Farfisa Compact Duo MKI power supply problem

[myster-spy]

Hi,

I'm trying to bring back in life a Compact Duo MKI but without a schematic is pretty difficult. I can find only MKII's schematic but they're not exactly the same. Different PCB numbers, tremolo circuit and I'm not sure about the power supply.

Does anyone have the MKI and has compare the power supply with the MKII's schematic?

Someone has salvaged the psu on this Compact Duo and I don't know how to convert it back to the original circuit. The "FAR" socket is missing as is the original cable. Someone has replaced it with a standard balanced cable and I'm not sure what did they have in mind. Nothing is connected at the end of the cable and I can't power the Farfisa. Also, some cables in the circuit are cut!

Right now the situation is like this,

[David Anderson]

I own a Duo Mk1 that I brought back from non-working condition and missing its F/AR power supply. I actually built a complete working replica of the F/AR. As far as I know, there is no difference between the power supplies for the two models of the Duo.

None of the Farfisa schematics are easy to read. I've put together some documents I could send you concerning the Mk1 boards. There is no known complete schematic for the Mk1 Duo unless one has surfaced in the last few years.

To put it bluntly, this is not a project that will be easy for someone who doesn't have some expertise in following electronic circuits.

For U.S. versions, the Duo has a cable permanently attached to the organ that plugs into the F/AR with a 7-pin Amphenol plug.

[Drawbar Dave]

Yes the F/AR is the same for both versions of Duo with the only difference being the Farfisa logo changing. I've rebuilt both types and still have a later version. When I owned a MK1 I initially built a power supply using a 24vdc enclosed psu and a 9 vac transformer. It had an output jack on it to connect to a mixer. This served me well until I found my first F/AR.

[David Anderson]

It was pretty much the same for me. I got my Duo running with two external power supplies while I built my F/AR copy. The thing is, you don't really want to supply +24VDC to the Duo. There are two dropping resistors, 75Ω and 22Ω in series between +24V and the Duo. The onboard Zener Diode inside the Duo works with those resistors to drop the supply to +9VDC, which is all the Duo needs.

So you really only need a +9VDC supply and a 9VAC supply to make the organ itself work. The 9VDC supply has to be able to supply 155mA. I can't recall what the 9VAC supply needs to be.

The F/AR has a +24V supply because you need it for the reverb drive circuit.

But it's issues like this that bring me back to my warning that this is not a great project for someone who doesn't come to it with a background in working on combo organs.

[Drawbar Dave]

In my case I already had a 24vdc supply and I used dropping resistors as per the schematic.

[myster-spy]

Thanks for your help!

But over here someone has messed up pretty bad on the psu. Can anyone take any photo of his Compact Duo MKI psu? It would be very helpful.

As I understand there are two ground points in the Farfisa otherwise I can solve this with only one external power supply at 9Vac to power the lamps and the multitone booster and then regulate it for approximately 9Vdc to power the whole organ. But it seems the Swell pedal and the PA-27 pcb (according to MKII's schem) have their own separate ground! But does the MKI has a PA-27 pcb? This is the tremolo circuit. Surely has a Swell pedal though that in my Farfisa is missing but does it need a separate ground?

[myster-spy]

I have a few questions as I'm a newbie to Farfisas.

The Brilliant knob seems too weak. I can't hear too much difference when it's engaged, is this normal?

Also, when the green Upper Manual knobs are engaged are the white ones active? Or only one of them are active?

And, what does the All Booster knob do exactly?

[myster-spy]

Ok I fixed almost everything except the Percussion. I powered the Farfisa with a single 9Vac external power supply and regulated it with a simple p-filter to 9Vdc. No problems here.

Also, from the schematic Brilliant knob seems to take effect only on 2 2/3. I thought it was a common brilliant knob aka bright switch as you find in guitar amps and the effect was on all voices simultaneously.
About the green vs white voices and Multi + All Booster knobs, some cables were cut. I solder them back and everything is perfect. :)

But, I tried everything and the Percussion knobs doesn't work. Recapped the pcb, different transistors, nothing. It's frustrating. Can anyone help over here?

[Drawbar Dave]

On a MK1 the percussion only works on bass notes and pedals.

[myster-spy]

Thanks Dave! Yes I know but percussion doesn't work. Only noise with the note when I press Percussion and louder noise when I press Short.