Lightweight Farfisa F/AR unit for Compact Duo, Transicord

THE F/AR UNIT

The Farfisa F/AR unit is an external PSU for the Transicord electronic accordion and the Compact Duo organ - the former because of lack of space and weight, the latter because the organ was already so heavy. It also provides a transistor preamp and a massive reverb spring, switchable from the Transicord or Compact Duo. These are now rare and expensive, as well as being heavy, awkward and old tech. This project replaces the F/AR with a cheap and lightweight unit incorporating a digital reverb, the popular PT2399 chip.

UNNECCESSARY FUNCTIONS

Some things about the unit are no longer needed. For example, as the output goes to an amp of some kind, there is no real need for volume, treble and bass controls - the amp will have those. Likewise the external signal input jack.

WHAT'S LEFT

Stripped of these, the F/AR provides, on a Hirschmann MEB100 10-pin socket (in Europe) or an octal socket (in the US):

• A 24V DC (unregulated) power line and ground,
• A pair of 9VAC lines which drive the bulbs forming part of the volume and treble controls,
• An audio input line and signal ground,
• A reverb control line connected to the wet reverb signal.

F/AR European pinout:

• Pin Function
• 1 Audio 60mV pp
• 2 GND (connected in common)
• 3 Wet Reverb
• 4 GND (connected in common)
• 5 +DC power
• 6 GND (connected in common)
• 7 9 VAC terminal 1
• 8 9 VAC terminal 2
• 9 GND (connected in common)
• 10 GND (connected in common)

F/AR US pinout:

• Pin Function
• 1 9 VAC terminal 1
• 2 GND
• 3 Audio 60mV pp
• 4 +DC power
• 5 Wet Reverb
• 6 --
• 7 9 VAC terminal 2

There's a lot of useful background information on the ozvalveamps site, which I acknowledge as the basis of this project.

A REALLY SIMPLE PSU

The organ and accordion each contain a 9V Zener diode on the input, stabilising the supply voltage. So any stabilised voltage above 9VDC ought to do the job, provided it contains a current-limiting resistor - the Ozvalveamps site says that a 12R 0.5W resistor is correct to drop 3V. I used a 5W resistor I happened to have around.

On the other hand, the AC voltage supply goes to bulbs intended to be driven at 9VAC but rated at 12V and, of course, they will take either AC or DC. So a single DC power source supplying between 9-12VDC can be used to replace all four power supply lines from the F/AR, as demonstrated in this excellent and helpful video from "organ69". I used a cheap-as-chips 12VDC wall wart.

I added a red LED and current-limiting resistor just to show when the device was switched on.

A REPLACEMENT REVERB AND PREAMP

There's a really neat little reverb chip called the PT2399, analysed here. It gives you a (controllable) delay time of between 10 and 300-odd mS, allowing you to emulate reverb or echo, and is often used in cheapo karaoke machines, for which many different PCBs are available from eBay, Alibaba and so on. For this project, I had two requirements for the PCB:

• It has to run off a single-sided 12VDC power supply (many PCBs either want a bipolar +/-12V source or an AC source),
• It has to include a preamp, so as to accept instrument input levels (some PCBs operate at line level only), and
• The reverb level has to be accessible (some PCBs only let you control the reverb time).

Beyond this, many PCBs also include microphone input jacks, and stereo input and outputs, neither of which we need.

The one I selected is branded YX-203, cost £6.35 from eBay.

It provides a reverb level potentiometer, and allows you to fit another 20k-50k pot for reverb time, if you remove the R27 SM resistor, which I did.

Surprisingly, however, it didn't work out of the box (bag, in fact), and a Google search revealed why - this clever hobbyist spotted that R24 and C22 had been soldered the wrong way around. It is a bit of a pig of a job, but I managed to de-solder these and swap them, using my normal soldering iron and a Stanley knife blade to lever the chips up. After that it worked fine.

To connect to the board you need 3x 0.1" spaced 2 pin connector cables, and an MES-100 plug (or octal equivalent if you have a US-type Farfisa connector). And electrically, that's it.

My original plan was to connect the reverb "wet" wire from the connector to the but I couldn't find a suitable point on the PCB to access just the "wet" reverb signal connection point. That wire is connected, in the organ or accordion, to the REVERB ON/OFF switch, which selectively drops it to ground, removing the wet signal.

BOXING IT ALL UP

I bought a "Split Aluminum Alloy Enclosure, Metal Surface Bright Electric Project Enclosure Power Junction Box Electricity Box for Power Supply for Electrical Control" for £4.77 from Amazon. The PCB almost fits neatly in sideways - I cut off the side ridges with a Dremel and it went in perfectly. I drilled holes for an (old) jack socket, the LED, the 12V power barrel jack, the instrument cable and the two control pots. Job done. Demo on Youtube.