I recommend looking at the voltages around U760. Its supplies +5V (pin 15) and
-5V (pin 4) are generated locally, that should be the first place.
What were the voltages at pin 15 and pin 4?
Pins:
1. -3.17V
2. -0.67V
5. -3.15V
7. +8.54V
8. 0.00V
These voltages are so off (except pin 5!) my concern is either you are not looking at the right nodes or the voltmeter was in a different mode. Pin 1 voltage of -3.17V is 7V below what it should be, pin 2 voltage is more than 6V below what it should be and doesn't match to your earlier measurement of base voltage of Q780 (there is only 47 ohms between pin 2 and base of Q780).
While I was probing pin 5, the display changed to this:
/g/TekScopes/photo/259498/1?p=Created,,,20,2,0,0
/g/TekScopes/photo/259498/0?p=Created,,,20,2,0,0
I just killed the scope for good. Grrr...
I¡¯m quite sure I was very careful, and didn¡¯t touch anything... ?
Either probe tip touched another node and made a short, or your DVM has an issue. Probing node 5 with a DVM is safe, and looking at all other voltages being so far off says there may be an issue with the measurement setup.
Still, it is not dead. Debugging goes back to measuring the same voltages again with a known good setup.
I don't know your location but if the scope can make it to SF Bay Area (aka Silicon Valley) I don't mind taking a look.
Ozan
