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Old 10th January 2011, 11:05 AM   #1
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Default Have I fried an OPT?

In wiring up my Edcor CXSE OPTs on my Simple SE in Triode mode, I incorrectly connected both plate and screen leads where the diagrams show the single light blue wire. Naturally, this didn't exactly sound wonderful, and on investigation I noticed my error.

I then connected them correctly, and now it sounds great, except one of the OPTs has a hum, and it has damage to the leads - at the ends, they have clearly got very hot as the plastic shows signs of melting - not much, but enough to indicate a problem.

As an experiment, I then swapped the OPT's primaries round on the board, and the hum moved to the other channel. I'm not sure if this is what I was expecting or not - I only swapped the board ends, not the connections to the speaker terminals.

Even though both transformers were initially connected incorrectly, only one of them has damage/hum. I'm worried therefore that there's something wrong on one channel on the amp. I did Ty's tests and checked running voltages and all was well, so I suspect it was just 'good' luck that only one fried, but I wouldn't want to replace the fried transformer only for the same to happen again.

I was wondering if anyone more qualified that I has any ideas on anything else I can check, or can better understand what the hum moving with the OPT's primaries means.

And what about the fact it sounds perfect except for the hum? Is there perhaps something I can do to fix it?

Thanks!
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Old 10th January 2011, 12:27 PM   #2
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Quote:
Originally Posted by theskintman View Post
As an experiment, I then swapped the OPT's primaries round on the board, and the hum moved to the other channel. I'm not sure if this is what I was expecting or not - I only swapped the board ends, not the connections to the speaker terminals.
If you suspect you've damaged a transformer and want to swap them between channels to determine if the transformer really is the problem, you need to swap all the transformer's connections. If the hum is generated from somewhere on the board and you only swapped the transformer leads at the board side, your hum would move to the other channel.

In fact, if we mentally swap your left and right speakers, your test "proves" that both transformers are just fine. Your problem is coming from somewhere in the board or the tubes.
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Old 10th January 2011, 02:55 PM   #3
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Are both output transformers at equal distance from maybe hum inducing components like power transformers? You could switch output transformers and check if the hum moves along to the other channel.
Also you can try to turn transformers 90 degrees to see if hum induction lessens.
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Old 10th January 2011, 10:09 PM   #4
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Originally Posted by Ty_Bower View Post
In fact, if we mentally swap your left and right speakers, your test "proves" that both transformers are just fine. Your problem is coming from somewhere in the board or the tubes.
It was one of the input connections - I just rewired them and the hum has gone. I also tried it with a different source. Turns out my preamp wasn't helping because it has a hum to it at low levels and I haven't connected a volume control to my SE yet so it was down very low indeed.

I am a relieved man. Thanks!
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