Thomas Vox repair boards work in the field...

[R.G.]

Any Vox amp fanciers out there?

OK, this is getting to be a reality. I got back enough reports from beta testers on the rebuild boards to have some assurance that someone can build one of these and retrofit it into a busted amplifier and have it work.
See:Repair boards for the following Thomas Organ Vox models have been field tested and work properly
Here's what happens in a Beatle/Guardsman/Buckingham/Viscount:

instead of this:

Another board covers the small combos and Berkeley II.
I feel like a proud pappa. :icon_lol:

I just ordered a 17"x7"x3" Bud chassis base from Mouser to put one of the boards in. I think this will just plug into the effects loop of an amp and give a Vox front end.

On other fronts, a former owner of a Vox UL730 is testing out the preamp section from a UK Vox done on the same kind of PCB. When I get word back from him, it will be possible to do a UL700 preamp front end for other amps.

[olddawg]

Damn that looks like a lot of work!

[J M Fahey]

I guess I'm missing part of the movie.

Are those PCBs you designed and sell, where old VOX amps may be rebuilt?
Not clear here or in the linked page.

If so, very nice and congratulations.

[R.G.]

Those are the ones I designed - but they weren't for sale for anything other than cost. Some very brave beta testers were generous enough to dig in and try them out. What's different is that the boards are now proven to be workable and complete functional replacements in actual amps.

I was especially happy with the top picture - that installation was done entirely from the bare board by a guy in the midwest USA. He was kind enough to send the picture of the completed repair.

[loudthud]

What's the story on the ICs and TO-220s. There weren't any of those parts in the originals.

[R.G.]

What's the story on the ICs and TO-220s. There weren't any of those parts in the originals.

Correct, there weren't any of those on the originals.

The original circuits used resistor-capacitor dropping networks to get approximately +24 and +18V. The actual voltages varied from model to model depending on the current drain, and the quality of the power changed depending on the age and condition of the half-dozen or so big electrolytics on the tagboards under the circuit board. I decided that three-terminal regulators would be a good way to give solid, quiet power to the circuits and not suffer the drift and slow degradation in power quality that R-C decouplers give. They seem to work well and are very quiet. And since the whole power network is on the PCB, it eliminates about a dozen wires carrying power and ground to/from the decoupling caps in the basement.

The two ICs are CD4052 CMOS switches which are set up to implement a 4P3T switch. There is a 4P3T rotary in the originals that has a clot of signal-carrying lines going to/from it. The wiring is (1) confusing to work on, (2) fragile and (3) allows hum and noise to creep in. The two ICs do the exact switching function, but do it on the PCB so the signals don't have to go out to the switch, reducing the opportunity for noise and hum.

And yes, for old-school guys who can't abide any newfangled CMOS switching technology that's only 40 years old, I put pads and jumper spaces on the boards so a die-hard could run the signals to a hard-contact metal 4P3T rotary and leave out all (both) of those ICs if they really wanted to. The original 4P3T rotary switch was prone to broken wafers and dirty contacts. If even one section of the original relay still works, it operates the CMOS switching just fine instead of going off to find another 4P3T rotary to replace the original.

Something that is not all that easily apparent is that the ground on that PCB is a nearly-star configuration for low noise; and you can't break off a ground wire to make it hum-my like the originals.

For the V1141 board (that picture is one, you can tell by the reverb transformer on the chassis), there are only two other differences from the original circuits. The distortion-switching relay used to be on the chassis, with wires going to and from it, and then the signal from those wires being amplified (hugely) by the distortion and amplifier after it. I ditched the chassis-mount relay in favor of a $3.00 PCB-mount relay; that's the orange rectangle on the PCB at about the middle of the right edge. This moves the relay to the signal, eliminating wires, and also eliminates a failure prone component that's not easily replaceable, if it can be replaced at all. The circuitry around the relay is original, just the actual relay changed to miniature and on the PCB.

The other difference on this board from the original V1141 PCB is that I also ditched the MRB relay, which is identical to the distortion relay in the originals. I thought about using another PCB mount relay, but instead I borrowed the JFET switch for the MRB switching from big brother - the V1143 Beatle model. Fewer wires on and off the board, and in this case, a $0.50 JFET in an original circuit - albeit from a later model.

I think those are all the differences. Other than that, if there is a resistor in the original, or a cap, there's a matching-value resistor or cap in this one. Where there was an NPN, there's an NPN in this one that is the device I'd use to replace an original in a repair. Same circuits.

[soundmasterg]

Very cool RG! I was open to testing the 700 series boards but I was busy with school (graduated now) and you were looking for some boards and we never followed through on it I guess. I can still do some testing if you need it, but it sounds like you have that in hand already? I will be wanting to get some 700 series boards for sure when these are ready!!

Greg

[R.G.]

Actually, I've only had the one beta tester for the 700's. They're still in the "needs user-certified" classification. Send me a PM or an email.

[soundmasterg]

Actually, I've only had the one beta tester for the 700's. They're still in the "needs user-certified" classification. Send me a PM or an email.

Will do RG!

Greg

[soundmasterg]

Actually, I've only had the one beta tester for the 700's. They're still in the "needs user-certified" classification. Send me a PM or an email.

I sent you a couple PM's R.G. but no reply yet so I guess you are busy or haven't seen them. I don't have your email. I would love to help you out with beta testing the 700 series! you can reach me at soundmasterg at yahoo dot com. Thanks!

Greg

[R.G.]

Sorry Greg - I've been drowning lately in my real life. I'll contact you.

[soundmasterg]

Sorry Greg - I've been drowning lately in my real life. I'll contact you.

I figured that might be the case but I wanted to check. Thanks R. G. I'll wait to hear from you!

Greg