>> >> Any clues appreciated. It’d be nice to feed the 6VAC input from another >> source but I’m not sure my VARIAC will go that low. > > > The gain pin is presenting the output of the error amplifier, an increase > there is what you would expect as the output V falls, as the difference > between the ref and target V increases. > It's 'trying harder' to raise the output V to correct the error. > It implies that the reference and the error amplifier are functioning.
OK, I can see that. > > "6VAC outputs do go a bit erratic" is not a helpful description. I thought they dropped continually but they’re just constantly adjusting around 6VAC. Annoyingly I’ve just run the whole system for the last 40 minutes and while the troublesome supply has dipped below 5V at times it hasn’t collapsed. I’ve been formatting floppies, copying files between floppies and rebooting. Only now has floppy access started to fail at 4.8V but the machine itself still runs. Screen output flickers though, but nothing catastrophic like the other day. > > A wide range of stepdown transformers .. 8V, 10V, 12V, 18V, 24V .. could be > used in conjunction with your variac to get 6VAC for an alternate feed. > > If you have a variable DC supply of adequate current you can feed approx. 6V > * 1.4 = 8.4VDC into the 6VAC input instead. > I’ve got a couple of 5A bench supplies if necessary. > Is the failure load-level (current-draw) related?: > Does the out V remain stable if the supply is unloaded or very lightly > loaded? > Does it take longer to drop at a lesser load? > If so it's likely thermal-related. V out is stable with no load, as soon as the ST is turned off. I could use it to run the floppy drive but that’s such a low load I don’t think it would cause failure. Worth a try I guess. > Is anything heating up unduly? > I can’t tell without the aforementioned jury-rigging extra wiring, in normal usage it looks like this - http://www.binarydinosaurs.co.uk/atariSTPSU-2.jpg Cheers! — Adrian/Witchy Binary Dinosaurs - Celebrating Computing History from 1972 onwards
