After restoring a Motorola R-390 a few months back I ran into a very perplexing problem with the power supply.
The symptom was no DC output although the expected voltages were present at the 26Z5W pins. After a lot of fruitless checking, where everything tested OK, I finally discovered that the HV center tap on the power transformer(pin 6)was not grounded as per the manual diagrams. Further investigation showed a new wire connected from this pin to pin 14 on P118. Further tracing of wiring showed this to be connected to the DC fuse which I normally don't bother to install. Putting a fuse in this restored the power supply to normal operation.
I then found the B+ fuse had been shorted out at the fuseholder. I assumed this to be some mod to transfer the fusing from the hot side to the ground side of the system. Why I have no idea. I forgot about it and went on to other things. I even forgot to make a note of it. Today I fired up another R-390 which I have just restored and ran into precicely the same fault. Only problem was I had forgotten the fix. How Dumb can I be. Anyway after about 20 mins head scratching the light went on and I replaced the DC fuse. Now all is well. Even the voltage regulator works!!
I record this now in case anyone else runs into the same problem.