{Collins} Re: 75A-4 Buzz at Turn-on



Hi Bill,
I have heard this on some other power supplies that I have. My best guess it that it probably is transformer noise. When you first turn the rig on, the cold filaments look almost like a dead short. As the filament heats up, their resistance also rises and they draw less current. ( That is why we sometimes use "soft start" circuits on big expensive final tubes.) The initial shock of the high current is really taxing the transformer to the max but as things heat up the current demand slacks off and the noise tapers off. I would make sure that the transformer is tight by checking all of the mounting and end cap screws. If everything is tight and the laminations are tight, then the only thing to do would be have the transformer reworked and / or dipped in a transformer lacquer by someone that works on things like motor rewinding shop. A lot of work and messing around and probably the reason the former owner chose to just live with it since it probably really doesn't harm anything.
 My 2 cents
       73
Tony - W9JXN



bscurlock wrote:

When my 75A-4 is turned-on, there is a very loud buzzing noise that last for
about 15 seconds or so, and then fades out fairly quickly.  This radio has
done this since it was new according to prior owners.  It sounds a lot like
transformer noise when something is shorted out.  It is very scary until you
get used to it.

Has anyone else experienced this noise at power turn-on?  Does anyone know
what is the cause?

The reason I am asking at this time is that I just put the receiver on my
test bench to check out prior alignment, and if there is something I can do
to eliminate this noise, I thought this would be a good time to do it.

Thanks.

73, Bill, K5GCW

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Sponsored by the Collins Collectors Association http://www.collinsradio.org
Nets: Tues: 3.805 Mc-2000 Central / Thur: 3.875 Mc-2000 Central
Fri: 3.895 Mc-2000 Pacific / Sun: 14.263 Mc-2000 UTC
1st Wed (of the month) AM Net 3.880 Mc-2000 local (ET, CT, MT, PT)
Sun AM Net: 29.050 Mc-1200 Central






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