Don't worry, I'm not trying to get you all to fix my problem for me, just thought I'd ask if anyone had any clever ideas before I go to bed. I'll dig the damn thing apart tomorrow morning and try to fix it.
All of a sudden, my beloved (OK, it's a love-hate relationship) Tek 2445A has developed a weird malfunction: whenever both channels 1 and 2 are switched on (and regardless of whether 3 and 4 are also on), the scope will eventually switch to displaying just one ground trace on the screen. Sometimes it takes a few seconds, but it always happens. The rest of the scope continues to function, and the processor seems completely unaware that anything has happened (it's not clicking any relays to switch to this mode, no LEDs change, or anything). It does not appear to be the ground trace
of a specific channel, as no channel position control moves it. It is dead center on the screen. The on-screen text remains, but it just shows a single ground trace and no other signals. Pressing any of the buttons in the "Vertical" group brings back proper function (even just the bandwidth limit button). It only does this if both Ch 1 and 2 are on, and does not do it in X-Y mode. The trigger is still working, as the built-in frequency counter continues to display the correct value.
I'll dig into the service manual tomorrow, but does anybody have a hunch? Is it more likely to be something in the digital circuits, or the analog? I'm suspecting whatever sort of register chip outputs the configuration bits into the analog circuit - really hoping I'm not dealing with a corrupting ROM or something
Though IIRC it runs a full checksum validation on boot, which it appears to pass.
Edit: Now, it managed to sit for a whole minute, but when it switched, it did it differently - now it appears to be in Add mode!
Edit 2: I think I was wrong about the checksum test, it only seems to do that for the cal constants. Either way, I've got it running the self tests in a loop and it is passing them all repeatedly.