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Chip Amps Amplifiers based on integrated circuits

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Old 30th September 2020, 05:49 PM   #11
Mooly is offline Mooly  United Kingdom
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Complete novice alert! *don't read if easily annoyed by stupid questions*
Try and understand the reasoning... the transformer has two 18 volt windings which if connected in series give 36 volts in total however because these are AC voltages we also have to get the phase correct.

If the phase is incorrect we are still always able to measure 18 volts from the centre point to either end but when measured end to end the voltages cancel. In that case we reverse one of the windings.

Lets start from the beginning and be sure each pair of windings is identified correctly

1/ With the transformer OFF and NOT connected to the mains measure the resistance from the RED lead to each of the other three in turn.

Only one of those three leads should have continuity and it will be a low ohms value. That lead and the RED lead form one winding.

There should be no continuity (so an open circuit) to the other two leads from this pair.

2/ The pair that are left should read a similar resistance to the first pair just identified above. This is the second winding.

This second pair should also have no continuity to the first pair.

3/ Now switch your meter to AC volts and measure the voltage of each pair of windings we have identified. Both should be similar and around 18 volts.

4/ If the windings are connected in series they should add and give 36 volts end to end. If they don't then swap just one pair around.

(Also make sure the ends are clean bare wire and not covered in insulating varnish which can give misleading results)
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Old 1st October 2020, 10:18 AM   #12
GooseyGander is offline GooseyGander  France
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Hello Mooly and thanks for your reply. I have followed your instructions very carefully and here's what I found: The red and yellow leads are a pair when I tested for resistance. However, when I test each pair with AC volts, I get 19v on the red/yellow, nothing on the green/blue pair.
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Old 1st October 2020, 10:48 AM   #13
Mooly is offline Mooly  United Kingdom
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Are you absolutely certain that the green and blue wires are scrapped back to bare copper?

It sounds crazy but this can be somewhat difficult to achieve. Be 100% sure you have the insulation scrapped off. Use sandpaper to do this.

If there really is no continuity between those two wires (the green and blue) then just for completeness see if there is any continuity between either of them and the other pair. There shouldn't be but lets cover everything.
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Old 1st October 2020, 11:04 AM   #14
GooseyGander is offline GooseyGander  France
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Yep, all clean and bare copper (they are now cos I sandpapered them to be sure). Still nothing.
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Old 1st October 2020, 11:11 AM   #15
Mooly is offline Mooly  United Kingdom
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Well it sounds for all the world that the unit is faulty and the winding is open circuit... which is something I've never encountered or even heard of happening as a fault mode. No continuity and no voltage output are conclusive proof though.

Don't go deliberately bending the wires but have a look and see if there looks to be any fracture where the leads comes out of the core... like would happen if it had been waggled back and forth many times.

(Primaries can become open for various reasons but never secondaries with their thick wire)

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Old 1st October 2020, 12:38 PM   #16
GooseyGander is offline GooseyGander  France
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Right. Well, I went off and had my lunch, thought this can't be faulty as it's straight out of the box and hasn't been fiddled with or anything, so it can't be faulty, especially as you've said you'd never heard of such a thing. So I test both pairs again and lo and behold, 19v on both pairs. So WTF? I don't understand it but am going to get another multimeter anyway as I don't trust this one now! I'll test them again in a bit just to make sure!
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Old 1st October 2020, 02:03 PM   #17
Mooly is offline Mooly  United Kingdom
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right then... so its looking good.

Its very important to be sure that not just the transformer leads but also the meter probes are clean.

When you tested the secondary for resistance and got an open circuit then the next instant step to take (well what I would do anyway ) would be to quickly short the meter probes as confirmation the meter is good.

If that was OK I would remeasure again and if still not getting the expected result I would then see if the meter read a short circuit if the probes are placed on the same bit of wire (probes just a few mm apart).
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Old 1st October 2020, 04:36 PM   #18
GooseyGander is offline GooseyGander  France
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Tested yet again and now the exact same result: red/yellow=19v and blue/green=19v.
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Old 1st October 2020, 05:51 PM   #19
Mooly is offline Mooly  United Kingdom
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Excellent. So now just wire them in series and measure end to end. 38 volts is what we want. If it is near zero reverse one set of wires.

When you have it correct the centre join then becomes the 'zero' point and you have 18 volts from this point to each end wire.
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Old 2nd October 2020, 10:28 AM   #20
GooseyGander is offline GooseyGander  France
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Right. I've joined yellow and blue and I now get 38v on the red and green pair. Can you explain what I do with them now? I assume the yellow/blue stay joined? Do the red and green go to the rectifier?
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