Need help on Gould 1604 Timebase repair

[XPModder]

Hello,
I have a Gould 1604 DSO that has been working fine for a few years now. Some time last summer however, the timebase stopped working.
The power was switched on and off for a couple times in quick succession, due to me trying to close the case and hitting the plastic rod, that connects the front power button with the actual power switch.
Ever since then, the timebase has been stuck. There was no magic smoke released at any time. Not even an unusual sound. It just doesn't work any longer...
Everything else seems to work fine.
So here is a better description of the actual problem:
The timebase seems to be at 200ms/div. That is the rate at which the trace gets drawn. When I change the timebase, the text on the screen updates correctly and shows what the timebase is set to. The actual timebase of the trace however does not reflect this and is permanently stuck at the 200ms/div mentioned above. (I stopped the time that it takes the trace to go across the screen once and that was close to 2 seconds...)

With the service manual including the schematics I found that the shift register U602 (74HC595) has only outputs A and B doing anything when I change the timebase. All the other outputs are permanently low. This does of cause explain why the timebase is not changing as it should be controlled by the other outputs of the shift register.
After a few months I was able to get my hands on a different working scope and I used that to look at the clock and data lines of the shift register. With that I found out that even on the serial data goind in to the shift register only the first 2 bits ever change and the others are always 0.
I also looked at the register clock input of the shift register, but that never does anything whatsoever, as in it is always 0 and never changes...
I do not think that there is any problem with the VIA that generates all these signals, as the X-shift works perfectly fine and that uses the same data and clock lines as the timebase.

I also checked all the pins of the shift register for any shorts or low impedance to ground, but theres is no such problem. The pins have at least a few k-Ohms to ground.
I also measured the supply rail for thje shift register, which is fine and even measured all the supply rails coming out of the power supply and they are all good.
I thought it would be some capacitor somewhere, as I have previously replaced all the tantalums next to the front ends as one of them had blown and I also had to replace a broken high voltage capacitor on CRT-board before... Also the relatively rapid power cycling that seems to have caused the problem kind of indicated to me that it may have been a capacitor somewhere that could not handle the stress any longer. As I have not found any problem with any of the supply rails nor have I found any obviously broken components however, it seems that it is not actually a capacitor at fault.

I am a little bit lost right now, as the only remaining explanation I can come up with is that there is some kind of software error, so the CPU does not instruct the VIA to send the data for changing the timebase.
Does someone have any suggestions as to what may be the problem and how to fix it?

I attached the service manual of the scope. The schematics for the timebase are on page 110 of the pdf. The signals are coming from the VIA (U4) on page 113 of the pdf. The description of the timebase is on pages 51 and 52 of the pdf. (The original pdf was minimally better in quality but too big to upload here. You can easily find it online though)