Re: [VAC] Univolt Converter

[Jerry Flanders <jflanders2@home.com>]

For what it is worth, I once opened up and repaired one from a '76. It used
the "ferroresonant" regulation principle - you might be able to look that
up if you want to read up on it. 

Briefly, this type of regulation circuit depends on establishing a resonant
circuit (the combination of the transformer AND the capacitors inside it)
and a saturated magnetic circuit (the physical design of the transformer).

What happened in mine was that one of the two capacitors failed, upsetting
the resonant condition. I replaced it by going to an electrical supply
house and getting a new capacitor of the proper electrical specs and
installing it. That made it work OK.

I could never understand why the manufacturer recommends against working on
these in the field - they are not very complicated.

The part of the circuit that I could work on was so obvious a schematic
would not really have helped - there were only 2 or 3 components in the
entire box, and you could see just what hooked where. 

BY THE WAY: If mine failed today, I would probably convert it over to use
one of todays easily available 13.8 VDC adjustable power supplies intended
to power Ham radio stations. I have one in my ham radio station made by MFJ
(www.mfjenterprises.com - list price is maybe $150) that puts out 35 amps
and is adjustable up to maybe 15 volts. It is highly regulated and puts out
extremely clean DC with only a few millivolts of ripple. It is metered both
for voltage and current, and is much quieter than the univolt. I have not
yet looked into what is required to use it in this application - a blocking
diode on the output may be needed.

Jerry W4UK