Restoring a VOX Beatle Solid State Super Reverb

[loudthud]

yes it does have a 3 prong mains cable. my voltage it 117 volts ac.

That shoots down most of my ideas. These things are kind of tricky to work on. If you can't find a broken ground wire, you might look at the power amp. Does it have significant DC offset on the output (with no speaker connected) ? This is usually an issue with transistor or resistor matching on the output stage.

R.G. Keen is your best resource. Look over everything you can find that he has written.

[johnk_10]

yeah, i'm aware of RG Keen's info on them. i didn't see where you measure the DC offset. i believe that mine has the original output transistors but i'll have to check again.

[R.G.]

The whole Thomas Vox line has lurking AC hum issues. This has several underlying causes, and it's hard to eliminate it all. Someday maybe I'll get there.
The underlying issue seems to be the way the signal grounds are connected. I have had to install isolated input jacks and ground the jacks to the PCB; change the preamp output RCA jack using an isolated RCA jack with 10 ohm connection to the preamp chassis; re-wire the signal grounds to the local bypass caps, and locally bypass the DC indicator lights - their ground return runs down the same wire as the rest of the preamp and inserts ripple offset into the preamp-vs-power-amp grounding. These helped a lot.

For offsets, measure at the speaker output to ground on the "Big Head" amps; the speakers are DC coupled on these amps. In the smaller amps, some use a single polarity power supply and a series speaker capacitor. There is a difference between measuring offset with and without a speaker load. The offset is best thought of as a mismatch in how much current the top vs bottom transistor is trying to pass. If the speaker load, a few ohms of DC resistance, is connected, the offset is pulled to lower levels by the low resistance. Without the speakers' DC resistance, the offset is bigger as the loading is just the bias networks on the output devices. I like to tune the offset in with no speaker load.

DC offset with speakers really needs to be under 50mV-100mV to protect the speakers from overheating.