Thomas Vox Repair Boards

[R.G.]

are you anticipating that folks will pull the old parts and then stuff the new boards..., or are you thinking that folks will buy new parts and stuff the new boards with new parts? A combination of both new and old parts perhaps? Thanks.

I was not thinking that pulling the old parts would be a useful thing to do. It's possible, but it's a huge amount of work and you're harvesting parts that are already 50 years old.

The old saying goes that if Necessity is the mother of invention, Laziness is the father. I'm all for doing things the easy way, and it's much easier to put in new parts than to carefully unsolder the old ones.

With one exception, the full suite of new parts is about $30-40 depending on where you buy them. I did the estimate on buying them in 1s from Mouser. The exception is the inductors used for the MRB effect if your amp has that, and the Tone-X tone controls in the bass preamp.

Those inductors are 0.5H, and the simplest replacement is a wah inductor. Of course, wah inductors are in the $10-30 range themselves. So it makes sense to remove the inductors and save some cash. If the inductors show low DC resistance after you pull them out of the old PCB, they're almost certainly good.

The other parts on the old board are of dubious goodness. The electro caps are probably toast. The carbon comp resistors would add a nice vintage hiss to the sound, but I'm not as fond of that hiss as I used to be. The film caps are probably good, but they're HUGE compared to what you can get today for $0.25 each. The old transistors might be good, but they often go hissy too, and using new ones will be an advantage in getting quiet operations. And of course, the wires are what we're trying to get rid of.

That was my line of reasoning, and so I laid out the board for modern parts, which are usually smaller. It's possible to shoe-horn in the old ones, but the cost in time and frustration is high, and the benefit is dubious, in my opinion.

My idea was that you could take the new PCB, fill it with new parts that make the same circuit as the old schematics so it sounds the same, then just clip out the old board and wires and solder the new board in.

[rooster]

Alright, understood. Thank you for the pics of the new vs. old board, btw, I did get them at my email. Thank you! I will spend some time with the pics on a larger screen, I've only been able to open them on my cell phone. Are the boards showing the resistor and cap values? That would be sweet.... Ah, but I have now read your Vox PCB manual/guide and see that all the info is there. Wow, that's some amazing work, RG!!

I do like the sound of those old Philips silver foil caps, but as you say, that's work. Wow, this is QUITE a task you have undertaken, RG!

[R.G.]

The "silver foil" caps are aluminum foil with polystyrene insulation. Good caps, low leakage, low memory, low dielectric absorption. You can still get polystyrene caps, but they're expensive. Most PET/Mylar will be as good in audio circuits. Back then, they couldn't make the polystyrene thin enough consistently enough to make them lower voltage, and hence smaller. They're better today if you can find them.

[rooster]

Roger this on the Silver foil caps, RG. I never knew they were polystyrene! That's a surprise. Hm. Man, I have a friend who's been pulling them from organs over the years - he has hundreds of them in various values. I need to consider a house call. It's crazy to think that they were so cheap and available at the time and now they are expensive.

[R.G.]

I just got back the first build report on a repair board installation in a Berkeley II.

Worked. ( Imagine much screaming of "yeee-hah" here.)

[rooster]

Wow, super cool! You know, RG, the more I think about this 'rebuild' I am blown away by the amount of work this entails. Your love of this project is truly amazing. Frankly I am sitting on the fence about this project for myself at this point.

[rooster]

Oh, BTW, I recently ran into a non-working Thomas Organ wah pedal. As I was stumped as to what was wrong, I googled the subject and - maybe no surprise - a post you had made on some forum came up. You told the owner of that wah that it would most likely be the 4uf cap or perhaps a cold solder joint or a broken trace. You said, 'Resolder every connection on the board.' Well, I followed your advice and sure enough, as I was resoldering the connections on the board I found a trace that had been broken. It was actually one of the .22 polystyrene caps, someone had pushed down on it (with a LOT of pressure) for some reason.

Anyway, RG, thank you - again - for your advice and help!! I say 'again' because you have been a great help to me many times over the years. You Rock, RG.

[R.G.]

Frankly I am sitting on the fence about this project for myself at this point.

You should continue sitting on the fence until any decision is obvious. This is not a fix for minor troubles.

The criteria is "Is this amp headed for the dumpster or being parted out if it doesn't get the insides all replaced?". If yes, think about replacing the guts.

And you're very welcome to any help I can be.

[R.G.]

Got a few pictures of the Berkeley installation in shape to see. Here's a good overview:
[img:816:612]http://i.imgur.com/vte9cZG.jpg[/img]

This one has the original inductor for MRB salvaged and placed in the space for inductors in the upper left.

The original power filter cap was left in place for looks (upper right corner of the chassis) but disconnected. The terminal strip for the rectifiers was used as a tie point for the wires leading to the new board's AC input to the the on-board rectifier and filter caps (right side of the PCB).

I was impressed with the craftsmanship the owner put into building the PCB and getting it wired in. Nice he did work there.