Tektronix 7A13, debugging

I have a Tek 7A13 amplifier plug-in that shows a problem. The 1V:100mV:10mV/div ranges work fine, but others don't.

With both inputs at GND, trace centered on the screen, changing to a 2x or 5x voltage range brings the trace down a couple of divisions. If a signal is injected in 2x or 5x ranges, nothing happens. A signal can be injected this way:

• bring to (+) 0.000V the internal voltage generator
• set both inputs to GND and POSITION the trace on mid screen
• set IN+ to Vc: you're now comparing the voltage generator to GND
• slowly increase the voltage

If the range setting is working, the trace will move up and down together with the voltage. Return the generator to 0.000 and change range: if the trace jumps, there's a problem.

And here the original paper documentation becomes useful. Since only three ranges work on my unit, I started my search from board 1, where the selector is located.
Amongst all poles, there's one that drives two relays, namely K47 and K48, in all ranges except those that work. They control a resistive network on board 3. Ouch!, BTW, this is also described in the "Circuit Description" section of the manual!!!

So there is the possible culprit: a relay, a contact on the rotary switch or a connecting cable between boards.

I will inspect it soon...