Farfisa "Compact"

The Farfisa "Compact" organ line was manufactured from 1964 to 1970, and there were many variations. Their unique sound stems from the way the transistor divider circuits act on the high frequency "square" wave oscillator, resulting in narrow, rounded, negative-going pulse waves.
The model I have here is probably from around 1966 (hard to say for sure as the serial numbers aren't consistent). Although they made a big deal about it's "electronic transistor"  circuits, it also contains two twin-triode valves. Of the four valve stages available, one is for the preamp, two for the reverb driver and one for the reverb recovery.
To service one of these beasts, the legs need to be in the open position, to allow access to the 5 wing nuts in the wooden base, so that the internal frame that holds the keys and the electronics can be lifted up out of the flightcase.

!!!Now's the time to re-check you unplugged it from the mains!!!

The trick here is to find the four screws holding down the PSU chassis, with it's gigantic transformer  and two valves sitting atop. Just loosen these enough to allow them to slide in their slots, moving the PSU towards the front of the machine so that the pots on the rear side clear the body cut-out. Then you can lift the internal frame up and rest it on the rear edge of the case, allowing access to the voice boards and (sorta) the key contacts. You can't really put this thing on your bench unless you do a much more comprehensive  dismantle, so you'll need plenty of room to get it as close as possible to an area you can work off.

Above: with the internal frame resting on the rear edge of the case, you can see the 3 groups of 4 voice boards ("master oscillators"), one for each of the twelve notes, and the filter boards to the right of these.

Above: the large transformer and the two ECC83 valves sitting atop the PSU chassis, on the left with the bass (pedal) trimpots board just in front of this.

This baby had been 'in the family' since the late seventies, and had given trouble-free service all that time. However now it wasn't making any sound except buzzing. Inspection immediately revealed a few electrolytic capacitors had given up the ghost, and lifting up the PSU revealed the charred remains of what I think was a spark suppression capacitor near the power switch.

Replacing these, and a few other electros in the PSU, and we had organ sounds again, but they weren't quite right...

When diagnosing faults with the sound of the organ voices, it helps to isolate the oscillator divisions. Start with all tabs off, except one of the 4' voices. Play all the notes chromatically.

There might be an obvious crackle suggesting the contacts need cleaning. Or, there might be a dead key altogether. Is it the contact, or the voice electronics? Switch the 4' tab off and the 8' tab on. Did the dead key stay dead, or did it just move up an octave? If it moved, it could be that specific divider output on the master oscillator boards for that note. I had a dead "A" that moved with the footage selection. This was traced to the 1uF electrolytic cap at the output of that division, on the master oscillator board for "A". Each division of each of the twelve notes has such a cap on the output. 

Shortly after fixing that, all my "D" notes went wonky, sounded horrible across all octaves. Getting the scope onto the boards I found the shape of ALL of the D divisions looked like this

Whereas the shape of the healthy notes looked like this

The top frequency of the D oscillator, the square wave from which all the lower notes are divided, looked OK, much like the other notes. So whatever was wrong, it had to affect ALL the divider circuits. I couldn't see, from looking at the schematic of the master oscillator board, what that one component could be. 

But inspecting the voice board itself revealed a cap that wasn't marked on the schematic, a 50uF 12 volt electrolytic that was bypassing the 8 volt rail (top line of the schematic above). This rail is a common connection to ALL the emitters of the divider transistors. Ah ha! Sure enough, replacing this fixed the sound, and the pretty pulse waves were back again.

Above: the "D" oscillator board. The blue electro was the unmarked cap.

Key contacts will likely need cleaning. Each key (except for the bass octave) has a contact for 4', 8', and 16' footages. Cleaning these contacts will test your patience, I assure you. Make sure you allow a LOT of time. Beforehand, you will need to fashion some kind of tool. Now, this could be something as simple as an old-school pipe-cleaner with a couple of judicious bends made in it. That didn't work for me, I needed something more rigid. But it needs to be very narrow, to get past the crowded linkages at the rear of the key assembly. Yes, 3 contacts per key. And you can't really see what you're doing. Be very careful, if you damage one of the delicate springs (that live in that tiny space where you can't see) that actually make the contact, it will be a major nightmare to replace!           

If anyone knows of a shortcut...?

Above: by carefully sliding out the PCBs on their harnesses, you can gain slightly easier access to the rear of the keys to clean the contacts... if you can make it past the white plastic linkages.

So having fixed the oscillators and cleaned the key contacts, I then realise the reverb has gone. The reverb in this machine is a source of great character. The springs are suspended inside a box that itself is suspended below the base of the flightcase, beneath the keys. Hitting the box produced the usual "reverb explosion", however no organ sound was making it to the springs. I deduced from this that something on the driver side must have failed, while the pick-up circuitry was still functional. Two halves of valve 2 are involved with the driver amplification, and half of valve 1 with the pickup recovery amp (see below). There was a seemingly very healthy 100 volt signal at the driver side output to the transducer. Still, I tried replacing the valves - no change. The driver and pickup transducers are actually ceramic gramophone cartridges. In the place of the stylus, is a wire that is soldered to the springs.

These sort of cartridges appear to have some sort of piezo material inside them, that decays over time. I couldn't find anything like them for sale anywhere.However, Jaycar sold "buzzers" that contained piezo elements, so I cracked one of these open and experimented with it, after bringing out some new connections from the PSU so that I could have the box on the bench. (About here is where you realise it's going to be much easier if you adapt the reverb wiring to accomodate some Molex connectors, rather than crouching under the machine with a hot soldering iron in one hand..) There were definitely some "sweet spots" for the transducer, and the wire needed to be tight against it. I tried it both in driver and pickup positions.

I settled on using the disc as the new driver. It sounds to me like there is some "plate" effect as well as a fair bit of distorted "direct" sound going on with this arrangement, but in any case, it sounds great, certainly just as characterful as the original!  In fact, I liked it so much I converted the output jack in the PSU chassis that had been dedicated to the bass or pedal octave, to an audio input jack for the pre-amp and reverb, summing it with the internal organ sounds, making that dirty, springy, valve-y goodness available to all!

I'm grateful for the all the assistance I received with these repairs, from the heavy hitters on the Sound On Sound DIY forum!