Re: help replacing an old ac power transformer.
- From: John Popelish <jpopelish@xxxxxxxx>
- Date: Sun, 09 Oct 2005 14:44:09 -0400
fotoobscura@xxxxxxxxx wrote:
Hi. I have a very old transformer/power supply that blew that i'm trying to replace.
All it says on the outer box is:
120vac 75 watts 50-60hz
The transformer has 4 leads...two reds and two blocks.
The transformer was made sometime between 1974-1979.
I went to Rat shack to buy a new transformer which has three blacks and two yellows. Based on the way the wiring *was* setup I rewired the transformer to the new one and flipped it on.. Nothing..not even a hum or anything. The radio shack lists "center-tapped secondary" if thats any help.
There is one black wire "loose" and I am unsure where it goes or if it should go anywhere. There's nowhere else for it to go unless its supposed to also connect to the ground (the ground already existed for the plug to the box the transformer is in). Is that last wire a ground?
Transformers can be made in almost infinite variety. We have to figure out the approximate output voltage and current rating of the old unit. We may get a clue of the voltage from the big electrolytic filter capacitor voltage rating after the bridge rectifier. The rectifiers or capacitor may also be bad. In fact, your transformer may be fine. Once the output voltage rating has been figured out, the current rating can be approximated based on the power rating. As a last resort (after you are absolutely sure the transformer is bad) would be to take it apart and count the turns on primary and secondary and compute the output voltage that way. The wire gauges are also an indication of the current rating needed.
.
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