I had a 7704A power supply that would lose -15V after it had been running awhile. It might be after minutes or a few hours but it would fail and the screen go blank. I could never find anything wrong with the power supply out and on the min load.
Since the power supply did not go into tick mode I guess that the problem must be in the pass circuit of the -15V. I had previously replaced the pass transistor. I decided to replace it again just in case. I happened to see R93, the 0.12 ohm resistor in series with the pass transistor. It checked okay but I decided to jumper the resistor and test again. It worked fine. The -15V went to -15.1V but it worked for 24 hours. I have replaced R93 with a resistor from a donor and it is running fine.
Looking at the resistor with a magnifying glass I can see a very small crack.
