Source for replacement caps in H744 regulators

Nigel Johnson Ham g4ajq1 at
Fri Jan 7 14:35:54 CST 2022

I never had this problem with an H744 power regulator, but once I had a 
horrible intermittent problem in the RK05 power supply where the caps 
were 'wafting in the breeze' of the fan.

Metal fatigue had created almost microscopic cracks around the part of 
the land where the screws attached, causing a disk fault every week or 
so.  It was in  one of four drives and the only way I found it was to 
swap the drives one by one and wait, since the emerg signal was paralleled!

I told the chief engineer that the problem was the same thing that 
caused planes to crash, and he suggested maybe the cargo door had fallen 
off!  (that puts it in 1972 since it was AA Flight 96 that had just 

Try reheating around the screws.



Amateur Radio, the origin of the open-source concept!
Skype:  TILBURY2591nw.johnson at

On 2022-01-07 15:21, Rob Jarratt via cctalk wrote:
> 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?
> Regards
> Rob
>> -----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.
>> JRJ

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