Thomas Vox repair boards work in the field...

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R.G.
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Thomas Vox repair boards work in the field...

Post by 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:http://geofex.com/Article_Folders/Repai ... 20Amps.htm
Here's what happens in a Beatle/Guardsman/Buckingham/Viscount:
[img:1082:487]http://geofex.com/images/Buckingham%20U ... hassis.jpg[/img]
instead of this:
[img:1082:468]http://geofex.com/images/Viscount%20Bd% ... 150dpi.jpg[/img]
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.
Mark
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Re: Thomas Vox repair boards work in the field...

Post by Mark »

Nice work R.G. unfortunately those old phenolic boards can crack which is a major hassle to repair. Was this your motivation for coming up with these boards?
Yours Sincerely

Mark Abbott
R.G.
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Re: Thomas Vox repair boards work in the field...

Post by R.G. »

It was sure a big part of it.

The overall intention was to keep Vox amps out of dumpsters. At the moment, I only know of four techs in the USA that will take in a Thomas Organ Vox amp for repair, although I'm sure there are more that I just haven't heard of. What usually happens is that an owner has a problem with their amp, tries to get a tech to fix it, and is told "Sorry, I don't work on those." They get dumped, stored for a day that never comes, or dismembered and parted out.

The proximate cause was exactly that. A fellow with a Royal Guardsman that had a broken board contacted me and pleaded for help.
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M Fowler
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Re: Thomas Vox repair boards work in the field...

Post by M Fowler »

I know of two techs in my area that work on these Vox amps but I won't :)
R.G.
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Re: Thomas Vox repair boards work in the field...

Post by R.G. »

M Fowler wrote:I know of two techs in my area that work on these Vox amps but I won't :)
One of the things I tried to do once was to just get a list of techs that *would* work on them, so I'd have someone to refer people to.

First - would you PM me or email me with those two tech's names? I'd love to refer people to them.

Second - I'd love to convince YOU that you can make one of them work by clipping off all the wires and wiring up a new whole board in them. :D
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rooster
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Re: Thomas Vox repair boards work in the field...

Post by rooster »

RG, I still marvel at your effort here. The new board looks fantastic. 8)
Most people stall out when fixing a mistake that they've made. Why?
Mark
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Re: Thomas Vox repair boards work in the field...

Post by Mark »

R.G. I noticed your version has op-amps in it, while I'm sure no one notices the difference, what was the motivation to use op-amps?

Was it real estate or were the transistors used leaky germanium types which are impossible to find?
Yours Sincerely

Mark Abbott
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M Fowler
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Re: Thomas Vox repair boards work in the field...

Post by M Fowler »

RG I sent you a PM with the info.
R.G.
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Re: Thomas Vox repair boards work in the field...

Post by R.G. »

Mark wrote:R.G. I noticed your version has op-amps in it, while I'm sure no one notices the difference, what was the motivation to use op-amps?

Was it real estate or were the transistors used leaky germanium types which are impossible to find?
Actually, none of the above. :lol:

There are no opamps in it. The circuits are the same, resistor for resistor, cap for cap, transistor for transistor with some very minor exceptions:

- No self respecting power supply designer would make a power supply like the originals today. Instead of using dropping resistors and capacitors to get different voltages, I dropped in three-terminal regulators. This let me put the whole power supply for the preamp board actually on the preamp board, not on wires to the tag strips in the "basement" under the PCB. Fewer wires, lower noise, more reliable performance.

That's what those TO-220 packages are.

- The two 16 pin dips you see are not opamps, they're CMOS analog switches. The originals have a 4P3T rotary switch used to select which channel gets the reverb. That's at least six wires onto/off the PCB all carrying signal and therefore prone to noise, hum, and breaking. I replicated the switching, but using CD4052 switch chips, on the PCB itself.

