Wow! Thank you. This is immensely helpful.
To put tings into perspective, i measure about 3.1 V at 2R105 when the trigger level is set to 0 V with the same settings. From there in keeps increasing as i increase the trigger level before it wraps around to negative.
Trace centred, trig level 5 V: -3.8 V
Trace centred, trig level 4.1 V: -3.9 V
Trace centred, trig level 4 V: +3.8 V
Trace centred, trig level 0 V: +3.1 V
Trace centred, trig level -4 V: +2.4 V
Trace centred, trig level -5 V: +2.2 V
There is certainly something wrong here. Based on this, it makes sense that the scope does not trigger. The measured trigger levels are set so high that even at -5 V set trigger level, i would need a voltage probably in the 6-8 V range. That being said, i was still not able to get it to trigger even if i applied a 15 V signal. The front end would probably start clipping at those kind of levels i would guess. Maybe i have so much offset in the trigger level that the front end start clipping the signal before i ever reach the required signal level to make the scope trigger.
I guess that i need to digg more into the DC GENE circuitry and the output from 17IC2. Perhaps identify the part of the muxed signal that corresponds to the trigger level(s) and check if it also have a significant offset in the output from 17IC2. 20IC1 could also be interesting as it adjusts the DCG offset voltage and DCG gain. In theory it should be possible to adjust the DCG offset in software (if that possibility exists in the service menus), but i will not start messing with that yet as the potensial offset error is so large that i have i hard time imagining that it is just a matter of calibrating the DCG offset.
I guess that i would need to decode the signal going into 17IC2 in order to actually check and if needed adjust the offset as it would be hard for me to know if there is an actual offset error from the IC without knowing what voltage is commanded over the digital input. That being said, i could probably assume that the digital bit is correct for now.
I think i have some work ahead of me
Again, i am really grateful for these measurements. It seems like this could be a major step into figuring this out. Once i have gone over all the remaining tings that i want to probe on these chips, i will do a quick reflow of them, just in case.