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PostPosted: 13 Oct 2017, 03:00 
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MrJackson wrote:
C4 is not electrolytic, but polypropilene, if I’m not mistaken. I’ll check tonight.


Yes C4 is a polly cap its across an electrolytic, I think its to reduce the ESR of the electrolytic cap (to increase frequency response).
I assume the electrolytics the right way round? (on your PCB)

Link from your share point:
https://photos.google.com/share/AF1QipN ... 1EaHJVOVFn


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M. Gregg

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PostPosted: 13 Oct 2017, 03:17 
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I’m pretty sure C5’s the right way round. It would’ve popped otherwise, I think.


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PostPosted: 13 Oct 2017, 03:20 
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MrJackson wrote:
I’m pretty sure C5’s the right way round. It would’ve popped otherwise, I think.


The reason I wondered if C4 had another function is because the ACF seems to use the supply ripple injected into the bottom triode grid.
But just try removing them. If I was doing this I would have put a grid stopper on the bottom triode.
However the circuit doesn't seem to use one.

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PostPosted: 13 Oct 2017, 03:27 
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Something you could try is to add some loading across the electrolytic like 470K. 500V.

It could be the supply is ringing.
But we are drifting into other situations. (Design)

However that's not really the point it should work as designed.
You could do with some input from others things to try ect.
Rather than go off on a tangent looking at design problems.

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M. Gregg

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PostPosted: 14 Oct 2017, 05:44 
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I've shorted the inputs on the board... still humming - that takes the input wiring/switch out of the equation.

Had a good look at the caps, they're not buzzing, but I'm almost sure it's the power supply diodes that are emitting the darn buzz. I feel I'm getting somewhere though.

What would cause the diodes to buzz?


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PostPosted: 14 Oct 2017, 06:40 
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Finite recovery time. That's why most of us use fast-recovery types in designs.

Put a 10n disc cap across each one and see if it goes away. That's the usual cure for standard rectifiers.

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PostPosted: 14 Oct 2017, 07:00 
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There’s a 10nF cap across each diode already.


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PostPosted: 14 Oct 2017, 07:37 
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Your absolutely sure the mechanical noise isn't the end caps/covers on the power transformer?

Edcor power transformers have a tendency for the metal covers to vibrate with the magnetic field.
Just gently push on each side of the power transformer with a finger. If it stops then you know.

If it is the power Tx you can remove the end caps and put a filling of either silicone sealant or rubber glue or sorbathane inside the end bell about 3mm thick. let it dry and reassemble. (just inside the cover not the flanges that mate to the laminations). Watch you don't lose the insulator grommet's on the laminations or you will get a shorted turn after re-assembly.
Don't fill the end bell, the silicone should not touch the transformer coils.

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PostPosted: 14 Oct 2017, 07:56 
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The noise was definitely coming from the PCB. The trafo was almost 10cm away, resting on a box, and the noise was getting lower as I got closer to it.

I will try to gently squeeze the bell covers.

Also, there's another guy over at diyAudio who reported the same simptoms as mine, on an ACF-2 octal. He also had the 9 pin version and that was ok.


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PostPosted: 14 Oct 2017, 08:29 
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Just curious.
Does the transformer have an electrostatic shield?

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