H744 regulator whistle

From: jpero_at_sympatico.ca <(jpero_at_sympatico.ca)>
Date: Sat Jan 12 14:14:28 2002

> Date: Sun, 13 Jan 2002 00:57:51 GMT
> From: pete_at_dunnington.u-net.com (Pete Turnbull)
> To: Discussion re-collecting of classic computers <classiccmp_at_classiccmp.org>
> Subject: H744 regulator whistle
> Reply-to: classiccmp_at_classiccmp.org

> I have two H744 regulators in my PDP-11/40, along with an H7441, H745, and
> H754. I also have two spare H744s and one each H745 and H754. All the
> H744s emit a fairly loud whistle, and it's driving me nuts. I've seen a
> note somewhere that a loud whistle is caused by the coil de-laminating, but
> I don't think that's the case. I tried swapping the two original H744s for
> spares, no difference. I also tried swapping the coil from the spare H745,
> which runs almost silently, and that made no difference either.

Pete,

Sounds like PSU is reacting to overloading or medium shorts
downstream of that PSU outputs. Also can happen with dried
up capacitors (too high ESR).

I remember that too well when I shorted out a switching VCR's PSU
thanks to bent pin. :-) Rising thin WHISTLE then *snap!* with mini
fireworks (chopper transistor blowing up) then silence.

Blown bits tally: one of resistor series, chopper, chopper
transistor driver, fuse, straightened out pin. VCR functioned
afterwards for years. Even that friend's vcr is funai, with my mods
to improve it's quality, that thing lasted longer than it should be.
(!?) :-)

Cheers,

Wizard
Received on Sat Jan 12 2002 - 14:14:28 GMT