Topics

Help with 7A18 problem

Bill Carns
 

I am new to the group here. I have a 7603 mainframe with two 7A18 plug-ins
and a 5B53A Dual Time base. I have been using this scope reliably for over
30 years. Love it.



Recently over a long (about maybe 6 months) period of very light use, I
noticed a problem and wound up using just one of the inputs on the vertical
plugins. When I finally figured out that things were definitely going south,
I did some diagnostics and then tested the mainframe by swapping in two
other (one 7A18 and one 7A18A) vertical plugins and they all work just fine
- both DC and AC up to very high frequency.



After this, the problem channels are consistent.



Probes compensated and adjusted on 5 other good channels all compensate
basically the same. But, when put on a bad channel (3), they compensate
(not quite completely - at the limit of the compensating probe cap ??) quite
differently and they all look the same.



The bad channels also perform poorly in terms of waveform (seem to be almost
differentiating) and they all look pretty much the same.



I am guessing I have lost something that is close to the probe end of the
plugin. (Since it affects compensation)



Does this sound like a common failure and does anyone know where to start
looking.



Bill



Wimberley, TX

512 618 2762 (Cell)

512 847 7010 (Home)

Albert Otten
 

Hi Bill,

How do the bad channels perform when you use the calibrator signal straight forward as input?
If the problem still exists then there are likely bad contacts in the V/div attenuator. This usually produces rather pronounced differentiating or spikes. If the problem is present at some, but not all, of the V/div settings, then usually you can recognize a pattern and conclude from the schematics which switch contacts might be bad.
The Gain adjustments (front panel, central with V/div) won't have much effect on waveform shape.
It remains strange however that you experience problems in 3 channels. On the other hand, I think you say that that the problems showed up one after the other during a longer time period?

Albert

Bill Carns
 

Albert, I much appreciate your response. I too was suspecting switch contacts. Yes, out of the two plug-ins that were in the mainframe when this occurred, 3 of the four channels eventually developed similar symptoms.

The calibrate signal as a source shows the problem and I am looking at the calibrate signal through the 10X probe (A TEK probe). Clearly, something has checked out very close to the input jack schematically and this would include the input to the amp board. But, for these symptoms to occur on three of the four channels with almost no use, and certainly no abuse, it must be in passive components and the atten. circuits are suspect.

When the bad channel is used and I attempt to compensate the probe, it does not compensate completely and it looks basically the same on all three bad channels as I said.

I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: Saturday, March 02, 2019 8:32 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Hi Bill,

How do the bad channels perform when you use the calibrator signal straight forward as input?
If the problem still exists then there are likely bad contacts in the V/div attenuator. This usually produces rather pronounced differentiating or spikes. If the problem is present at some, but not all, of the V/div settings, then usually you can recognize a pattern and conclude from the schematics which switch contacts might be bad.
The Gain adjustments (front panel, central with V/div) won't have much effect on waveform shape.
It remains strange however that you experience problems in 3 channels. On the other hand, I think you say that that the problems showed up one after the other during a longer time period?

Albert

Adrian
 

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Bill Carns
 

Major helpful Adrian. Thanks

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Adrian
Sent: Saturday, March 02, 2019 9:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Bill Carns
 

Also, does anyone have a parts 7A18 or 18A that has good knobs. I need one main Display Mode knob, and a "Identify" small little grey push on for the push button switch. One of the spares I have has busted knobs there but works just fine.

Thanks

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Adrian
Sent: Saturday, March 02, 2019 9:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Colin Herbert
 

I have a parts-donor 7A18 with a good DISPLAY MODE knob and both IDENTIFY buttons, but I live in London, UK. I think you will find that those knobs and buttons are common to the 7A26 and maybe some other dual amps, too (and the -N variants, which don't have readout ability and may thus be cheaper, though possibly less common, have the DISPLAY MODE knob, but no IDENTIFY button). Option 6 on the 7A18 replaces the IDENTIFY button with a DC OFFSET control.

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.

You really need to get a copy of the Service Manual which will help you lots. You can find a pdf (I prefer the original paper manuals, especially for the use of fold-outs) on the TekWiki site.

If I recall correctly, there are threads on Tekscopes relating to cleaning these cam-operated attenuators, but you will have to do a search yourself, unless someone else gives you the location(s).

