Tektronix 2465B Square wave noise on active readout. #photo-notice



I'm new to the group, and a beginner electronics hobbyist. Coming here looking for help repairing a Tek scope. I see most people have problems related to the power supply but, as far as I've been able to tell, it is working kind of fine.
It might have been repaired or refurbished already: It had a missing screw on the top metal shielding, and the caps seem new. No surface mount electrolytic either.

So, I recently bought a Tek 2465B for a very low price, it was sold as "good condition" and as working. I had the opportunity to test it for a brief time, but not knowing almost anything about the operation of this scope, I just checked the probe compensation square wave and fed a couple of waveforms from my AD2. It seemed to work fine, and apart from a slightly bent pot under the screen and a somewhat noisy fan, the apparent condition was good.

Then, testing it more thoroughly at home, I found a problem with the readout data and cursors, it is shown in the following video:


The problems are as follows:

1 - Readout intensity potentiometer in the "off" state: It works fine. Trace is stable, and the readings are more or less accurate.

2 - Pot turned clockwise, "scale factors on": The trace depicts a waveform that looks like a digital signal, an irregular square wave.

3 - Pot turned counter-clockwise "scale factors off":
- Measurement lines on: The same waveform as in point 2.
- Measurement lines off: A normal squarewave.

4 - Auto button pressed: Trace disappears on the upper side of the screen, readout and cursors activate. It can be moved down without problems, but the readout intensity has to be turned to "off" again to display the input waveform.

Some notes:
- This happens only to channels 1 and 2. Channels 3 and 4 work well with auto set up and everything else.
- Measured voltages are within specs according to my cheap DMMs (2 anengs and one WHA5000 something).
- Photos added in linked album.

Album: https://groups.io/g/TekScopes/album?id=264175

Also, don't know if it's relevant, but there is a jumper resistor and a small wire on U975 that goes under the ribbon connectors on the side of the scope.
I tried to look for U2810 and U2900, but the board in the service manual shows an earlier version that looked like an addon card, don't really know where it is on my scope.

If someone could help pointing in any direction to troubleshoot, I would be really grateful. Maybe this is a documented fault, but I haven't been able to find this reproduced in any of the forum threads or videos I looked at.

Thank you very much,

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