Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe


Stephan Lonis
 

Hi all. I received a 7b92a recently. I checked all jumpers for correct settings. When I put it in my 7904 mainframe it shuts off the crt and graticule illumination. The trigger light on the 7b92a is constantly on but it renders the mainframe useless. I have a 7b53a and 7b85 and those work fine but this plug-in does not. Anyone ever come across this problem. I have a service manual and operators manual for the 7b92a. Any ideas of why it would do this would be greatly appreciated or something to check in the plug-in itself. I have the schematics for it on hand in the service manual. Just got the 7904 up and running a couple weeks ago as it had the power supply problem but everything in the power supply checks out fine after replacing some bad capacitors.


Tom Lee
 

I would check for shorted caps in the plug-in. Something with that large a systemic effect sounds like a PS problem, so that tends to point to capacitors as at least one thing to check.

Sent from my iThing, so please forgive typos and brevity.

On Feb 6, 2021, at 7:08 PM, km6rso@gmail.com wrote:

Hi all. I received a 7b92a recently. I checked all jumpers for correct settings. When I put it in my 7904 mainframe it shuts off the crt and graticule illumination. The trigger light on the 7b92a is constantly on but it renders the mainframe useless. I have a 7b53a and 7b85 and those work fine but this plug-in does not. Anyone ever come across this problem. I have a service manual and operators manual for the 7b92a. Any ideas of why it would do this would be greatly appreciated or something to check in the plug-in itself. I have the schematics for it on hand in the service manual. Just got the 7904 up and running a couple weeks ago as it had the power supply problem but everything in the power supply checks out fine after replacing some bad capacitors.





Jean-Paul
 

Inspection for burned or leaking Tant caps is a first step...

Jon


n49ex
 

If you haven't run across it yet in numerous threads here, the aging Tek scopes are notorious for shorted tantalum capacitors. So, I would look at your 7904 manual first to see which power supplies are associated with the outages you are seeing, and then look for power supply filter/bypass capacitors in the 7B92A that are on that power supply. Probably a 5V supply.

Reinhard


 

It's been a while since I've been inside a 7904, but I'm sure it has some accessible power-supply test points. With power off and the plug-in not installed, measure the resistance of each test point to ground. Then, with the power still off, insert the plug-in and measure the test points again. They may measure under 100 ohms, but should not be under 10 ohms. This should identify which supply the plug-in is shorting and suggest which caps on the plug-in to look for. Tantalums don't always show external distress when they short so dont rely on a visual check.

--
Bob Haas


Dave Daniel
 

Ummm... I’m not sure hot-plugging a 7xxx plug-in is a good idea since they were not designed for that. The result will depend on the sequence of contact between the plug-in board fingers and the 7904 connector contacts when the plug-in is inserted (hint: they will not all make contact at the same time).

All hot-plug systems of which I am aware have different length pins/fingers which serve to control how the circuits on the hot-pluggable board are activated as well as active circuitry to protect things during power-up.

DaveD

On Feb 7, 2021, at 12:07, robeughaas@gmail.com wrote:

It's been a while since I've been inside a 7904, but I'm sure it has some accessible power-supply test points. With power off and the plug-in not installed, measure the resistance of each test point to ground. Then, with the power still off, insert the plug-in and measure the test points again. They may measure under 100 ohms, but should not be under 10 ohms. This should identify which supply the plug-in is shorting and suggest which caps on the plug-in to look for. Tantalums don't always show external distress when they short so dont rely on a visual check.

--
Bob Haas





Colin Herbert
 

It seems to me that Bob was quite clear that the power should be OFF. Maybe you just didn't read that bit thoroughly? There are often, if not always, notices on the mainframe to warn against installing or removing plug-ins while power is ON - It's a no-no.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dave Daniel
Sent: 07 February 2021 18:43
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Ummm... I’m not sure hot-plugging a 7xxx plug-in is a good idea since they were not designed for that. The result will depend on the sequence of contact between the plug-in board fingers and the 7904 connector contacts when the plug-in is inserted (hint: they will not all make contact at the same time).

All hot-plug systems of which I am aware have different length pins/fingers which serve to control how the circuits on the hot-pluggable board are activated as well as active circuitry to protect things during power-up.

DaveD

On Feb 7, 2021, at 12:07, robeughaas@gmail.com wrote:

It's been a while since I've been inside a 7904, but I'm sure it has some accessible power-supply test points. With power off and the plug-in not installed, measure the resistance of each test point to ground. Then, with the power still off, insert the plug-in and measure the test points again. They may measure under 100 ohms, but should not be under 10 ohms. This should identify which supply the plug-in is shorting and suggest which caps on the plug-in to look for. Tantalums don't always show external distress when they short so dont rely on a visual check.

