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Old 24th April 2016, 06:52 AM   #1
MVZIK is offline MVZIK  Singapore
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Default Preamp Transformer overheats


Mine is an AI L1 that uses 6922. Lately I realized the power supply section is running very hot. too hot to touch. I thought it was due to aging tubes but things didn't change even changing to a pair of new EH6922. Can anyone tell me what are the possible causes of transformer temperature to rise?

So far there is no blown fuse and the amp is still working even the PS section gets very hot but I'd stopped using it now to avoid causing further damages.

Please help guys. Thanks .
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Old 24th April 2016, 07:05 AM   #2
ericj is offline ericj  United States
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Not familiar with the product but i went looking for pictures.

Huh. I have those exact blue axial 0.01uf caps.

Anyway - my general guess is that maybe one of those heatsinked transistors has died in a bad way and is causing an overload in the power supply. It is interesting that it still works because a responsible designer would employ a fuse or circuit breaker to prevent excess current from damaging the device or causing a fire, etc.

If you have or can get a non-contact (infrared) thermometer you should be able to narrow down which parts are excessively hot.

But it may be advisable to seek authorized, professional repair service.
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Old 24th April 2016, 08:16 AM   #3
MVZIK is offline MVZIK  Singapore
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Hi ericj,

Thanks for the reply, they are LM317T and LM337T. How to know if they are faulty?
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Old 24th April 2016, 09:03 AM   #4
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If a regulator had failed there would be serious sound problems. To prove the transformer has not got shorted turns, disconnect the HT winding an insulate to make them safe, remove the valve and then run it. If the temperature increases with no load, replace the mains transformer. If you email your circuit to Majestic Transformers, Parkstone, Dorset, they may have the drawings still or at least make you a new one. Electrical Transformers Company, Poole, Dorset UK
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Old 24th April 2016, 09:18 AM   #5
MVZIK is offline MVZIK  Singapore
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What's HT ?
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Old 24th April 2016, 09:21 AM   #6
MVZIK is offline MVZIK  Singapore
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It's does have cracking sound and sound of crumbling of PP polybag(you know what I mean?). One side is more obvious than the other.
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Old 24th April 2016, 12:31 PM   #7
Enzo is offline Enzo  United States
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HT means "high tension", another term for high voltage.
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Old 24th April 2016, 01:25 PM   #8
DF96 is offline DF96  England
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In the UK we call it HT. Some other places call it B+.
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Old 24th April 2016, 02:07 PM   #9
MVZIK is offline MVZIK  Singapore
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Got it. B+ is more commonly used in Asia.

Last edited by MVZIK; 24th April 2016 at 02:10 PM.
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Old 24th April 2016, 03:06 PM   #10
djn is offline djn  United States
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I had the same over heating with my pete millett low mu preamp. Turned out the 6.3A supply was for 5a and all the tubes added up to 8a. I installed a separate heater transformer.
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