Gather round folks and I will spin a yarn of confusion.
So I acquired a Thurlby PL310 PSU (least that's what it said on the front) a while ago. It had an issue where the voltage adjustment at the low end (0-3V) was very jumpy:
Although useable, I decided after a while that I should really get round to fixing this. A quick Google relieved a few others who had suffered the same problem with a common fix: Replace the old wire wound coarse adjustment pot with a new, better, carbon track one. And whilst your at it replace the fine adjustment one.
There was a pretty google hit of somebody who had posted pics of their repair that was on a site which had not been maintained/dues paid, but can't seem to get the same hits from the same search terms anymore...
I also read that the voltage adjustment pots were not wired as you would typically expect in a linear PSU. They appeared to be part of a feedback network and being used a variable resistors/rheostats rather than potentiometers changing the set point.
I've a copy of the service manual here if anyone wants it: SINCLAIR THURLBY PL SERIES POWER SUPPLY
Now if you look at the Thandar range of PL series power supplies you will find that the pots are wired as pots:
Also IC7-A is an LM324....which I couldn't find:
No LM324 here! So massive pitfall there! Turns out the Sinclair branded and Thandar branded PSU, SHARE THE SAME MODEL NUMBER!! But why the flipping hell did Thurlby do this!?!
"I know! New company structure, New PSU design, Lets name it EXACTLY the same as the old product!!"
...yeh well done chaps...could have at least bloody versioned it!
Anyway, massive model number flaws aside, this was a distraction from the task at hand, but I though I should make the uninitiated aware.
The task at hand was to replace the potentiometers:
Rummage in the spares box and a 50k pot (coarse) and 1k pot (fine) were acquired.
After fitting them and giving them a little test, the voltage was stable and the fluctuating output had gone completely.
It should be noted as well that if you're testing this after fitting new pots, and you are resting the front panel PCB near the AC line switch, you will see the same fault you are trying to cure! Seems the Mains interference is large enough to cause the same issue.