Power Designs TP325 Startup Issue

[amosborne]

I have a Power Designs TP325 linear power supply that appears to be having an issue starting up on the 6V/5A channel. I've attached the schematic I have below; the channel at issue is on the bottom half.

Background

A visual inspection of the circuit components shows no visible sign of damage. With a multimeter and the circuit powered off, I measured the resistance across all BJT's, diodes, and capacitors and found no obvious short circuits. Upon powering on, the output (dc+ to dc-) is -0.56V.

Here's how I understand the principle of operation of this power supply:

• C103 is the primary secondary side power supply, rectified to approx. 11V. Q106 controls as a Darlington parallel series pass transistors Q104/Q105 (with load balancing resistors R112/R114).
• C102 is the auxiliary secondary side power supply, rectified to approx. 30V. This supply drives the feedback amplifier by floating above the positive output. Zener diodes VR101/VR102/VR103 generate bias voltages about the positive output (+11.2V, +6.2V, and -5.6V respectively). Q102/Q103 operate in feedback to series regulate the auxiliary supply by shedding power in parallel with R104.
• Q108/Q109 drive the voltage feedback error amplifier. Q110 senses a current limiting condition and drives Q107 in the feedback error amplifier. Q111 detects an overvoltage condition to enable SCR CR109. Q101 drives the fault condition lamp on the front panel.

When probing the circuit while powered on, I find that the auxiliary power supply never finds its bias with the Zener diodes. VR101 is forward biased rather than in reverse breakdown, and Q103 (and therefore Q102) are biased into the cutoff region. Q110 is also in cutoff.

I know this must be a startup issue because over the course of my live probing, the power supply started working! While probing about Q103 and Q107, my multimeter must have held enough charge to turn on Q103. Once that happened, the power supply was fully operational (under no load, I did not test with a load). Power cycling the supply returned it into its faulty state, and I've been unable to return it to its working state again.

Questions

• What is the order of events during startup? I am having a very difficult time describing how I think the Zener diodes are supposed to find their bias. I take it that C107 is supposed to charge through R104 up until Q103 turns on or the Zeners enter breakdown but I have no idea what is supposed to happen first.
• What component issues may impede the startup of this linear power supply? My best guess would either be a leaky diode/capacitor, C102 has unacceptably high ESR, or there is a soft-short somewhere else, but because I cannot describe what the startup order of events are, I also cannot describe has such a bad part might impact that sequence.

I realize the schematic sucks and I'm working on trying to draw it in a way that is more sensible. This is proving to be challenging for me as well.

[amosborne]

I found the issue - R104 was failed open. This explains why the supply would not initially start-up but would if if Q103 was forced on by an external voltage source.

[alm]

Nice that you found the problem so quickly! According to the manual, R104 is a wire wound resistor. I don't think they generally fail unless they are grossly overloaded. Did you see any reason why R104 failed, like signs of overheating, or C107 failing short (and possibly being replaced)? Or does it look like a random part failure?

[amosborne]

I did think about that but could find no evidence for why R104 failed. If I force the supply on by turning on Q102/Q103 manually, it works fine (although Q102 gets a bit more toasty than it would if R104 were still there).

I am going to replace with a 680 Ohm, 5W resistor and see how it goes.