OK, I am looking at all this and I got something, if you can do it, and I am not sure IT will do it. In XY mode, if you can put damnear anything in, say a sine to deflect, hitting trigger view should result in a display. Whether it be a line of whatever, if you get something that clears everything up to U520. Actually U520 is a suspect here, but let's not condemn too quickly.
Now after U520 is that tunnel diode, shunting the "`A' GATE" signal. The 475 print doesn't but the poorly scanned 475A print shows a voltage of zero at that point. Also (page 6 of 14) it gives a waveform. If that is not there U520 is an even bigger suspect.
The A GATE goes to Q572, which is in a diff pair with Q574 which means that passing the signal is dependent upon Q574 being biased on so the emitters can couple. The collector of Q572 is the place to check, if a signal is not there you need the DC voltage. U600 can affect the bias on Q574, so that needs to be checked if Q572 collector does not have a pulse.
The circuit of U600 is a bit murky, it has input and output but then that diff pair. Hard to tell exactly how it works. It does seem like the outputs to those transistors are open collector or similar, but it has both Vdd and Vss so there is no way to know if it is supposed to pull up or down.
RIGHT NOW, I have a 475 on the bench, and it will stay fairly handy some for waveforms or voltages if you need. You should be able to disconnect U600 from the B of Q574 if the bias is not right. If that lets the signal pass it does not automatically mean U600 is bad. Can't condemn those things out of hand, but if it IS bad, I am planning on - even if I have to borrow the money, to go to HGR and buy up me some tekscopes. They are local and if they like cash I might just have more parts than I know what to do with. Bear with me though, I was injured and unable to work for a long time and I am just getting past that, and I might just devote myself to my own business, scopes and audio. If that works out I will be a pretty valuable resource. I would like to do that. I want to go down there and but EVERY Tek scope they have. I am sure some are junk, but just ONE unobtainium IC would make it worth it.
Moving right along, the signal goes to the base of Q902.
That 475A schematic is not that good, but good enough. You can see what happens after Q902. See where the signal stops when you think it shouldn't.
From there the circuit gets complicated. At that point we need to know where the trigger or gate signal is and is not. Pursuing all the possibilities would be counterproductive. Get to that point, and if the signal is not there go backwards.
Also remember this problem is obviously thermal. Got some Freon or even 91% alcohol ? Or solvent that is same on this thing ? you get it narrowed down and use a Qtip to apply an "evaporant" to cool the part. Shooting the stuff on 100 components tells you nothing. The trick is to catch it when it first acts up. Of course the problem has to be localized, so definitely try that once it is narrowed down.
Keep us posted. Once we now a few things we can proceed, unless you get it fixed just by that. Any component can fail from heat, and not overheat just heat. None of this is high power so you are not looking for much heat, like you might use an infrared thermometer to find. It would not work.
So basically, where does the trigger (or gate) signal stop ?
Also, I assume by what you wrote that "auto" trigger does not result in a trace. This I believe is an important piece of information but to get into it will take a bit of time. I haven't yet found out exactly where the "auto" is accomplished, but I will. Sometime tomorrow probably I will get into that and come back with more. Maybe tonight. Who knows ? I might find a dead giveaway there. (once I find the damn thing)
Fret not, I almost NEVER fail. Once I sink my teeth into something I tend to stick with it unless it is about to pull my jaw off my body. Just the way I am.
Keep us posted.