Silverface Champ LOUD Hum/Buzz

[Pryde]

Hello folks

I have a silverface champ AA764 amp, schematic:

http://ampwares.com/schematics/champ_aa764.pdf

On start up there is extremely loud hum/buzz even with volume on 0 and nothing plugged in. Volume control does effect the sound when turned. Pulled preamp tube and hum remains. Hum goes away when power tube is removed.

-Tried different speaker
-Chopticking all connections does not effect anything.
-Checked all ground connections, no problems.
-Cleaned/tensioned pins, jacks, pots
-Swapped all tubes (rectifier, power, and preamp) and hum/buzz remains with each new tube.
-Checked all pin voltages on tubes per schematic and all are in spec (used dummy load to avoid noise).

The amp is all original with no mods. It has the 20/20/20 can cap and original coupling caps. Would a bad filter can produce extremely loud buzz/hum like this? I have no problem doing a cap job but want to rule out other possibilities.

Thanks for the help!

[Mick Bailey]

There's a fair chance its the multisection cap. May be something else, but I'd check this first. If its on the way out you'll get very high ripple right through the supply nodes into the preamp. That's why the volume control affects it too. Set your meter to AC and take a reading off the 20uf sections - post back the results. Being an SE amp it's far more sensitive to supply ripple than PP.

If you can measure ESR you may find the can sections going high resistance.

[Pryde]

There's a fair chance its the multisection cap. May be something else, but I'd check this first. If its on the way out you'll get very high ripple right through the supply nodes into the preamp. That's why the volume control affects it too. Set your meter to AC and take a reading off the 20uf sections - post back the results. Being an SE amp it's far more sensitive to supply ripple than PP.

If you can measure ESR you may find the can sections going high resistance.

OK I measured each node of the can cap and got ~55v AC on each one. I then measured preamp tube pin 1 and got 14v AC on it. Can I assume the can cap is toast letting that much AC through?

I have no way of measuring ESR unfortunately

Thanks for the replies

[eschertron]

OK I measured each node of the can cap and got ~55v AC on each one. I then measured preamp tube pin 1 and got 14v AC on it. Can I assume the can cap is toast letting that much AC through?

I have no way of measuring ESR unfortunately

Thanks for the replies

I have a 78 champ, and was just measuring ripple this weekend (a little preventative maintenance check). I got about 2vac on the first node, 115mvac or so on the second, and <1mvac (all RMS values) on the preamp node.
If you have some big electrolytics, clip one in parallel to the first node (observing voltage magnitude and polarity, of course) and see if your hum is reduced. I think you're on the right track.

[Pryde]

I have a 78 champ, and was just measuring ripple this weekend (a little preventative maintenance check). I got about 2vac on the first node, 115mvac or so on the second, and <1mvac (all RMS values) on the preamp node.
If you have some big electrolytics, clip one in parallel to the first node (observing voltage magnitude and polarity, of course) and see if your hum is reduced. I think you're on the right track.

Wow that's way less then what I am getting. the only high voltage cap I have is an 80uf. Can I test with it just so I know I am on the right track or is it to much for the rectifier tube?

BTW: the can cap is actually a 40/20/20. The schem calls out a 20/20/20.

[Jazz P Bass]

You can use the 80uf as long as the voltage rating is high enough.
This is only a brief test.
You will not need to play the amp.
Parallel the 80uf. If the ripple drops dramatically, then that cap is bad.

My lunch money is on 'bad cap can'.

[Mick Bailey]

I have no way of measuring ESR unfortunately

With 14v ripple on the preamp that kind of clinches it. Not exactly marginal!

[Pryde]

OK I clipped in a parallel filter cap on the first node and it dropped the AC to about 1.5v so that confirms the bad can.

It looks like my supplier has a 20ufx4 can. I can link 2-20uf tabs together and get the 40/20/20 I need from it.

[Enzo]

If it were mine, I'd clip off the terminals of the can cap and leave it there for appearance, then I;d mount three individual 20uf 450vf or 500v caps under the chassis wired to the points the can cap was wired to. Looks like a row of three connections at the end of the board. Ground them where the center taps are grounded.

Lots cheaper than a new can cap.

[Pryde]

If it were mine, I'd clip off the terminals of the can cap and leave it there for appearance, then I;d mount three individual 20uf 450vf or 500v caps under the chassis wired to the points the can cap was wired to. Looks like a row of three connections at the end of the board. Ground them where the center taps are grounded.

Lots cheaper than a new can cap.

I would go this route if it were mine but I know the owner wants it as original as possible inside and out so new can cap it is.