--
Bob Haas





Dave Daniel
 

Yes, you are correct. I mis-read his email. My apologies.

DaveD

On Feb 7, 2021, at 13:54, Colin Herbert via groups.io <colingherbert=blueyonder.co.uk@groups.io> wrote:

It seems to me that Bob was quite clear that the power should be OFF. Maybe you just didn't read that bit thoroughly? There are often, if not always, notices on the mainframe to warn against installing or removing plug-ins while power is ON - It's a no-no.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dave Daniel
Sent: 07 February 2021 18:43
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Ummm... I’m not sure hot-plugging a 7xxx plug-in is a good idea since they were not designed for that. The result will depend on the sequence of contact between the plug-in board fingers and the 7904 connector contacts when the plug-in is inserted (hint: they will not all make contact at the same time).

All hot-plug systems of which I am aware have different length pins/fingers which serve to control how the circuits on the hot-pluggable board are activated as well as active circuitry to protect things during power-up.

DaveD

On Feb 7, 2021, at 12:07, robeughaas@gmail.com wrote:

It's been a while since I've been inside a 7904, but I'm sure it has some accessible power-supply test points. With power off and the plug-in not installed, measure the resistance of each test point to ground. Then, with the power still off, insert the plug-in and measure the test points again. They may measure under 100 ohms, but should not be under 10 ohms. This should identify which supply the plug-in is shorting and suggest which caps on the plug-in to look for. Tantalums don't always show external distress when they short so dont rely on a visual check.

--
Bob Haas














Ed Breya
 

For 7K plug-ins, you can do it on the bench, just the plug-in alone, opened up for access. Probe around with an ohmmeter and check across each Ta cap for short or very low R, and mark any that show this. Also check for signs of overheating of series Rs and Ls in the power supply buses. Don't worry about which supplies and caps are which, or that you can't necessarily see or access every Ta in there. Go for the easy stuff first. After you've marked any bad ones, lift one lead - un-solder or clip close to the board (leaving some lead to tack it back) - and check the part itself, and at the board pads. Often, more than one part spot will show bad because there may be several caps on the same bus, and any one bad will cause all of the spots to show the same. Then you just figure out which ones are actually shorted and remove. Don't replace any until the plug-in can fire up and operate without crashing the mainframe. If you can't find any easy ones to blame, you'll have to dig deeper and look for more. In my experience, shorted Ta caps in plug-ins are almost always on the +/- 15 V supplies - that's where they're used the most. Also, don't be surprised if another cap shorts after fixing. Sometimes they chain react, where eliminating the shorted one allows the supplies to return, and the next weakest one pops, and so on. I remember once in an EIP counter, I had about half a dozen or more Ta caps on the +5 V supply light up and burn, one after another - after clipping the burnt one, the next would go within a minute or so.

Ed


Eric
 

If I remember correctly there is a separate small 5V rail especially for the lamps and lights. You have to add the connections for a 7603. As it does not have it. Most likely this is what is being dragged down. It wont effect the scope. I have bad news on the trigger light though If I remember correctly that is driven from a custom tech IC. I had one of those burned out in an A plugin it did share the IC with a 485 of which I had a parts unit for. If you are looking for shorts on the main 5V supply I do not think you will find them. At least for the lamps.

Eric

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Dave Daniel
Sent: Sunday, February 7, 2021 1:43 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Ummm... I’m not sure hot-plugging a 7xxx plug-in is a good idea since they were not designed for that. The result will depend on the sequence of contact between the plug-in board fingers and the 7904 connector contacts when the plug-in is inserted (hint: they will not all make contact at the same time).

All hot-plug systems of which I am aware have different length pins/fingers which serve to control how the circuits on the hot-pluggable board are activated as well as active circuitry to protect things during power-up.

DaveD

On Feb 7, 2021, at 12:07, robeughaas@gmail.com wrote:

It's been a while since I've been inside a 7904, but I'm sure it has some accessible power-supply test points. With power off and the plug-in not installed, measure the resistance of each test point to ground. Then, with the power still off, insert the plug-in and measure the test points again. They may measure under 100 ohms, but should not be under 10 ohms. This should identify which supply the plug-in is shorting and suggest which caps on the plug-in to look for. Tantalums don't always show external distress when they short so dont rely on a visual check.

