Mar 3, 2019, at 4:55 PM, Reed Dickinson <reed714@...> wrote:

I just re-read your trouble note and I suspect you have identified the problem when you said the trace is only 2/3 of the normal width. This points immediately to a defective U1321 voltage tripler. Unsolder the lead from T1320, pin 12, tripler and power on, look for normal waveforms around Q1318. If your tripler is defective a replacement 3X can be had in an NTE539 or ECG539 (an NTE538 or ECG538 would work too).
By momentarily connecting the base of Q1318 to +5 through a 1K resistor you have bypassed all the regulation that you get from Q1306 through Q1316 and you have proved that the transformer and associated circuit are working. The shortened trace you saw was probably due to the unregulated HV giving a shorter trace or a defective voltage tripler U1321. I would still look for problems around C1304. When you power up the time constant of R1304 and C1304 is about 680ms. This means that you have under a second for the HV to stabilize since the base of Q1306 is being pulled toward +110V until it is nailed at +0.6 by the conduction of CR1304. After about 1 second the base of Q1306 should go to about +0.6V and oscillations should occur via Q1318.
To troubleshoot the regulator circuit pull F1318 out and disconnect P1313, the transistor Q1318 is now out of the circuit. Connect a 25K pot between +15 and - 5 and connect the wiper of the pot through a 10K resistor to the Q1306 base node. Connect a sensitive voltmeter to the - side of C1316 and power on your scope. Connect another voltmeter to the Q1306 base node and vary the pot. When the base of Q1306 is around +0,6V you should see some variation at the - side of C1316 (a 4.7K temporary resistor to ground will be necessary to give a little current through Q1316), If no variation is seen when rotating the pot then you have a problem in the regulator circuit, troubleshoot it and you will find the problem. If a variation is seen then the regulator is working and you need to fix problems on the secondaries of the transformer. Before using the following shotgun replacement procedure unsolder the lead from T1320 pin 12 going to the 3X voltage multiplier. If the multiplier is bad it would put a load on the oscillator circuit and kill the oscillations. Next, use the shotgun approach and replace C1320, CR1320, CR1321, C1322, C1323, C1328 and CR1329. I recommend you use HV caps rated at least 5KV as the surge of HV during the first second is high.
Good luck:
Reed Dickinson
On Sunday, March 3, 2019, 12:01:12 PM PST, SuddenLink <bob.ripley@...> wrote:

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: 2247A PSU Troubleshooting

 

On Mon, Mar 4, 2019 at 12:17 AM, Nicholas Keller wrote:


I measured 120-122VAC up to the diode ring. Does this
suggest that C2202 is bad?
I guess you realise you're measuring in an area that is directly connected to the mains, no isolation? Take great care! *Do* use an isolation transformer and *float your 2247A*.

Did you measure the 77 V across C2202 or between C2202 and chassis? The latter would be incorrect, because the chassis is floating (with exception of some C's) against C2202 (since that has a galvanic coupling to the mains).

Raymond

Re: 2465-erratic cursors

Chuck Harris
 

Is it an actual 2465, or the A or B model?

I ask because the actual 2465 uses a very funny
pot for the Delta controls. It has no stop, a
conventional resistor wafer, but two separate
wipers that ride on the resistor wafer with an offset.
The program watches the voltages out of the two wipers
change as you rotate the control, and calculates the
position of the cursors... which takes a little time.

Anyway, pots get dirty, and need a little lube from
time to time.... Especially pots that get cranked on
as much as the cursor pots do. There really is nothing
much else it could be... especially if other pots are
not affected.

If you have the A or B model scope, the CPU is over
taxed by all the stuff that has to be cyclically checked
and dealt with, and that causes a delay between when
you turn a knob and when it makes the function happen.

And, all three scopes will get a little put out when you
turn a knob more quickly than the CPU can process the
info and make its response.

-Chuck Harris

@DC912 wrote:

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