See attached schematic.
Here's my problem: VR2 and VR3 work fine, but VR1 only produces ~4v measured from its output to its return line. I checked the resistance and it's showing 0.7 ohm from output to return, all others read high. Based on the attached schematic, what's the most plausible explanation? It's possible I overvalued one of the circuits, so assume components could have failed due to that. What could "fail open" and create a path to ground?
Here's my problem: VR2 and VR3 work fine, but VR1 only produces ~4v measured from its output to its return line. I checked the resistance and it's showing 0.7 ohm from output to return, all others read high. Based on the attached schematic, what's the most plausible explanation? It's possible I overvalued one of the circuits, so assume components could have failed due to that. What could "fail open" and create a path to ground?
Is LED3 lighting up when powered?
The regulator VR1 must be bad, there's nothing else connected to its output that could give such a very low ohms reading.
The regulator VR1 must be bad, there's nothing else connected to its output that could give such a very low ohms reading.
This started when I replaced VR1 and 2 because I had killed then by accident. Everything worked fine. Then, some time between then and the next time I turned the unit on, VR1 stopped producing voltage. I also observed that it was very hot. So I ordered replacements.
I replaced VR1 and observed no improvement. I swapped it for another new regulator, same behavior.
So I doubt it’s the regulator itself.
I replaced VR1 and observed no improvement. I swapped it for another new regulator, same behavior.
So I doubt it’s the regulator itself.
Is LED3 lighting up when powered - is relay RY1 working?
If you think the VR1 is not the source of the problem, remove it from the board and test resistance again on the board between the same two pads. Also test the op amp separately to confirm.
Is R31 ok?
How about U3, maybe it is damaged (or the related diodes/capacitor).
If you think the VR1 is not the source of the problem, remove it from the board and test resistance again on the board between the same two pads. Also test the op amp separately to confirm.
Is R31 ok?
How about U3, maybe it is damaged (or the related diodes/capacitor).
U3 may be shorting VR1. It is suspect because it is exposed to the tubes and therefor potentially high voltage.
That would leave the grids of V14 open and unpredictable results, likely the 305VDC (19) would rise to +412 to +618 VDC. You could remove the 6L6s. But there are issues like it is unclear how "common" (18) connects to "...return" (14). Disconnecting the load may cause strange results.
Unload VR1 from the other circuit branches first, and check the resistance again. Then if it's OK look to the other branches for the short. Unpowered, of course.
The plus input of U3 is unprotected, at least place some diodes between the input and the gnd and plus rail, same as the minus inpout protection. it looks like maybe these diodes were meant at the plus input or the design was borrowed somewhere, and the minus input was the active input.
Please use English. 
What is Vr1 used to supply power to? I think it is short-circuited in the consumption part, leading to such voltage drop. You should check it, try checking TL072 (U3) to see if it is dead..
What is Vr1 used to supply power to? I think it is short-circuited in the consumption part, leading to such voltage drop. You should check it, try checking TL072 (U3) to see if it is dead..
VR1 is some sort of start-up circuit powering part of the HV regulator, in addition to the V1, V2 filaments.
Not clear how it is intended to work. Maybe there is some info in the service manual.
Not clear how it is intended to work. Maybe there is some info in the service manual.
Hmm ok, I see.
I'm not 100% sure what happened. I know I fried at least one of the timers (U8 or U9) by making the wrong connection through the umbilical. I assumed I'd fried a couple regulators too, but this is making me second guess.
The whole thing worked for a bit, I could play a record through it, then VR1 went kaput.
I'll try swapping out U3 and the diode tomorrow when the parts come in.
I'm not 100% sure what happened. I know I fried at least one of the timers (U8 or U9) by making the wrong connection through the umbilical. I assumed I'd fried a couple regulators too, but this is making me second guess.
The whole thing worked for a bit, I could play a record through it, then VR1 went kaput.
I'll try swapping out U3 and the diode tomorrow when the parts come in.
Based on my descriptions of what I likely did wrong, would you check any of the other components in that VR1 circuit? I assume ICs are more susceptible to failure than passive components, but I don't want to replace the IC six times only to realize there was fried resistor or something.
If I had to repair this power supply, probably would:
unpowered: cable disconnected
measure all the resistors in circuit for value
measure all the diodes/Zeners in circuit for good PN junctions
check tubes V13,14,15
powered, with fuse F2 removed to disable HV regulator output: cable disconnected
check DC voltages for all Zeners, regulators, and op amps
see if LED1 and LED3 light or not, they should.
unpowered: cable disconnected
measure all the resistors in circuit for value
measure all the diodes/Zeners in circuit for good PN junctions
check tubes V13,14,15
powered, with fuse F2 removed to disable HV regulator output: cable disconnected
check DC voltages for all Zeners, regulators, and op amps
see if LED1 and LED3 light or not, they should.