Good Luck, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 02 March 2019 16:28
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Also, does anyone have a parts 7A18 or 18A that has good knobs. I need one main Display Mode knob, and a "Identify" small little grey push on for the push button switch. One of the spares I have has busted knobs there but works just fine.

Thanks

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Adrian
Sent: Saturday, March 02, 2019 9:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Colin Herbert
 

Oops, typo alert:

...moving and stationary contacts, then *gently* pulling the paper strip out.

Read "then" for "they".

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 02 March 2019 18:58
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem


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.

Good Luck, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 02 March 2019 16:28
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Also, does anyone have a parts 7A18 or 18A that has good knobs. I need one main Display Mode knob, and a "Identify" small little grey push on for the push button switch. One of the spares I have has busted knobs there but works just fine.

Thanks

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Adrian
Sent: Saturday, March 02, 2019 9:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Bill Carns
 

Very good info. Prior to your post, I just went out and opened the really bad one and looked at the "Switches" and all I saw was a lot of plastic discs laminated. I guess those are the cam units. I will go see if I can see underneath and what it takes to get at it. Have you cleaned those with the attenuator board still in place in the plugin chassis frame? I will heed your warning re the circuit card material. I am a big fan of Deoxit, but will stay with the IPA.

Regards,

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Saturday, March 02, 2019 12:58 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I have a parts-donor 7A18 with a good DISPLAY MODE knob and both IDENTIFY buttons, but I live in London, UK. I think you will find that those knobs and buttons are common to the 7A26 and maybe some other dual amps, too (and the -N variants, which don't have readout ability and may thus be cheaper, though possibly less common, have the DISPLAY MODE knob, but no IDENTIFY button). Option 6 on the 7A18 replaces the IDENTIFY button with a DC OFFSET control.

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.

You really need to get a copy of the Service Manual which will help you lots. You can find a pdf (I prefer the original paper manuals, especially for the use of fold-outs) on the TekWiki site.

If I recall correctly, there are threads on Tekscopes relating to cleaning these cam-operated attenuators, but you will have to do a search yourself, unless someone else gives you the location(s).

Good Luck, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 02 March 2019 16:28
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Also, does anyone have a parts 7A18 or 18A that has good knobs. I need one main Display Mode knob, and a "Identify" small little grey push on for the push button switch. One of the spares I have has busted knobs there but works just fine.

Thanks

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Adrian
Sent: Saturday, March 02, 2019 9:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Albert Otten
 

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

Bill Carns
 

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

Jim Ford
 

Hi, Bill.I have a 7A18 that I don't need.  IIRC, it works just fine.  You can have it for shipping if you like.  I'm in Southern California.   Where are you located?I will be back home in So Cal early next week, and I will confirm the condition of the 7A18. Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Bill Carns <wcarns@...> Date: 3/2/19 2:26 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Help with 7A18 problem 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 OttenSent: Saturday, March 02, 2019 3:53 PMTo: TekScopes@...: Re: [TekScopes] Help with 7A18 problemOn 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

Bill Carns
 

Jim, I really appreciate the effort and the reply, Yes, That would help as I am just not sure how effectively I can clean these contacts without some serious disassembly.

Let me know the shipping and your PayPal address and I will get it off to you.

Regards,

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jim Ford
Sent: Saturday, March 02, 2019 8:02 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Hi, Bill.I have a 7A18 that I don't need. IIRC, it works just fine. You can have it for shipping if you like. I'm in Southern California. Where are you located?I will be back home in So Cal early next week, and I will confirm the condition of the 7A18. Jim Ford Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Bill Carns <wcarns@...> Date: 3/2/19 2:26 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Help with 7A18 problem 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 OttenSent: Saturday, March 02, 2019 3:53 PMTo: TekScopes@...: Re: [TekScopes] Help with 7A18 problemOn 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

