R2001B low voltage power supply.

[jonnyc]

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

[DJP126]

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.

[jonnyc]

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

[DJP126]

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

[jonnyc]

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

[jry]

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

[jonnyc]

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