Hi
I know threads on this subject are two a penny, and the problem can't be fixed remotely anyway, so I guess I'm looking for moral support rather than practical advice, but here goes anyway.
I have this oscilloscope that I have about 20 hours invested in, fixing broken switches and troubleshooting what turned out to be a duff dual JFET. Now that's all fixed, most of the time it works fine, but if I flex or tap the PCB, sometimes the traces vanish. Ah well, I'll case it back up and hopefully that'll hold the shaky bits together. No such luck of course - doesn't work at all.
So, back apart it comes. The problem is, it doesn't trigger a sweep. X-Y mode works fine. I've traced the trigger signal as far as I can, but the point where it disappears is at a component in a large and complex feedback loop, so that doesn't get me very far.
I've examined every solder joint under a microscope. I've resoldered the main timebase resistor network rotary switch contacts on the PCB, just in case. I've previously cleaned the tiny little wafer switch contacts as best I can - but you can rotate the switch without the trace coming back anyway. Similarly, none of the other rotary or pull switches consistently cause or fix the problem.
I've gone tapping around both when it's working and when it's not working, trying to find a loose wire or component. Nothing.
The only thing I can think of is a tiny crack across a trace somewhere which is normally held closed but can be opened if the board flexes. How the hell do I find that? I'm out of ideas!
Bleat over
John