Thanks again for your reply! I got a quite nice progress I think.
At first a look at the workbench. Quite uncomfortable to work at that lecroy thing. I bet at the lab they had riser cards to work at a single card without having the whole scope around it.
Second: I had to change the scope I use to probe around in there . I know, it's not quite the best choice, BUT, I know that that thing is quite good in calibration and its absolutely reliable. My father used it for years until I got it. I have some other scopes around (like Hameg HM412 + 512, Tektronix D10/5103, Tektronix 422) but that little Hameg thing works perfectly. I changed it because I didn't realy believe in what the 412 used before showed me. Guess I have to go through that at next
However, I probed around a bit, looked at the "S1" zero Ohms resistor connecting the output stage with the integrator and linearity correction part, compared that with the working channel (at this point I noticed already that both channels are identical, nevertheless thanks helius for pointing that out!) and noticed that the signal at those bridge resistors are quite near - with the little difference that the working channel had like 1,5VPP and the not working one had 50mVPP. If you look at the graphs you could assume that that isn't a real signal but rather a interference. Which it indeed was, as I saw after another hour of probing around, thinking etc... but then I noticed, that the trimmer for XO was missing
I never found it in the package, nor flying around in the scope, so I wasn't aware that there was something missing. Bloody transport damage
So I fixed that in a very professional manner (in need of a 500 ohms trimpot..), shown in the fifth picture, which fixed it.... nearly. just look at the last picture. I feel quite reassured now, knowing that the eeproms and custom DACs and whatever working in there is not destroyed at all. Overall this half sided error should be somewhere around the IRF9523 (Q58), at least a simple measurement shows that at testpoint T19 a signal simmilar to the signals at the working channels testpoint is measureable, but at testpoint T18 there is nothing. So i measured the resistance over the IRF and noticed that there is quite a difference between the working channel and the not working one. Measured over Gate and Source of the FET i get around 330 ohms. the working channel is at 430 ohms as you would expect, looking at the diagram. But again, measuring the 430 ohms resistor (one side desoldered) going to the gate of the FET, that thing is perfectly fine. now I'm confused again.
By the way - don't you all love disconnecting those high voltage plugs at CRTs?