Vox Buckingham distortion

[Haldy]

In a effort to rehab this amp, the electrolytic caps have been replaced in the power section and the preamp section. Several issues remain. The most annoying one is this: with the preamp output cable only connected by its tip the amp sounds ok, but there is a bit of a hum which is constant and unchanging regardless of volume setting. If the preamp output cable is fully seated, the hum goes away but so does the volume at the speaker and the sound is distorted (garbled). Any suggestions?

[R.G.]

In a effort to rehab this amp, the electrolytic caps have been replaced in the power section and the preamp section. Several issues remain. The most annoying one is this: with the preamp output cable only connected by its tip the amp sounds ok, but there is a bit of a hum which is constant and unchanging regardless of volume setting. If the preamp output cable is fully seated, the hum goes away but so does the volume at the speaker and the sound is distorted (garbled). Any suggestions?

The grounding on these things is very tricky. They had to work very hard to keep hum out of it, and the grounding reflects that.

It is most likely that either the ground in the 9-pin connector between the preamp and power amp chassis has one of its grounds not making contact, or that one of the caps in the limiter circuit is put in reversed, or the input cap to the power amp is reversed, or some of all of these.

I just got through re-capping a Viscount chassis; same circuits, different cabinet.

Which model?

[Haldy]

Vox Buckingham V1121, sn 1049517. The amp head was in rough condition(dropped or knocked over). Didn't work at all. At one point during rehab most of it was working except for the reverb. Working without a diagram; and it appears others may have had a go at getting it working before me. Replaced caps with same types and sizes as installed. In an earlier post, we determined the reverb tank was replaced with an incorrect unit. The first work done was a rehab on the power section which seemed to work well after test (except for hum). The preamp section has been frustrating. A lot of contact cleaning and of course the recapping. I'll look at he grounding issue you wrote about and repost.

[R.G.]

That's a big project. It's good that you're a determined kind of guy.

The power unit should not hum by itself. The way I test these in isolation is to do something that I recommend no one else do - it's too dangerous. I temporarily solder a power cord to the actually AC contacts inside the power unit, on the terminal strip under the filter caps and transformer end. This makes any work on the amp as dangerous as petting a rattlesnake on your workbench. But it eliminates the funny way ground goes to the preamp unit and lets me test the power amp truly in isolation.

The Thomas Organ schematics do not show you the ground wiring between the two units. They just note the ground symbol wherever it was handy for the drawing. This is a problem because Thomas did a good job on the grounding, at least as good as they could. The speaker return, for instance, is floated off the chassis and goes right back to the power cap junction. This keeps speaker return off signal ground and prevents oscillation. The shielded cable from preamp to power unit carries signal ground to the power amp circuit.

For reference, do this quick test: clip your voltmeter (-) lead to the preamp chassis, then with the (+) lead, test each pot lead for DC. NONE of the pots has DC on it except the tremolo balance and limiter pots. Any DC means that one of the replacement capacitors is inserted backwards.

If the unit has had mechanical abuse and earlier tinkering, it is very likely that one or more wires are broken inside the preamp. This can be slow work, but it is possible to track down the broken wires.

Contact me by personal message with your email address.

[Haldy]

Thanks, R.G. I'll try the tests you recommended. I did find three broken wires in the preamp section so far. Been testing with a signal into the input jack and tracing with an o-scope. One was just "hanging" near the reverb channel selector switch and I took my best guess at where that one looked like it belonged. I'll spend some more time on it soon and repost in a couple of days.