Partial Failure - Resistance Decrease Over Time with Power

[Rjc987]

Okay, so I’ve never seen this before and was hoping someone could help. I have a 250v 3A  adjustable power and current supply (Schematic below) that suddenly will only put out ~150v at full. It starts out at about 170v and quickly drops to 160v in a few seconds then slowly to about 150v over a minute when powered on from cold. I probed the voltages and they all look correct before the LM317 - getting 350v + coming out of the transformer, etc but I noticed when I probe the pins on the wire wound pot (37k) controlling the LM317, it starts out around 22k and quickly drops to 21, 20, 19 … down to around 16-17k which I assume is the problem- the LM317 is seeing the pot as if it were set to ~half. I immediately suspected a cap in parallel increasing In resistance as it charges but I pulled all the caps and they’re all good. Since the voltage control is working fine just at half power I assume the LM317 and the BU406 are both good. Other than the caps and trannys there’s only a few diodes and resistors in the circuit so what am I missing here?

The voltage section is circled in red and the pot is in green. Sorry for the bad quality schematic but it’s all I can find.

[soldar]

I cannot make sense of the diagram. It looks like the base of the transistor only goes to the capacitor. Maybe others can help.

[fzabkar]

The base of the transistor appears to be connected to the junction of the zener diode and its series resistor. This will limit the voltage differential between the input and output of the regulator.

That said, ISTM that the pass transistor will be dissipating a massive amount of power.