Thanks in advance for the excellent help I have received already just by reading.
In an nutshell: 7904 from fleabay advertised as good work but is not - no trace no beam find no sawtooth. Seller very kindly offered me partial refund as seller doesnt want the BA back.
Teardown -> -15V supply at -0.6 V (yes, I know that number ;)) all other supplies that I had the tools to check are within 1%. Pulled power to each board 1 by one -> still -0.6V. Of note, pulling Z axis board power beam unblanked but no sweep or nuthin.
Pulled PS and it is in tik tik mode when disconnected as it should be. It also goes into tik tik mode if no plugins, which I do not think its supposed to do.
All supply resistances much much greater than listed in manual - I assume autoranging DMM is driving the semiconductors different than Tek forsaw a half century ago
-15V transistor fell apart while desoldering from board - not sure if stigmata of overheating or I was just cavemanning it. Replaced and replaced its driver. Still no love.
- Pulled main interface board thinking C60 might be culprit - nope! no short. Ugh now gotta reassemble.
- Measured resistances of the tantalums on the rectum fryer board - seem all OK or at the very least not dead shorts.
- have to get it together to test further. I might be able to borrow a variac. I don't really feel like putting together a fancy dummy load setup just for this one thing (lets all remember I thought I was buying a working scope to use on my OTHER projects) but would be willing to spend a little on some resistors to finish the troubleshooting.
but -17 seems ok and during the burst the - 15 is at about -17. The waveform, though, isnt pretty as shown in the manual - it looks more like the -17 or so is actually the envelope of a much higher freq signal.
The ribbon cable ends are turning to dust in my hands.
Any assistance you might offer in next step troubleshooting, or some minimal load current values to keep it out of burst mode while troubleshooting would be great help.
Thanks in advance!
Bill
