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Ugh.... another Tek 2445A problem

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c4757p:
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.  :-BROKE

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.

SLJ:
First thing I'd do is check and clean the switches.

c4757p:
Really? (Despite griping that I was going to do it today, I never got around to it  ::)) I didn't think to check there - like I said, the scope doesn't even seem to know that it's happening - but I will definitely take your suggestion. Always helps to check everything.

Skimask:
I've got a 2246A, so it could be completely different, or could be practically identical...
Anywho...a few years ago, the front panel switches started acting stupid as well...sometimes not working, sometimes press this switch and something happens "over there", pretty much random as far as I remember.
Took it apart and cleaned/reseated the front panel connector.  Problem solved.  At least I think it was the front panel switch connector.
Whatever it was that I disconnected, cleaned, and reconnected, took care of the problem and haven't had a problem since.
Gots ta luv the freebie fixes...

c4757p:
Found it! Had nothing to do with the panel controls. It was U650 - display sequencer. Luckily I was able to track down a donor at my school.

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