The rotary switch now only switches the control signal to the CMOS chips, which do the same switching the old rotary used to do. I know there are purists who will say that this causes an unacceptable loss of "tone" :lol:, so I put two extra pads and jumper locations on the PCB so die-hards can leave off the analog switches and do it the old (hard!!) way.

The rotary switches in the original are prone to bad contacts, and although they can be replaced, the CMOS setup only requires one of the four sections work to reused the old switch mechanism.

The other change from the original on that PCB is in the relays. The original V1141 and it's smaller brothers used two relays. One bypasses/enables distortion, the other does MRB. The relays have the same issues as the reverb switching - analog signals go off the PCB somewhere else on the chassis, and raise issues of hum pickup, noise and breakage. I put a $3.00 micro relay on the PCB for the distortion switching; it's the orange rectangle on the left edge of the main PCB.

I also adopted the JFET switching from the next-generation V1143 to switch the MRB effect, replacing the off-board relay. The relays are no longer available; replacements could be found, but would be more expensive. So I sidestepped the relay issue.

The fundamental design objectives for this board were:
1. Have it mount exactly like the original.
2. Declare war on wires: any wire that did not absolutely have to be there was removed. The wires were the problem with the originals.
3. What wires had to be there would all come from one edge, eliminating the Gulliver-in-Lilliput tie-down of the board with wires from everywhere.
4. Wires would go from the board to the control/jack/switch and be as short and direct as possible. Ideally, there would be no wires crossing. For all the pots, for instance, there are three wires from PCB to pot, and they are always in the order CW/wiper/CCW on the PCB edge.
5. Wire paths were set up so that, unlike the original, the PCB can be rotated up 90 degrees so you can get at the solder side and component side for repairs without breaking wires in laced bundles.
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JazzGuitarGimp
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Re: Thomas Vox repair boards work in the field...

Post by JazzGuitarGimp »

What are the two cream-colored, squarish-with-a-doughnut doomafloppers that are cocked up at an angle?
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davidjon_99
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Re: Thomas Vox repair boards work in the field...

Post by davidjon_99 »

JazzGuitarGimp wrote:What are the two cream-colored, squarish-with-a-doughnut doomafloppers that are cocked up at an angle?
I was going to ask the same thing. They look like something you'd put in a smore.
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dorrisant
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Re: Thomas Vox repair boards work in the field...

Post by dorrisant »

Great to see these staying alive. I didn't realize that most were getting trashed. Makes sense now.

Working Vox Organs = Smiling customers.
"Education is what you're left with after you have forgotten what you have learned" - Enzo
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rooster
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Re: Thomas Vox repair boards work in the field...

Post by rooster »

Damn, RG, you've actually created a reissue amp that Vox (Thomas Organ) walked away from. I love the work you've done, I'm amazed at your depth on the subject, and now I need to hear it. YouTube?
Most people stall out when fixing a mistake that they've made. Why?
R.G.
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Re: Thomas Vox repair boards work in the field...

Post by R.G. »

JazzGuitarGimp wrote:What are the two cream-colored, squarish-with-a-doughnut doomafloppers that are cocked up at an angle?
Those are the original Thomas Organ 0.5H toroidal inductors, salvaged from the original board for the retrofit. Thomas made these in a couple of different epoxy colors.

The oddity of their orientation is that I worried about the availability of 0.5H toroids, so I put space on the PCB to drop in reissue wah inductors, which happen to be about the same value, and (relatively) widely available. The hole patterns will take any wah inductor, and the originals on that funny slant.
... and now I need to hear it. YouTube?
The first beta tester owns a Royal Guardsman, and is working on a youtube video of his original versus the new board setup. He's also adept at doing videos for youtube, which I'm not. :D It's coming.
Mark
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Re: Thomas Vox repair boards work in the field...

Post by Mark »

5. Wire paths were set up so that, unlike the original, the PCB can be rotated up 90 degrees so you can get at the solder side and component side for repairs without breaking wires in laced bundles.
The most forgotten feature. Nice touch R.G.
Yours Sincerely

Mark Abbott
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