Colin Herbert
 

Hi Bill,

You are looking at the wrong side of the plug-in. Snap off the left-hand side-panel (looking from the front) and you will see the two attenuator assemblies at the front. They have four cross-head screws holding the thin ally covers in place. On each assembly, one of these screws has an earth-strap under its head. They should also have the lettering "CAUTION DELICATE BOARD MATERIAL SEE MANUAL SECTION 4". You can see the attenuator modules through the four oval holes. When you lift the top, you should see the four attenuator modules themselves. Turn the VOLTS/DIV switch and you will see the contacts between the attenuator modules moving up and down. The attenuator modules can be removed by just pulling them up with your fingers, but only remove one at a time, otherwise you may forget which one goes where. This gets you access to those contacts - there are two to each attenuator module and the moving "finger" contacts have two "halves" which are further split into two, to give a total of four very fine and delicate contacts. To clean a set, arm yourself with some shortish, ~2mm wide strips of paper (they only need to be as wide as one half of the contact) and moisten one with IPA. Turn the VOLTS/DIV knob until the contacts are seen to open and carefully insert the paper strip without bending any part of the contact. Turn the VOLTS/DIV switch until the chosen contacts close and then carefully and slowly pull the paper strip out of the contacts. Whatever you do, don't pull the paper strip up vertically, as that will be likely to bend the contact and ruin it. Space is limited and it is very easy to pull the paper strip up inadvertently. After doing this a couple or so times on one of the contacts, repeat it on the other half, then move on to the contact set on the other side of the attenuator module and repeat, possibly using another, clean paper strip. It should only be necessary to clean the contacts of one attenuator module, but I suppose that another one might need its contacts cleaning. The rule here is that if the contacts are ok, don't clean them, as the chance of messing things up increases. As Adrian suggested, it may be that the contacts on an attenuator module may not be making well and that re-seating it is all that is needed. These have a total of six pins each which have to push-fit in six holes on the circuit-board. This can prove to be fiddly as it is difficult to see the holes that the "legs" push into.

Let me stress-

1) get the Service Manual and read the appropriate section(s)
2) check if the 5mV /DIV setting is ok, if it isn't then solve that problem before anything else
3) ascertain which attenuator (if any) needs its contacts cleaning.
4) don't clean contacts that don't need cleaning (if it ain't broke, don't fix it)
5) don't use anything other than IPA, you will do irreparable damage
6) use great care; once you've done this, then you will know how to do it in future
7) look up the info on TekScopes to get as much knowledge about this as you can (it's similar to the "measure several times, cut once" principle)
8) if you are unsure, leave it alone and ask someone who has done it before to help you.

Another issue is replacement of the attenuator assembly lid. The earth-strap must be re-connected and the springy "fingers" on each side of the lid need to connect to the appropriate place below the attenuator assembly. This isn't helped by the lack of space between the attenuator assemblies and the framework so that you can't see where the "fingers" should go too easily and the presence of a four-contact ribbon-cable bearing a ferrite ring. The fingers on both sides need to be located so that they are pushing outward and contacting the metalwork below the attenuator assembly and this is fiddly, to say the least. You might need to remove the top section of the framework to see the location better. This is held in place by four screws, two at the rear and two at the front. Sadly, the front two screws' heads are hidden by the thin front panel. This pops off easily, but only when all of the knobs have been removed.

Do you still want to do this job?

I haven't cleaned any attenuator contacts on any of my 7000-series amplifiers, because it hasn't been necessary, but I have done this on my 464, which also uses cam-switches. I didn't mess that one up, thank goodness. As these contacts are gold-plated, it is doubtful that there is oxidation going on, but the dirt that gets removed may well have come from airborne muck such as cigarette-smoke.

Good Luck, Colin.

E&OE: I started this message last night and continued the following morning, so there might be typos despite having proof-read a few times.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 02 March 2019 19:12
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Very good info. Prior to your post, I just went out and opened the really bad one and looked at the "Switches" and all I saw was a lot of plastic discs laminated. I guess those are the cam units. I will go see if I can see underneath and what it takes to get at it. Have you cleaned those with the attenuator board still in place in the plugin chassis frame? I will heed your warning re the circuit card material. I am a big fan of Deoxit, but will stay with the IPA.

Regards,

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Saturday, March 02, 2019 12:58 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I have a parts-donor 7A18 with a good DISPLAY MODE knob and both IDENTIFY buttons, but I live in London, UK. I think you will find that those knobs and buttons are common to the 7A26 and maybe some other dual amps, too (and the -N variants, which don't have readout ability and may thus be cheaper, though possibly less common, have the DISPLAY MODE knob, but no IDENTIFY button). Option 6 on the 7A18 replaces the IDENTIFY button with a DC OFFSET control.

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.

You really need to get a copy of the Service Manual which will help you lots. You can find a pdf (I prefer the original paper manuals, especially for the use of fold-outs) on the TekWiki site.

If I recall correctly, there are threads on Tekscopes relating to cleaning these cam-operated attenuators, but you will have to do a search yourself, unless someone else gives you the location(s).

