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Hello people of TekScopes,

I have a Tektronix 7B92A that needs repairing. It has a very severe horizontal deflection off to the right of the display, but the sweep is also jittery and not stable.

I've probed the sweep generators and I've confirmed they are not the problem as the sweep waveforms are beautiful.

I'm pretty sure the problem is in schematic No. 6, the output amplifier for the plug-in. The only problem is, this part of the circuit is on a section of the A6 main board that is under the A7 logic board. Because of this I can't reach any of the points I need to test.

I've tried removing the logic board and probe the A6 board that way, but that stops the plug-in from sweeping, which is useless for testing points on the output amplifier.

I could just solder a tiny wire to every point I need to check and do it that way, but some of the points I need are on socketec transistors and I don't want to risk solder filling up a socket.

Any tips?

Benjamin

I no longer need advice for this issue as the 7B92A has been repaired.

Turns out Q938 was the issue. Funny thing is I tested all the transistors on my 5CT1N and they all tested fine at the time.

What I did was I removed the logic board and only tested voltages rather than following the signal until something weird happens. Base of Q942 was at -8.5V instead of -6.9V so I did a little simulation in falstad and turns out if I remove Q938 the base of Q942 gets a higher magnitude of voltage.

Taking Q938 out and testing it on the 5CT1N, it showed no curves. It was entirely dead. Strange that it worked before when I pulled all the transistors and tested them. Must explain the jittery issue then I guess.

Question now is, will a simple 2N3906 transistor be a sufficient replacement for the SPS6765?

Thanks,

Benjamin

I was going to recommend soldering test wires onto the solder tails of the sockets on the bottom of the board, assuming they were through-hole.? Glad you got it fixed.
BTW, thanks for mentioning this plug-in, as I had the 7B92 on my test equipment wishlist (you do have one of those, right?), but a look on tekwiki for the 7B92A showed it had improved the 7B92's sweep aberrations.? So I've updated my wishlist after the visit to tekwiki (always a pleasure for test equipment addicts like us!) to verify the 7B92A exists.? I'd not heard of it before.
I did have a note in the wishlist that the 7B92 has that HF sync circuit that works better than the direct trigger.? Triggering being a bugaboo of mine on these vintage 1970s scopes, I'm sure I will appreciate the HF sync when I get my hands on a 7B92A.? The nice thing about the plug-ins, as opposed to the scope mainframes, is that they are small enough and cheap enough to slide under the nose of SWMBO (She Who Must Be Obeyed) without detection!
A 7B92A would I'm sure be great in my 7904 or 7104.? Although I really want a 7B10 and a 7B15 and a couple of 7A29s or 7A29Ps for the 7104.? Of course, getting one or two (or more!) 7A29P(s) would really lead to a 7854 or a 7912HB, and down the rabbit hole we go!
The 5CT1N is also on my wishlist, as I have a a 5103N D10 mainframe (working) and a 5113 parts mule that got mangled in shipping.? Also a 7CT1N if that pops up first.? And a 576, but the disadvantage there is the size.? I'm in Southern California where space is tight (and it would be difficult to obtain a 576 without SWMBO knowing about it!)? I do know a guy who has a 576 just collecting dust....
Ya, I'm salivating like one of Pavlov's dogs now LOL!
Well, best of luck to you, Benjamin, with the 7B92A and all that other great vintage gear you have!? At least we have a chance at fixing the old stuff, designed before the suits took over and messed everything up.? HP >> Agilent >> Keysight.? Tek is still called Tek, but it's of course nothing like it was.? And Broadcom Corp >> Broadcom Ltd/Inc./Avago, Hughes >> Raytheon Company >> RTX, etc., etc.? Those last 2 sad progressions are where I've worked for the last 19 years, unfortunately.
Hopefully somebody knowledgeable chimes in about subbing a 2N3906 for an SPS6765.? Can't help you there.? Maybe Tom Lee can?
Jim FordLaguna Hills, California, USA On Thursday, February 20, 2025 at 10:58:23 PM CST, Benjamin Badrakh via groups.io <benbadr2009@...> wrote:

Hello people of TekScopes,

I have a Tektronix 7B92A that needs repairing. It has a very severe horizontal deflection off to the right of the display, but the sweep is also jittery and not stable.

I've probed the sweep generators and I've confirmed they are not the problem as the sweep waveforms are beautiful.

I'm pretty sure the problem is in schematic No. 6, the output amplifier for the plug-in. The only problem is, this part of the circuit is on a section of the A6 main board that is under the A7 logic board. Because of this I can't reach any of the points I need to test.

I've tried removing the logic board and probe the A6 board that way, but that stops the plug-in from sweeping, which is useless for testing points on the output amplifier.

I could just solder a tiny wire to every point I need to check and do it that way, but some of the points I need are on socketec transistors and I don't want to risk solder filling up a socket.

Any tips?

Benjamin