H744 +5 supply - easy to fix?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Mar 3 18:15:01 2004

> I finally got my TS11 racked this morning and after a good cleaning
> found that it has no +5v. (I must say, I love DEC manuals...)
> Looks like the H744 supply is not working. Anyone know about these?
> Are they easy to fix? Should I just pull it, find a schematic and start

I've fixed them. They're not hard to fix. Yes, it's a switching
regulator, but it's a benign one, running off 30V AC from an isolating
transformer. That can source enough current to blow transistors apart
(see below) but it's a lot nicer than a switcher running off the mains

Whatever you do, don't disable the overcurrent trip. If you do, you can
bet that murphey's law will cause the corwbar to fire, and then you'll
have a lot of silicon blown apart as the current through it goes sky
high. Don't ask how I found this out, suffice it to say I had to replace
a handful of transistors....

Anyway, start by checking the fuse (yes there is one). Then make sure
you're getting a sensible votlage across the large capacitor. This should
be about 40-odd votls IIRC (full wave rectified version of the AC input).

Then check the chopper transistor, other transistors, etc. The IC -- a
good old 723 -- rarely fails unless there are other faults too.

Oh yes, make sure the screw terminals (large capactiors, inductor block)
are tight and making good contact.

> probing?
> (anyone have one they want to part with? :-)

I will pretend I didn't read that :-)

Received on Wed Mar 03 2004 - 18:15:01 GMT

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