Source for replacement caps in H744 regulators

Rob Jarratt robert.jarratt at
Fri Jan 7 14:21:49 CST 2022

Weirdly, the whine has disappeared. This is after I put the suspect capacitors back in. Because I had previously removed them for reforming, I wonder if one of them was not properly screwed in?



> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Jay Jaeger via
> cctalk
> Sent: 07 January 2022 16:02
> To: cctalk at
> Subject: Re: Source for replacement caps in H744 regulators
> On 1/6/2022 7:03 PM, W2HX via cctalk wrote:
> > My 2c. I am not familiar with a "whine" but certainly a "hum." Sometimes if
> a power supply has seen a lot of heavy load over its lifetime, the heat
> generated can begin to do things to the transformer. And once that heat has
> done its "thing" to the transformer, it stays that way. And no replacing
> external components will change the hum. However, there are some
> transformers with bolts and nuts that hold the laminations together.
> Sometimes they can be tightened to reduce the hum. I don’t know this PS
> specifically and whether it falls into this category or not.
> >
> > I don’t know if what you are hearing is transformer hum, but if it is, you
> may just have to live with it.
> >
> > 73 Eugene W2HX
> > Subscribe to my Youtube Channel:
> >
> This does not fall "into this category".  This is typically high frequency (in
> excess of 10KHz) whine, not 60 cycle, 120 cycle or even
> 400 cycle "hum".
> My experience with many PDP-11 machines going back to the mid 1970s, and
> those in my collection, is that this whine from the *switching* power
> supplies is very common. For some people, it may be above the frequency
> that they can hear.  For me it is not (I could also hear burglar alarms in excess
> of 20KHz back in the day, though I doubt I could now, at age 70.)
> My *guess* is that it comes from the inductors in the switching circuit, and is
> *mechanical*, induced by the switching waveform, which in turn is
> dependent upon load.  If I had one that was really bad, I'd be tempted to put
> on a glove for insulation and hold those to see if the mechanical pressure
> made any difference.

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