Reverse Engineering a Farfisa F/AR Power Supply/Reverb Unit

[Rhodesplyr]

I recently made the dubious decision to buy a Farfisa Compact Duo in non-working condition for restoration. I'm a keyboard tech, but I'd never worked on or played a Farfisa, so this was part curiosity. The price was right at $150. That may seem like a deal, but when you consider the hours it took to get the thing working again, it evens out. To add to the fun, this is the first version of the Compact Duo, and no complete schematic is known to exist for it. (And, yes, I'm a member of the Yahoo Combo Organ group.) Though they look a lot alike, the two versions are fairly different under the hood, much more so than, say a Hammond B-2 and B-3.

In an apparent attempt to get people to buy their amplifiers--and perhaps to save weight (or copy the Hammond BV), the Compact Duo gets its power supply via an umbilical connector from either a Farfisa combo amp or an outboard power supply, the F/AR, that also contains the final line out stage and spring reverb. The problem is that these sometimes get separated from the organ, which is the case with mine. Jerry-rigging a power supply isn't difficult, but I'd like to have the sound as vintage-original as possible.

But the only lead I've found on an F/AR unit is one someone wants $200 for--one not positively known to be in working order and with part of the case replaced with a piece of sheet metal. This sounds like the point of diminishing returns to me.

The basic circuit of the F/AR is not hard to copy, and Accutronics has provided me with the specs of the original spring reverb tank. The only thing I can't get specs on is the interstage transformer Farfisa used to drive a push-pull transistor pair that, in turn, drives the reverb tank. The only other place I've seen this circuit used is in the power modules of my 80W Peterson design Rhodes Suitcase Piano--both of which have self-destructed and required new power transistors over the years.

That's prompted me to take a close look at the reverb driver circuit and to notice how lossy Farfisa's early (1965) transistor design is. The signal is supposed to be 350mV at the signal take-off point, but with 82k and 5.6k resistors biasing the base of TR3 that appear in parallel to ground to the incoming AC signal, the signal is down to 25mV at the base of TR3. It occurs to me that there might be better ways to get a 1V p-p drive signal to the reverb tank, while keeping the rest of the circuit stock.

So, I'm thinking about, perhaps, since I mainly work with and on tube gear, a single triode or twin triode (copying Fender) through a matching transformer into the reverb tank. With the large grid resistors possible with a tube, it would appear almost invisible in comparison to the load the original reverb driver puts on the line stage, though I could maintain the appearance of the original load to keep frequency response, gain, etc.... stock. The only downside would seem to be the necessity of a small high voltage B+ supply.

I'm just looking for any feedback here. I suppose it could also be done with a JFET based IC, but I have less experience designing with those.

David

[loudthud]

I would think something like LM384 would get the job done. Any Power amp IC that can handle the 24V supply.

[olddawg]

Originally Posted by loudthud

I would think something like LM384 would get the job done. Any Power amp IC that can handle the 24V supply.

+1 As long as you filter and regulate the power supply sufficiently it should be better and cleaner than the original setup. It would get it usable and up and running for very little time and money. Then you could take your time to look for the vintage piece.

[farfisa]

Hi David,

I don't know if this will help, but I have relaunched farfisa.org after years of neglect and one of the things I've dug up so far are some of the schematics I have.

Farfisa Compact Duo Schematics Farfisa.org | Farfisa Combo Organ Museum

Good Luck!
Jim

[Rhodesplyr]

Originally Posted by farfisa

Hi David,

I don't know if this will help, but I have relaunched farfisa.org after years of neglect and one of the things I've dug up so far are some of the schematics I have.

Farfisa Compact Duo Schematics Farfisa.org | Farfisa Combo Organ Museum

Good Luck!
Jim

Hi Jim,

In fact, I'd already downloaded those schematics from your site. They helped, but the first version of the Duo is actually quite different from the later version, which is what you have listed. No complete schematic for the early version is known to exist. I'd wager that it does exist--somewhere--but no one has made it available, for free or for sale.

The basic circuit of the F/AR is not hard to copy, and I think the LM384 IC will work fine as a reverb driver. The most difficult part is just finding a box to put it all in and trying to make it not look too homemade.

David

[farfisa]

Hi David,

That's great! Good luck with the build, I've seen it talked about over the years. When you finish and want some press written about it, feel free to get in touch and I'll write up a blog about it.

Jim

[Rhodesplyr]

I've got one more question on the F/AR schematic and wanted to see what others thought.

In terms of the control pots, the Reverb Volume, Bass, and overall Volume pots are marked 'A' on the schematic, but the Treble pot is marked 'B.' I'm assuming the 'A' pots are audio taper, but I'm not sure about the 'B.'

Considering that this schematic was drawn in 1966 based on an Italian design but with writing in English, do you think that 'B' means linear taper or reverse log/audio taper?

There seem to be at least three different code conventions for potentiometer tapers all using the same letters. Totally insane....

David