Good Luck, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 02 March 2019 16:28
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Also, does anyone have a parts 7A18 or 18A that has good knobs. I need one main Display Mode knob, and a "Identify" small little grey push on for the push button switch. One of the spares I have has busted knobs there but works just fine.

Thanks

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Adrian
Sent: Saturday, March 02, 2019 9:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Colin Herbert
 

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

Bill Carns
 

Ahhh, You are a wealth of knowledge and I sure appreciate you taking the time. (By the way, no typos found). I do a lot of very fine restoration work, am one of the founders of the Collins Collectors Association, and am a Physicist and EE by background, so I know when to go slow - as you say. Actually, I had gotten those covers off and explored those contacts visually. I did not get as far (did not know you could) as to pull any of the attenuator modules out. I have also not read all you recommend yet, so only part way through your instructions here in the last post.

It is pretty funny. I write a lot and for over 10 years published our CCA Signal Magazine. A lot of my writing is historical, but a lot also regarding service and restoration..... and I saw myself many times writing those "Go Slow, Read the Manual, and do not go where you are not comfortable and get help if you aren’t" instructions.

That is one of the reasons I am asking a lot of questions and no damage so far (that I know of). And, yes, those covers are a little puzzle.

I will read more, and tomorrow perhaps, or Monday, try removing one of the attenuator modules. I may take a break, and try putting the covers all back including external ones and doing some more characterization of the issues. I must admit, out of inexperience, I did not look at all the right things before exploring physically.

More in a while and thanks again.

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:25 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Hi Bill,

You are looking at the wrong side of the plug-in. Snap off the left-hand side-panel (looking from the front) and you will see the two attenuator assemblies at the front. They have four cross-head screws holding the thin ally covers in place. On each assembly, one of these screws has an earth-strap under its head. They should also have the lettering "CAUTION DELICATE BOARD MATERIAL SEE MANUAL SECTION 4". You can see the attenuator modules through the four oval holes. When you lift the top, you should see the four attenuator modules themselves. Turn the VOLTS/DIV switch and you will see the contacts between the attenuator modules moving up and down. The attenuator modules can be removed by just pulling them up with your fingers, but only remove one at a time, otherwise you may forget which one goes where. This gets you access to those contacts - there are two to each attenuator module and the moving "finger" contacts have two "halves" which are further split into two, to give a total of four very fine and delicate contacts. To clean a set, arm yourself with some shortish, ~2mm wide strips of paper (they only need to be as wide as one half of the contact) and moisten one with IPA. Turn the VOLTS/DIV knob until the contacts are seen to open and carefully insert the paper strip without bending any part of the contact. Turn the VOLTS/DIV switch until the chosen contacts close and then carefully and slowly pull the paper strip out of the contacts. Whatever you do, don't pull the paper strip up vertically, as that will be likely to bend the contact and ruin it. Space is limited and it is very easy to pull the paper strip up inadvertently. After doing this a couple or so times on one of the contacts, repeat it on the other half, then move on to the contact set on the other side of the attenuator module and repeat, possibly using another, clean paper strip. It should only be necessary to clean the contacts of one attenuator module, but I suppose that another one might need its contacts cleaning. The rule here is that if the contacts are ok, don't clean them, as the chance of messing things up increases. As Adrian suggested, it may be that the contacts on an attenuator module may not be making well and that re-seating it is all that is needed. These have a total of six pins each which have to push-fit in six holes on the circuit-board. This can prove to be fiddly as it is difficult to see the holes that the "legs" push into.

Let me stress-

1) get the Service Manual and read the appropriate section(s)
2) check if the 5mV /DIV setting is ok, if it isn't then solve that problem before anything else
3) ascertain which attenuator (if any) needs its contacts cleaning.
4) don't clean contacts that don't need cleaning (if it ain't broke, don't fix it)
5) don't use anything other than IPA, you will do irreparable damage
6) use great care; once you've done this, then you will know how to do it in future
7) look up the info on TekScopes to get as much knowledge about this as you can (it's similar to the "measure several times, cut once" principle)
8) if you are unsure, leave it alone and ask someone who has done it before to help you.

