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BWD Minilab 603B regulator not behaving as expected

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clay1905:
Hello,
This is not a repair how to request, but a request for knowledge, or at least, where I might find it.
Recently I acquired a bench power supply, maybe for just the right price too. But only if I can fathom a few issues.
In the power supply circuit is a LM317T and a LM320T in a mirror image arrangement. They should output +15, & -15V about a common 0V ground.
The LM320 output is -17.97V. This is with the trim potentiometer turned fully one way.
About the only component in the schematic that I think will make much difference is if R320 drifts. However, it measures to within 1%  of 820 Ohms.
The outputs from this supply are connected to the various circuits via wire links on the board. In order to see what influence downstream circuits may be having, I lifted the -15V link. This had the effect of lowering the voltage even further, down to -30VDC.
If the output were weak, I would think that there's a current path that has become less of an impedance, or a short has developed. But no. The voltage is more negative and I don't quite get how that occurs. In my rather limited experience, regulator IC's should be capable of a pretty stable and accurate output if the controlling dividing network is set up correctly.
I would very much appreciate any advice in trying to determine what is happening here.
Attached is the schematic of the circuit in question. Pencilled in are actual voltages found, except the -30V.

gbaddeley:
Possible bad connection on r320 or r321, or the trimpot is bad. You have a classic piece of 1979’s Aussie test gear. Could be showing it’s age.

clay1905:
The trimpot looked good a while ago when I lifted it to check. I do know all the resistors in this network test within tolerances tighter than their bands.
It's odd, and I wish I knew where the supply has been. All the tinning on the component leads is quite grey. I wonder if it's spent time somewhere near the seaside. if the tinning is so strongly oxidised, what else? I'm hoping I don't become the proverbial dog chasing its tail.
Now this is significant. After lifting the -15V wire link, I have measured a resistance across the gap. Only 1500 Ohms. There's only a very slim chance, but I want to be sure that this is not due to the 25V connections. Otherwise I've traced all the nearby tracks, and they're all isolated from this supply. Such a low resistance over a circuit board doesn't look good for the future.
I will lift the trimpot again to test, and as I go I'll remake all the solder joints around this supply network. Mostly though, I think I'll need to thoroughly investigate the stray 1500 Ohms.
It would be a bit of a disappointment if I can't save the supply. From what I've read, this was the last of the "Old Tech" model line, with supplies designed around things that glow warmly. Just the ticket for a nostalgia buff such as myself.

clay1905:
Rats.
This is starting to look less like a technical matter, and more of a physical problem. A seriously leaky circuit board is going to be a tricky thing.
It might work out that making a copy of the circuit board is the best solution. The board isn't very tightly populated, so I think it's worth keeping in mind. If the resistance isn't somehow a spurious measurement, I'll probably need to move this to the appropriate place. I'll be back with more info soon.

gbaddeley:
What is the output on the LM320 when R321 trimpot is jumpered, or set to 0. It should be close to 12 V.
You could try changing to LM337, and lowering the GND to OUT resistor to 100R.
Oxidisation is not good news, there may be bad joints or failed components.

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