[RFI] Switch Mode Power Supply

Roger Parsons ve3zi at yahoo.com
Fri Oct 28 08:52:46 EDT 2005

My H/B amplifier uses DC on the tube heater because
(1) there was some hum modulation with AC and (2) I
didn't have a 5.5V high current transformer. It has
worked quite satisfactorily for 15 years, but a couple
of weeks ago the ex-computer supply that I have been
using decided to die. The fault is somewhere in the
very complex discrete component low level circuitry,
and I don't have a schematic. 

The simplest solution seemed to be to find and fit
another supply. So I did this and am now using a
Lambda LYS-K-5-0V, which is also pretty old. The
supply produces S9+ hash all over the place, which
rather defeats the object.

I found that by-pass capacitors on the power leads
help a little. A high permeability toroid around the
power lead doesn't change things at all. Disconnecting
the low voltage output leads reduces the hash a
little, but clearly the main source is either
radiation or conduction along the power cable. 

The power supply is open frame, but is fitted within a
steel enclosure.

Whilst I could probably find another suitable supply,
it might be just the same, and there is a distinct
limit to how often I could do that!

Any suggestions as to how I might manage to tame the


73 Roger

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