--
Bob Haas





n49ex
 

No sense hunting around all over. I would suggest go to the 7B92's connector in the back and directly check the power pins to ground. They are on the A connector side: +50 net pin 19, +15 net pin 18 (and it's most likely short C972), +5 lights pin 9, +5 pin 8; on the B side pin 18 is -15 (with it's most likely C976). Once you identify the shorted supply there (something in the few ohms or less region), THEN start chasing it down on that power net. Schematic and full maintenance manual is on TEkWiki  http://w140.com/smb/7b92a_sm.pdf

I've always thought the 7B92A was an absolute design masterpiece when you think about how much functionality they crammed into this space (for the time), and the astonishing analog triggering performance. One of my all time favorite Tek accomplishments, right up there with the (7904) distributed delay line CRT deflection system - and the roadrunner meep-meep cartoon on the first edition manual schematic of the vertical output!

Reinhard


Jim Ford
 

I second the +/-15 V recommendation.   Had an S-4 sampling head with -15 V shorted by a bad Ta cap, and it brought the mainframe to its knees before I found and replaced it.  I'd recommend at least a 35 V rated cap for replacement.   Good luck!         Jim Ford                                        Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: "Ed Breya via groups.io" <edbreya=yahoo.com@groups.io> Date: 2/7/21 11:41 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe For 7K plug-ins, you can do it on the bench, just the plug-in alone, opened up for access. Probe around with an ohmmeter and check across each Ta cap for short or very low R, and mark any that show this. Also check for signs of overheating of series Rs and Ls in the power supply buses. Don't worry about which supplies and caps are which, or that you can't necessarily see or access every Ta in there. Go for the easy stuff first. After you've marked any bad ones, lift one lead - un-solder or clip close to the board (leaving some lead to tack it back) - and check the part itself, and at the board pads. Often, more than one part spot will show bad because there may be several caps on the same bus, and any one bad will cause all of the spots to show the same. Then you just figure out which ones are actually shorted and remove. Don't replace any until the plug-in can fire up and operate without crashing the mainframe. If you can't find any easy ones to blame, you'll have to dig deeper and look for more. In my experience, shorted Ta caps in plug-ins are almost always on the +/- 15 V supplies - that's where they're used the most. Also, don't be surprised if another cap shorts after fixing. Sometimes they chain react, where eliminating the shorted one allows the supplies to return, and the next weakest one pops, and so on. I remember once in an EIP counter, I had about half a dozen or more Ta caps on the +5 V supply light up and burn, one after another - after clipping the burnt one, the next would go within a minute or so.Ed


Ed Breya
 

Reinhard said: "No sense hunting around all over. I would suggest go to the 7B92's connector in the back and directly check the power pins to ground."

I believe this is an alternative to my suggested method. You can indeed go through all the steps methodically, but to me, it's a lot easier to quickly check all the Ta caps in a plug-in, than it is look through the manual, determine which supply has the problem, identify the parts, and trace the power buses through the actual circuitry. I've been there and done that - it's a PITA, which is why I came up with my random direct search method. You know in advance what you're looking for, and it's easy to identify and poke around and check dozens of caps in less time (at least for me) than it takes to search around the manual and figure out where to start.After you've found the culprits, then you go to the manual and figure out what's what for replacement parts. You don't even need the schematics, and you don't have to care about which supply is affected - it's all about simply and quickly finding the bad caps, which are mostly right in front of your eyes, only an ohmmeter check away.

Try it both ways and see.

Ed


Tom Lee
 

The 7B92A (and NOT the 7B92) was indeed a masterpiece, brought to us by the eminent Bruce Hofer. He broke with the longstanding Tek tradition of a Miller integrator for generating the sweep and thereby solved many tough problems in one fell swoop. The non-A version marks the end of the old way of doing things. I thought my non-A timebases were all out of whack, but after studying the circuits, I realized that they were working as well as they could. It’s a great tutorial example of the limits of a Miller integrator.

Tom

Sent from my iThing, so please forgive typos and brevity.

On Feb 7, 2021, at 1:59 PM, n49ex via groups.io <n49ex=aol.com@groups.io> wrote:

No sense hunting around all over. I would suggest go to the 7B92's connector in the back and directly check the power pins to ground. They are on the A connector side: +50 net pin 19, +15 net pin 18 (and it's most likely short C972), +5 lights pin 9, +5 pin 8; on the B side pin 18 is -15 (with it's most likely C976). Once you identify the shorted supply there (something in the few ohms or less region), THEN start chasing it down on that power net. Schematic and full maintenance manual is on TEkWiki http://w140.com/smb/7b92a_sm.pdf

I've always thought the 7B92A was an absolute design masterpiece when you think about how much functionality they crammed into this space (for the time), and the astonishing analog triggering performance. One of my all time favorite Tek accomplishments, right up there with the (7904) distributed delay line CRT deflection system - and the roadrunner meep-meep cartoon on the first edition manual schematic of the vertical output!

