More Thomas Vox Repair Info

[R.G.]

I just got some more of the Thomas Vox repair books on line. They're at TheBookPatch.com, and/or thomas-vox-repair.com.

The list now includes:
- Vox Owner's Safety Net
- Vox Footswitches
- V1141 (Super Beatle) Repair Supplement
- V1181 (Westminister 60W) Repair Supplement
- V117 (Sovereign) Repair Supplement

I put them up as I get time to finish off the similar/different from amp to amp in print format.

[MWaldorf]

R.G. - would the V1141 repair doc cover the V1121 and V1131 as well? Mel

[R.G.]

Mostly. The way I got started down this road was noticing that the schemos for the various "Big Head" amps were nearly identical. The preamps for the V1121 and V1131 are built on the same basic circuit board(s) as the V1141. The part values are identical for the vast majority of the parts; some part values may be different, and Thomas would sometimes renumber the parts. So there are differences, but largely trivial ones, in the preamps.

The power amps are similar in design, but differ much more than the difference between the V1141 and V1131. The V1121 (and V1151) are the odd ones, as these use germanium outputs and fewer gain stages in the power amp chassis. So I guess the answer is yes, but see the 112 footnotes.

I have the common base for all of them down as the bones of each supplement, and the work on each supplement is to take the factory schemos, parts lists, and field upgrades or things I've noticed per model, and make each supplement have only the info for that model.

I do have the V1121/V1151 and V1131 near readiness, in case you have one to fix. I'd advise not trying to cross-bridge from the V1141, but wait for the actual model you have. Could save you some time.

The Sovereign V117 and V1181 came out first because I had a guy who was frenetically trying to get a Sovereign to work - and the V1181 basis for the Sovereign only had its field service changes internalized to make the Sovereign. So if you're needing the '21 or '31 - which one first?

[MWaldorf]

Gotcha. I've got a V1121 that's gotten noisy on the normal and tone-x channels over the course of a few months. My guess is dying electrolytic caps. If you've got a V1121 guide coming down the pike, I'll wait. In the meantime I've got my V1032 to finish up!

[R.G.]

What kind of noisy? Hiss? Pot crackle? Got a 'scope picture? Is the noise different in one channel versus the other?

Noisy channels have only a few causes. One is an input transistor or two going hissy. Another is a bum electro doing noisy leakage. Some varieties of RF oscillation get "detected" back down into audio as an angry-sounding hiss. Pretty much all the V11n1 amps have common circuits for the preamps.

I'll try to speed up the V1121. The V1131 is actually closest to done, but neither is going to take a long time.

[MWaldorf]

The noisy is hiss, not crackle. It started on the normal channel, then on tone-x. The tone of the signal isn't affected - no distortion, just background hiss. The brilliant channel doesn't seem to be affected. I haven't opened the amp up yet as I want to get the CRs done first. Oh, and a guy I know had a Series 90 V133/V130 that needs a little love - when's the book on those going to be ready?

[drewl]

The last one I worked on had hiss from a noisy transistor.

It was the final drive one before the transformer so it was present on all channels

[R.G.]

The noisy is hiss, not crackle. It started on the normal channel, then on tone-x. The tone of the signal isn't affected - no distortion, just background hiss. The brilliant channel doesn't seem to be affected.

As drewl says, that's likely to be a transistor going hissy, one per channel.

Could be a cap going leaky, expecially if the cap is an electro that's feeding from stage to stage, not a decoupling cap. The thing to do is when you open it up, replace all the caps (wait... did I say that again???) then if the hiss is still there, start tracking hiss.

In this amp, with one channel going bad at a time, you have the advantage of some isolation. The ability to think about which circuit connects to which others in what order was one point of those chapters on "architecture" and "blocks" in the Vox Owner's Safety Net. A fault which affects one channel/function only nearly has to be in that one circuit block. This is something that experienced techs learn and apply automatically. You're looking for something that's in one channel, but not the others. And remember, it may be two different somethings that have the same external appearance.

Running down hiss can get tricky, because you need a way to make it start or stop to know where it's being generated. The first Beatle I ever owned had horrible hiss, and it took me a long time to find it. It was everywhere after the input preamps, by oscilloscope. I finally hooked up a 100uF 35V electro cap with some clip leads on the ends, and clipped the (-) to ground. Then I would turn the amp off, move the (+) lead to some place in the signal chain where the cap would short the signal to ground, and turn the amp back on. If the hiss disappeared, I had just shorted the signal to ground somewhere after the hiss source. Turned out to be a hissy resistor in the reverb circuit.

Oh, and a guy I know had a Series 90 V133/V130 that needs a little love - when's the book on those going to be ready?

That may take a bit longer.

[MWaldorf]

Thanks guys. I think I read something about replacing all the electrolytics someplace, maybe once or twice.

I'll wait for the V1121 manual before tearing into mine. In the meantime I've ordered the footswitch manual - I want to add lights to the footswitch for my V1032.

[R.G.]

That will need you to bring out a source of power for the lights. Probably best to bring it out on the same pin the bigger amps use for power. IIRC the effect enables are on the same numbered pins for the Cambridge as the Big Heads. You might also want to put in an alternate action switch for the MRB while you're replacing the SP switches for DP switches in the footswitch housing.

[R.G.]

... and yes, I may have repeated myself on that electrolytics stuff.

[MWaldorf]

Thanks R.G. I know I need to bring a power source to the pedal. I'm not sure how much voltage (the V1032 has 31V, 27V and 17V sources) and what resistors to go with LEDs vs the vintage style bulbs. I've got DPDT on/off switches at least.

[R.G.]

A lot depends on what lights you use. Thomas used indicator assemblies with 24v or 28V incandescent lamps in therm. They powered these with 24V when they were first introduced, but went to -31V for all but the very first ones. Thomas put a 300 or 330 ohm resistor in series with the -31 to the footpedals. The incandescent lights then did their own resisting as well. While units similar to these can still be found, they're not common.

I think that Thomas picked indicators to use the power supply voltage with the most available current without upsetting the resistor/capacitor chain currents they already had. That means the highest-current ones available, and that devolved to either +31 or -31. I think they picked -31 to balance the loading a bit and not change voltages on the supply chain in the preamp when indicators came on.

Today, the decision tree revolves on two questions: (1) original looking bezels or not and (2) incandescent or LED. LED is not going to need replacement. But the incandescents Thomas used worked for a very long time because they were worked very much below their designed brightness. You can still get original-ish looking indicators. Mouser stocks panel mount indicators from VCC in LED, neon, and incandescent. The nearest to the original I can find is the 1090C series, 28V incandescents. These pull 40ma per bulb at nominal voltage.

And the 40ma per bulb leads to some considerations to be taken. With three bulbs in the 1031/1032, you're going to need 120ma. That's a lot for any of the outputs other than the +31, so I'd suggest first using the +31. LEDs are generally lower current devices anyway, so you can probably go to 10ma to 20ma LEDs per indicator for a similar look.

[52 Bill]

Mel- Do you have an original TO footswitch that you are adding lights to or is it something different?

[MWaldorf]

It's a Marshall knock off pedal with three switches and three leds, so nothing sacred. I've got an original TO footswitch also, but I don't want to mess with that.