Transformer repairs

Ade Vickers javickers at solutionengineers.com
Fri May 30 10:50:47 CDT 2008


dwight elvey wrote:

> Hi
>  First, I agree with Tony, the transformer is most likelty 
> OK. Your earlier description wouldn't be enough to say it was 
> good or bad.
> Measuring things on the primary side with a scope would be 
> difficult without using an isolation trasnsformer. The point 
> in the circuit that you'd need to use as ground reference has 
> about 150-170 volts DC on it. If you connected the scope 
> ground to this point you'd really blow things without using 
> an isolation transformer.
>  Next is that T3 is a pulse transformer and not an 60Hz AC 
> transformer.
> The input is from the regulator chip, pins 8 and 11. One 
> should see a 50-150KHz pulse on these pins.
>  This would be the first place I'd look.

My apologies - I previously stated that all T's were fed from the same AC
input, but that's wrong. Mea culpa, my memory is rapidly fading, and I was
going from memory...

You can probably follow things from the schematic better than I can from the
circuit board, however:

The T3 transformer is fed via Q1 from one side of C7. I'm guessing that Q1
is what's generating the pulses?


The other side of cap C7 feeds one half of T1 (power for this comes off the
+ve side of the REC1), the other side of T1 coming from the middle pin
(collector? Probably a different name on this device) of Q1.

So...... OK, maybe Q1 is fried? Yet I can see a signal, of sorts, going into
T1. I've not tried to measure it's amplitude, but it's definitely of mains
(50hz, in my case) frequency.

T3 - yes, that has a high-speed pulse going into it; I've not measured the
frequency, but it's rapid. However, T3 has movement on both sides of the
coil.


Can remove T1 from the board & conduct an isolated test? If I connect a 9v
battery (with a lightbulb in series, say) across the primary, would I expect
to see a pulse of some kind come out of the secondary if it is working
properly?



Cheers,
Ade.

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