Batboard

Does anyone know a way to test the low voltage supply of R2001A/B/C out of circuit?

What symptoms are you seeing and are you sure it's a supply problem? The seven output voltages can be measured on the sub-board with all the test points right next to each other. At the depot, we had a test fixture for troubleshooting and repairing the supply outside of the unit, otherwise it had to be in for proper loading and feedback circuitry.

The beast won't power up, either on mains or 12V. I did your "shotgun approach" and changed all the electrolytics for well rated low esr ones (and you were right; they were all very poorly!). Still no boot-up though. Now that I think of it I should have heard the relay click in when I connected the 12V. Don't remember hearing it. I'll go check now.
Been 15 years since I worked on these things, and it's a bit labourious now with the manual spread all over the bench! ANY help would be appreciated.

JC

Try (carefully) re-seating the ribbon connectors between the mother board and the front panel.

Turned out to be the SMPS chip, MC3420L.Trouble is the only suppliers I can find sell 'em by the hundreds!
I wonder if there's a modern pin-compatible alternative; perhaps from another manufacturer?
Suggestions welcome!

JC

Lot's on ebay ...try without the L suffix since the P version is more widely available. There is also the MC3520 which has a broader temp range for the same part.
the best deal would be the four for $5 of the MC3420LD

Thanks for that! However now no need; found one in a drawer and it's running again.While I was at it I set up the HV supply and the scope and it's as good as the day it was new!
I'd been told (by a Motorola tech) that the tube was shot.Real glad to find he was wrong; ......Hmmm.. wonder what I'll spend the $1300 on.........

JC