Jerry,
I repaired my ferro-resonant UniVolt once also; mine had a bad solder
joint. It has since failed again, and it appears to be the transformer
this time.
A better replacement for the UniVolt than your power supply is the
StatPower charger. It's designed for battery charging and will be much
kinder to your battery than any simple regulated power supply.
The StatPower has a 3-stage regulator in it: Bulk Charge, where it holds
the voltage at something like 14.6, current at a full 20 amps, until the
battery is mostly charged. It then goes into "Aborption mode", where it
tapers the charge until the battery is full. Lastly, it switches to
"maintenance mode", where it holds the voltage at 13.5 or so. Every 21
days, it goes into Absorption mode for a few hours to keep the battery
topped off and sulphate-free. There's also an equalization mode that can
be switched in manually.
A simple regulated supply like your MFJ, an Astron, or the UniVolt
charges the battery very slowly, at about 3 or 4 amps. Yet, if you leave
it plugged in continously when you're not using the trailer, it will
overcharge the battery, cooking out all the water. They'll work sorta OK
if you're using the trailer steadily and are plugged in while doing so,
but fall short if you expect them to recharge the battery after
boondocking and/or keep it plugged in at home.
<<Jim>>
Jerry Flanders wrote:
>
> 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
>
> At 01:01 PM 6/2/00 -0700, you wrote:
> >Does anyone out there have a schematic diagram for a Univolt Converter found
> >in a 1975 Airstream International 29' ?
> >
> >Ed
> >
> >
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