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Old 05-12-2008, 05:27 PM   #1
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1971 25' Tradewind
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Dead Univolt

Does anyone have a schematic for a Univolt Model 35, S/N 9425? The full wave rectifier that converts AC to DC has failed -- I would like to repair unit, and reuse it as a battery charger in my shop. I need the schematic to determine what the rectifier specifications are, what else may be wrong, and to help me understand how the circuit board controls the SCR. Or, if anyone has a good full wave rectifier (mine is epoxy encapsulated and is mounted on an aluminum plate for cooling) from a deceased Univolt, that would be a great help. The Univolt is out of my 1971 25' Land Yacht, and will be replaced with a converter having a smart battery charger in the near future.

Phil Weaver

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Old 05-12-2008, 05:34 PM   #2
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Can you just use a diode trio from, say an old GM alternator?

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Old 05-12-2008, 05:39 PM   #3
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Half-wave rectifyer

Originally Posted by Phil Weaver
Does anyone have a schematic for a Univolt ...
The SCR's exact function in my unit puzzled everyone who saw the schematic.

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Old 05-12-2008, 06:39 PM   #4
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The SCR must be to turn the voltage to the battery off when it reaches full charge. However I can't see how it would work the way it is drawn are you sure that D4 is not turned the other way? A SCR will conduct untill the polarity is reversed or you cut the power feed.
By the way my Univolt killed a brand new Optima gel battery, I think I would rather have a battery charger from Wally world. These old DC power supplies would work for a long time the new switching mode one's are easy to kill and hard to repair. The new battery chargers will not turn on unless they see that they are conected to a battery that has some voltage present.
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Old 05-13-2008, 06:01 PM   #5
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Terry, the alternator diodes are an excellent idea, and I may have to use them. However, if I can find an exact replacement part, I would not have to modify the Univolt to accept the new diodes -- life would be easier.

Thanks for your help.
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Old 05-13-2008, 06:28 PM   #6
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1971 25' Tradewind
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Tom, my Univolt regulation circuit has three transistors, many diodes, small capacitors, resistors, and a trim pot. There seems to be some type of voltage sensing (a fine guage winding) connecting the transformer's primary and secondary windings. To me it's overly complicated, I would not have designed it like that. The SCR turn-on time is regulated by this circuit, and I would definately like to have a schematic and parts list.

I looked at your schematic, it has a much less complicated regulation circuit. I am not sure what the purpose of R2 and D3 are. I think it would work just as good without them.

Still looking for info!

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