Noise floor rise on heathkit W4-AMs when power supply transformers were swapped - diyAudio
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Old 2nd October 2006, 03:54 AM   #1
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Default Noise floor rise on heathkit W4-AMs when power supply transformers were swapped

Hi. I just replaced the original power supply transformers on my Heathkit W4-AM williamson type amplifer. Since I've swapped the tranformers, the amplifiers have very noticeable increase in the noise floor (of varing levels sometimes nearly inaudable, sometimes loud enough to clearly hear when Im listening to music). I have not put a scope on the output, but I am suspecting it may be 60 Hz. The transformer I used was a Hammond 374BX.

My question is this. Could this new transformer be coupling into the signal stage of the amp? If yes, will it help if I rotate the transformer by 90 degrees? Anyother ideas?

Thanks,
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Old 2nd October 2006, 03:35 PM   #2
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Is the orientation of the xfmr the same as the stock?

The secondary voltage(s) the same? <--?

Did you do this on only one amp, or do you have the same effect on two?

Try swapping the phase of the B+ secondary first.

I presume you are hearing LF hum, or is it a buzz?

It is possible that the old electrolytic simply decided to become leaky.
Add capacitance, or replace via jumpers to test that theory.
Scope on the B+ line will tell you much.

If all that fails, try physically lifting the new xfrmr - electrically insulating - at least temporarilly from the chassis, to see if there is leakage causing hum.

That's a two chassis amp, right? Power supply and main amp chassis?

Could also be a bad solder joint where they plug together... since you say it varies. Try a ground jumper between chassis just in case.

And, of course, did you wire it properly??

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Old 2nd October 2006, 10:16 PM   #3
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Bear. Thx for the tips. Ill try some of them out. See my responses in line below....

Is the orientation of the xfmr the same as the stock?

-Its different.

The secondary voltage(s) the same? <--?

-yes they are

Did you do this on only one amp, or do you have the same effect on two?

-swapped both transformers, both amps are noisy now.

Try swapping the phase of the B+ secondary first.

I presume you are hearing LF hum, or is it a buzz?

-I would say its more of a buzz

It is possible that the old electrolytic simply decided to become leaky.
Add capacitance, or replace via jumpers to test that theory.
Scope on the B+ line will tell you much.

If all that fails, try physically lifting the new xfrmr - electrically insulating - at least temporarilly from the chassis, to see if there is leakage causing hum.

That's a two chassis amp, right? Power supply and main amp chassis?

-yes two monblocks. Power supply and amp on both chasis.

Could also be a bad solder joint where they plug together... since you say it varies. Try a ground jumper between chassis just in case.

And, of course, did you wire it properly??
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Old 7th October 2006, 09:37 PM   #4
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Also if that all fails, try lifting (with the unit OFF and unplugged) the power transformers electrically off the chassis. The assumption here is AC leakage to ground.

Look around for a ground wire that is changed, moved or otherwise not connected, or a component that formerly was present or connected to ground...

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Old 8th October 2006, 01:31 AM   #5
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Could it be that the old tranny had an electrostatic shield but the new one does not?
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