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Old 17th December 2007, 04:05 AM   #1
diyAudio Member
Join Date: Oct 2007
Default Help needed with vintage variable power supply

I picked up a vintage Voltbox power supply to use as a variac. I have a weird problem that I hope someone can help a noob with.

When I switch on the unit, the Line Voltage shows correctly as 120V. I now move the pot to say 50V - on the second switch (not the on/off but a second switch that toggles between Line and Load Voltage), the Line Voltage still shows as 120V (correct) but the Load Voltage shows as 0VAC.

I have used a DMM on the 3 2-prong A/C outlets that are provided and they all show 0VAC. I have also measured the voltage across the wires in the pot and they show the correct output voltage so I don't think the pot is faulty.

Interestingly, I can get the correct output voltage across one of the other outlets (I don't know how to describe it - more like an stereo jack outlet rather than a typical A/C outlet) but none across any of the 2-prong A/C outlets.

The wires seem fine so the only thing I can think of is that the Line/Load switch is faulty in some way - I don't have a schematic so I cannot verify this.

Does anyone have any other ideas of what could be wrong ? There are no visible caps/resistors/other parts - just the switches and the wires.

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Old 17th December 2007, 05:36 AM   #2
gootee is offline gootee  United States
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Join Date: Nov 2006
Location: Indiana
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If you are fully aware of the dangers involved, can you open the unit and measure the voltages at the switch's contacts? That should prove or disprove your theory.
The electrolytic capacitors ARE the signal path:
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Old 17th December 2007, 11:16 PM   #3
diyAudio Member
Join Date: Oct 2007
ok did that - when I switch to 'Line' there is 0VAC across all of the possible 3 combinations (between the 3 horizontal bars I see in the innards of the switch).

When I switch to 'Load' (yes, no typo here), I get 120VAC across the bottom and the middle and the bottom and the top bars and 0VAC between the middle and top bars.

BTW, the middle bar is the one that is connected to the Voltmeter. The other part of the Voltmeter runs to the line input (where the voltbox is connected to the mains).

Unfortunately, the rest of the wires are tightly wound together and it is impossible for me to decipher where each one runs to so am a bit stumped on where to check next. Any ideas ? thanks
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