Vox AC30 CC1 repair

[nickb]

I agree with you Something is wrong with your measurements; If there really were 2mV out of the rectifier, there would be no hum.

The first thing to identify is ground. The easiest place for now is barrel of the input jack. Next, make sure you are on a DCV range and probe the rectifier output - the +ve lead of the filter 10uf and 22uF caps is also a handy place.

Is the hum affected by any of the controls? If you remove V3 does it stop the hum? Inspect all the filter caps - any signs of leaks or bulging?

[blapo]

I agree with you Something is wrong with your measurements; If there really were 2mV out of the rectifier, there would be no hum.

The first thing to identify is ground. The easiest place for now is barrel of the input jack. Next, make sure you are on a DCV range and probe the rectifier output - the +ve lead of the filter 10uf and 22uF caps is also a handy place.

Is the hum affected by any of the controls? If you remove V3 does it stop the hum? Inspect all the filter caps - any signs of leaks or bulging?

I'm getting 46mv at C4 22uF condenser.
I'm wondering, is it correct to measure with the chassis as ground?
I've plugged on just the rectifier tube, the others are unplugged.
The hum is not affected by the potentiometers, just by the standby switch

Thanks,
Pablo

[nickb]

The chassis is connected to ground via a 10 ohm resistor in this particular case but not in general. That should be ok .
Are you sure you have your meter set to DCV?
Are you saying you still have hum with no tubes other than the rectifier installed?

[blapo]

Yes, DC. And just with the rectifier tube there's some high pitch humming :/

[nickb]

With just the rectifier is the hum different from what you had before?

Check your meter by measuring the DCV on a 9v battery.
With the amp off, measure the resistance from the rectifier cathode to ground (not chassis). It will take some time to reach a steady state.
Amuse me. Use the input jack as ground, just in case the 10 ohm resistor is open and try again.
Check the value of the HT fuse fitted.

[Randall]

All this probing around the rectifier by someone who is not sure of the basics makes me nervous.

[blapo]

All this probing around the rectifier by someone who is not sure of the basics makes me nervous.

Please don't be, I'm being reallycautious. I didn't want to say anything before because I'm embarrased of how uncertain I am with some things, but I've got a degree in telecommunication electronics. It's just I haven't done any electronics since college.

So yes, I said some dumb stuff like not knowing if the rectifier tube output has to be measured differential or to ground...Or not identifying properly a comfortable pin to measure trafo central tap, but I mean...I'm not gonna get electrocuted, licking capacitors or connecting the trafo output to my nipples



BTW I got a cheap GZ34 for €16, to check if that's the problem. I am also waiting if I can borrow an osciloscope from a friend.

Thanks,
Pablo