Hello! New guy here.
I have a 475A scope w/DM44 that I bought not working maybe 15 years ago. Replacing one of the power supply filter caps fixed it then. It's been great for occasional use since then. But several months ago it developed an intermittent problem where the display would go very bright and the fan speed would slow down for just a fraction of a second, then back to normal. Then it would be fine for a while (maybe the rest of my session, at least an hour or so), and repeat the very short glitch. One time it finally stuck in it's bad mode and I was able to troubleshoot it. All the supplies measured good (I don't have another scope to look at ripple, but no appreciable AC showed on my multi-meter). I was able to track down the issue as a shorted transistor (Q1344 in the CRT & Z axis circuit). I replaced it, and everything seemed fine again, for a few weeks. Now it is back to occasionally doing the bright display and slow fan blip.
I'm assuming this must be something in the +15v supply, since that is common to the fan and brightness circuits, and a low +15v might explain the Q1344 crapping out before. The problem lasts for such a short time, that even if I had it apart and was ready to measure things, I'd never catch it.
Does this sound like a common problem that has a known fix?
Can caps (tantalum or others) display a momentary short, then recover like this? If so, how can I identify a bad cap?
Could an aged solder joint cause this problem?
I have tried applying heat to various parts, but that does not reproduce the issue (or I haven't applied enough heat!),
It doesn't feel good just replacing parts wholesale, then waiting for maybe days for the problem to show up again, so I'd like to narrow it down to a small set of possibilities.
I have a service manual, and a 45 year old EE degree, so I ought to be able to fix this! :)
