Ok update on the Farnell TOPS1. It's knackered. Just how I like things.
So I stripped it down properly at lunch and catalogued the issues. Looks like there are a few bits of corrosion in it. What did I expect for £20? A project that's what. And I got one.
So first thing the power LED doesn't work. I checked the series diode and resistor and both were fine. But there was 8v across the LED which is wroooong. So I snipped off the heat shrink and found this lurking:
Made up a replacement and installed it and that works.
The 5V section works fine now I removed the philips cap which is good. But the +/-15V is knackered. It outputs 12.6v on each rail and this slowly drifts. On the negative supply, which everything is referenced to the current limit LED is stuck on. Quick finger poke around found a hot LM741C:
I tested VT6, VT3 and did some measurements on the current sampling resistors (R10) and R18 and found that the LM741 was slinging out 26mA trying to servo VT5 into meeting its objective. Despite its best efforts, due to the relatively low rail compliance of the LM741, it could only get a reference comparison voltage at the opamp input of 4V from the divider and it needed to match up to to the ref voltage of 5.3V.
So I figured VT5 was popped. Alas no it was the same fatality as the LED.
So I've written it up, written up a shopping list and will order parts this evening to replace both the remaining pass transistors and capacitors including new mounting hardware and wiring. Should cost about £5 delivered from bitsbox after I've dug through and filtered out stuff I already have.