Reinhard





n49ex
 

I agree in general, except in this case, given how densely packed the 7B92A is, it may be easier to know up front which power supply is the offender. And having to trace that power net through the unit using the schematic is a good learning experience anyway👿. And I would not be surprised if it is the entry filter cap, close to the connector.

Reinhard


Ed Breya
 

Yes Reinhard, if the bad caps aren't among the "easy" (to access) ones, you have to dig deeper, and possibly disassemble some things to get to them. Then having an idea of which supply and which sub-assembly is involved definitely helps to narrow it down, especially for the bigger, more complicated plug-ins. Murphy's Law would dictate that the bad caps will be the most inaccessible, which happens sometimes, but on single-wide ones, especially verticals, there aren't all that many Ta caps to begin with, and not many that are too hard to access. Horizontals tend to have more layers of stuff, but often can be quick-tested with simple disassembly if needed, or accessing from the back (board layout info from the manual helps here too) or other accessible points on the same nodes.

The quick test can actually be even quicker than I described, in cases where you know that all the caps are simply bypassing to ground, and not between supplies. I think this is almost always the case with 7K plug-ins. Instead of probing caps with two leads for the ohmmeter, one lead can be grounded, and the other one used to make two measurements - one from each cap lead. One will show ground continuity, and the other should be a reasonably high resistance, unless the cap or the line it's on is dumped to ground. You don't care which is which, only that one lead is ground, and the other far from it.

Ed


Stephan Lonis
 

Tracked it down to a bad tantalum on the 15 volt supply on the main interface board inside the 7b92a. Replaced the bad tantalum and now everything is working as it should.


Tom Lee
 

Glad that it was as simple as expected. That's a nice timebase, and I'm sure you'll enjoy using it.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/7/2021 22:28, Stephan Lonis wrote:
Tracked it down to a bad tantalum on the 15 volt supply on the main interface board inside the 7b92a. Replaced the bad tantalum and now everything is working as it should.




Stephan Lonis
 

It wasn’t exactly easy to replace but got it done. Had to place my
soldering iron underneath rotary switch towards the back of the 7b92a at
which point I was wishing I had more then two hands and melt the solder to
pull out the bad cap. It was a cap soldered alongside one of the coils
coming into 15 volts. I replaced a few other caps while I was inside the
unit with some higher temp, higher quality caps. In all probably 5
capacitors but the main culprit was that tantalum on the 15 volt rail.
After that plugged the plug-in back in and it was working after that.


Stephan Lonis
 

Yep, if I would have just checked every tantalum first instead of looking through the schematic to figure out what the 15 volt supply was connected to it probably would have been quicker. I think it was c976 or c972 something like that. I did check all tantalums after I found that one though and the rest checked out fine. I had time to spare this weekend and spent a good chunk of time on it. This is the second tektronix scope I own my first one was the 2465a. Just got a siglent dso 1104x-e, and have another pc digilent analog discovery 2. I seem to have a special love for the tektronix scopes though. They are fascinating to me and I really like them. This is the first mainframe I have owned. Picked it up at a ham radio auction for $50 I think a couple years ago. It sat for a while after I plugged it in and heard what sounded to me like a power supply problem. Fast forward two years after sitting and I decided it was finally time to repair it. Replaced the tantalums in the power supply and after that the plugins it came with worked with the exception of one which is a 7a24 which ch1 is not working. Purchased a refurb 7a19 in the meantime and have a 7a26 in the other vertical. Have a 7b53a and 7b85 also both working. I picked up this 7b92a on eBay with service manual and an operators manual, original for a good price. It said working pull but showed up DOA. Rather than go through the trouble of returning it I thought I would try and repair it as I did get it for a really good price that included the original manuals. I’ve been around electronics all my life. My Father is a service technician and repairs things daily. In fact most of my acquired equipment sits in his shop and I keep it there so he can use it and I just go over to the shop when I want to use it. This type of scope has been a bit new to us but the basic principles of electronic components are the same. Well that was a bit long but since I never really introduced myself to the group there is a little bit of background blended in.