Source for replacement caps in H744 regulators

Jay Jaeger cube1 at
Fri Jan 7 10:02:12 CST 2022

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
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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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