This will be a fun one. Whole power supply probably needs recapping. Then there’s the game of hunt the tantalum capacitor to play when it still doesn’t work. This journey is well worth it though. Good luck.
There’s a good thread here regarding the power supply and using adapter boards for the capacitor https://www.eevblog.com/forum/testgear/tektronix-76037613-repairs/
Yeah that's what it really needs. I'm actually planning to auction these off for charity here on the eevblog though (they are from work so can't sell them). I don't want to spend much on them because I won't be able to recover the cost, but hopefully whoever wins the auction gives it the full re-cap it deserves. Thanks for linking the other thread, it's good to have it here as a reference.
My first suggestion was going to be massive ripple from the power supply because of worn out input capacitors; I have seen it before.
Yeah it makes sense. I'd like to know what causes that pattern when the Readout Intensity is turned up. At first I was worried it was a problem with the CRT itself, but it almost looks like transistor Vds / Ids curves rotated 90 degrees, might just be a coincidence though.
I replaced C811 which directly follows the rectifier on the -18V line. I only had a 10,000uF which is really only a temporary fix as C811 was 14,000uF. I'm not sure if its ripple current rating is adequate for the long term either. I also added a wire to join the two -18V planes on the PCB that were connected by the multiple pins of C811. Unfortunately I forgot to take a photo of this while the module was out.
The ripple has improved a lot, now only ~1V on the -15V rail, probably due to the replacement being too small. The CRT intensity still doesn't adjust and the trace is still off centre etc. But the Readout Intensity adjustment doesn't produce that strange pattern anymore.
But, the -15V is now at -20V, so I assume the excessive ripple killed the regulator. Hopefully the -20V hasn't damaged anything.
A10 Rectifier:A12 Signals out: