Tektronix 7A19 Repair - Unobtainium 155-0068-00 suspected

[FlexibleMammoth]

Hi,
i have finally come around to revive a very old and very dirty Tek 7904 500MHz mainframe oscilloscope that I was gifted by my old physics teacher about 10 years ago. I am now debugging the 7A19 vertical amplifier, which shows no signal no matter the settings.

What works:
- All readouts (V/div, IDENTIFY, invert)
- Invert trace switch
- Coupling switch
- Fuse and attenuators

What doesnt:
- There is essentially no output signal. A 100mVpp signal yields about 7mVpp on each of the differential outputs (image 1).
- Position has no effect

I have traced the signal through the amplifier; it is a nice 100mVpp from the input, through the fuse, through the attenuator, through the fixed delay line and right up to the signal amplifier U100 and trigger amp U40, which are unobtainium Tek parts:
https://w140.com/tekwiki/wiki/155-0068-00

As a bonus, they are mounted right below one of the rotary switches, so I can't really measure at the pins. However, both devices get warm (but not excessively, just enough to "feel appropriate"). No other obvious hotspots using a thermal imager.

After the chips, the signal is strongly attenuated (image 1: Channel 3 is the 7A19 output; C1 and C2 are the vertical amplifier output), both before the polarity switch at R39, R49, R99, R109 as well as after the polarity switch right on the trace. Service manual is here:
https://w140.com/smb/7a19_sm.pdf

For reference, I have included the same measurement from a working 7A12 vertical amp (image 2: Channel 3 is the 7A12 output; C1 and C2 are the vertical amplifier output).

Any ideas? If both those devices are shot, I don't really see a way to fix this, since ebay has only one of them, and that one is 30€. Also, I am not sure how that would have happened - there is an input fuse that should catch the roughest abuse. ESD maybe?

Thanks,
Andreas

[perdrix]

The best place for advice on these puppies is the Tekscopes mailing list at groups.io.

David