Amplifone HV board - Solved a really wierd problem today...

     I had an Amplifone HV board that was producing no HV and smoking R13 (39
ohms), and I couldn't figure out why. I finally narrowed it down to the
oscillator curcuit, and the HV is fine now. This was the first time I saw a
555 go bad on an Amplifone HV board.
     This was crazy, I was checking through the MPSU07 (Q2), Q1 (3904),
thinking it had to be one or the other. Nope. I went through all the diodes,
and started measuring surrounding resistors. Nope. Then I was thinking the
small pre-amp transformer (T1) was bad, so I rang it out on the LOPT. It was
fine. Just for kicks, I rang out the WinTron. It was fine. Pulled and tested
the BU406D, it was fine, too. After checking all my traces, solder joints,
etc., I'm about ready to chew on the anode cap!
     I then thought about the 555, so I tried one of my Randy Fromm tricks
and checked for a junction drop on diode test at pin 3 of the 555. It gets a
junction drop! However, after scoping the 555, there was no waveform at pin
3. I socket and replace the 555, my HV board is fine again.
     Sorry for the big story, but has anybody ever seen a 555 do this on an
Amplifone HV board? This was the first time I saw this, and thought it was
really goofy.

Thanks,
MK
Received on Sun Jul 11 19:23:48 1999