Another issue is replacement of the attenuator assembly lid. The earth-strap must be re-connected and the springy "fingers" on each side of the lid need to connect to the appropriate place below the attenuator assembly. This isn't helped by the lack of space between the attenuator assemblies and the framework so that you can't see where the "fingers" should go too easily and the presence of a four-contact ribbon-cable bearing a ferrite ring. The fingers on both sides need to be located so that they are pushing outward and contacting the metalwork below the attenuator assembly and this is fiddly, to say the least. You might need to remove the top section of the framework to see the location better. This is held in place by four screws, two at the rear and two at the front. Sadly, the front two screws' heads are hidden by the thin front panel. This pops off easily, but only when all of the knobs have been removed.

Do you still want to do this job?

I haven't cleaned any attenuator contacts on any of my 7000-series amplifiers, because it hasn't been necessary, but I have done this on my 464, which also uses cam-switches. I didn't mess that one up, thank goodness. As these contacts are gold-plated, it is doubtful that there is oxidation going on, but the dirt that gets removed may well have come from airborne muck such as cigarette-smoke.

Good Luck, Colin.

E&OE: I started this message last night and continued the following morning, so there might be typos despite having proof-read a few times.


-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 02 March 2019 19:12
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Very good info. Prior to your post, I just went out and opened the really bad one and looked at the "Switches" and all I saw was a lot of plastic discs laminated. I guess those are the cam units. I will go see if I can see underneath and what it takes to get at it. Have you cleaned those with the attenuator board still in place in the plugin chassis frame? I will heed your warning re the circuit card material. I am a big fan of Deoxit, but will stay with the IPA.

Regards,

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Saturday, March 02, 2019 12:58 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I have a parts-donor 7A18 with a good DISPLAY MODE knob and both IDENTIFY buttons, but I live in London, UK. I think you will find that those knobs and buttons are common to the 7A26 and maybe some other dual amps, too (and the -N variants, which don't have readout ability and may thus be cheaper, though possibly less common, have the DISPLAY MODE knob, but no IDENTIFY button). Option 6 on the 7A18 replaces the IDENTIFY button with a DC OFFSET control.

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.

You really need to get a copy of the Service Manual which will help you lots. You can find a pdf (I prefer the original paper manuals, especially for the use of fold-outs) on the TekWiki site.

If I recall correctly, there are threads on Tekscopes relating to cleaning these cam-operated attenuators, but you will have to do a search yourself, unless someone else gives you the location(s).

Good Luck, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 02 March 2019 16:28
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Also, does anyone have a parts 7A18 or 18A that has good knobs. I need one main Display Mode knob, and a "Identify" small little grey push on for the push button switch. One of the spares I have has busted knobs there but works just fine.

Thanks

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Adrian
Sent: Saturday, March 02, 2019 9:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

I think you'll find the input attenuator switches are far from potted and can be cleaned fairly easily once you remove the four screws and take the cover off. I have restored normal operation with just an IPA clean and some few dozen operations. I have also cleared up strange compensation issues by simply re-seating the attenuator modules.

There are people here with far more experience than me on this topic though, I'm sure one will be along shortly!

Adrian

On 3/2/2019 2:53 PM, Bill Carns wrote:
I have not explored that switch area, but my past experience with TEK stuff tells me those switches are basically potted. Is there an accepted method of cleaning them, or am I looking at just exercising them a few hundred times carefully and hoping they clean up?

Bill Carns
 

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

Colin Herbert
 

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

Adrian
 

Guess I'm missing something here - or I have misled people with my terminology - I was referring to Colin's 'chiclets' when I talked of reseating the attenuator modules. No shafts need to be removed for this (on my 7A18s anyway) just the four screws holding the thin cover over them.

I agree with Colin - no Deoxit in the region of the attenuator PCB!

Adrian

On 3/3/2019 11:47 AM, Colin Herbert via Groups.Io wrote:
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.

Albert Otten
 

Hi Colin,

Diagram 1 shows exactly in which positions a given switch is open and closed. In other manuals there often is a legend in the diagram, telling which switch of a pair is closed when a black dot is present. For instance a dot means "top side of board" switch is closed, "bottom side of board switch" is open. Hence when just one switch is bad then this switch can be identified by systematically checking all V/div positions. Both AC and DC should be checked also. Annoyingly all V/div positions can show bad when more than one switch is bad, depending on which switches these are. For instance when both switches of a top/bottom pair are bad no signal can pass undisturbed.
I think Brian could only tell by heart that most position were bad, but that is not of much help.

Albert

On Sun, Mar 3, 2019 at 11:40 AM, Colin Herbert wrote:


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