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Old 8th January 2012, 02:13 PM   #1
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Default Salvaged transformer.

Hi.
I've salvaged a transformer out of a old vcr and I'm wondering if it could be used in a gc amp. I know it's 40VA and seems to have dual secondaries as it has 2x4 leads coming out. How would i have to take measurments? I know these questions maybe stupid but it is better to ask people than to frie it all
PS: Markings on it are:5214411 SOHH ZK and C-222-40VA.
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Old 8th January 2012, 03:00 PM   #2
sbrads is offline sbrads  United Kingdom
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I'm assuming this T/F isn't some fancy high voltage step-up one with high resistance windings.........

First make sure you've identified all of the windings using an ohms tester, the primary should be obvious from how it was wired in the VCR but it will be the highest resistance winding, probably >>15R. The dual secondaries will likely be a few ohms at most. Check if the 2 secondaries are isolated or not on the low ohms range of a DVM for future reference, both are OK but you need to know. Then, assuming you haven't got a 500v isolation tester, check on a high Mohm DVM range that there's isolation between primary and both secondaries, i.e. no reading or at least higher than 20M.

If the above is OK then power the primary up as safely as you can temporarily, insulating wires and using a 1A mains fuse, 3A at most. Measure the secondary AC voltages on the DVM for reference.

This won't tell you the phasing of the secondaries. If you need to know that (you won't unless they're the same voltages), then if they were isolated, wire them in series and the total voltage will double if the 2 joined wires were out of phase, or drop to near zero if the joined wires were in phase.
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Old 8th January 2012, 03:12 PM   #3
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Watch out if VCR had a gas tube display. May have a relatively HV secondary. Otherwise just issolate outputs and measure between all wires coming out. Write it down. Open leads won't fry things. Shorted leads will. When in doubt attach meter with clips to output leads and start with meter on highest range, then plug in transformer. Take reading, unplug and move to next set of wires.
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Old 8th January 2012, 07:35 PM   #4
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It had a gas tube display if I'm correct, that means that one of the secondaries is unuseable for a gc amp, but the other one may still be useable? One of the bridge rectifiers that it had was 100-800v, and the other 60v. that's what I managed to find out.

also, could this be used for a gc amp?
https://www.elfa.se/elfa3~ee_et/elfa...6-85&toc=19593
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Old 9th January 2012, 03:02 AM   #5
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What voltages do you need? What were the readings from the transformer leads? I don't know what a "gc" amp is, but it doesn't matter. You should know what voltages your project needs. Having a gas display doesn't eliminate a transformer, that's just an extra winding you may not need. I would presume the VCR had +/- supply rails. What does your project call for?
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Old 9th January 2012, 01:25 PM   #6
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A gainclone amp. It would need about +-25v after rectifier. I'll take the measurments later today. I've been wondering about one thing between the ac input and the transformer there were two little transformers(atleast i think they were transformerers) arent these supposed to up the voltage?
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Old 9th January 2012, 03:41 PM   #7
RJM1 is offline RJM1  United States
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It's been a while but I seem to remember the output voltages for a VCR power supply to be 12V for motors ( up to 1 amp) 5V ( another amp) for logic and 36V for the florescent display at a few milli-amps.
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Old 9th January 2012, 06:34 PM   #8
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So the transformer should be unuseable for this purpose. Where could I scavange a transformer for a gainclone amp? Or would this work:
https://www.elfa.se/elfa3~ee_et/elfa...6-85&toc=19593
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Old 9th January 2012, 07:02 PM   #9
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From my VCR repair days it was quite common for the display to be driven from its own little invertor.
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Old 9th January 2012, 07:21 PM   #10
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One way, maybe not the best way, to ascertain the current capacity of the windings of an unknown transformer is to load the secondaries with resistors.

At NO LOAD the secondary will have an excessive voltage. This will quickly stabilise as you add resistors.

What you are looking for is the point at which the voltage starts to fall to about 95% of this stable voltage.

For Example. If you measure Vs at no load and it is 24V. Try adding 240 Ohm 3W resistors one at a time across the secondary.

If one resistor gives you say 22V and the second 22V, you have reached a stable voltage.

Now add more resistors, one at a time until the voltage drops to about 20V.

You will now have an approximate value of load that can be sustained by that winding.

If you have multiple secondaries you might need to repeat the experiment wih all secondaries loaded.

THIS IS NOT EXACT - But it does give you an approximation.

For safety sake you can always down-rate your findings.
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