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 Forum index » Instruments and Equipment » General Discussion
Help with a Farfisa repeat percussion circuit
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Analogman



Joined: Sep 22, 2012
Posts: 7
Location: Vienna

PostPosted: Wed Nov 29, 2017 4:58 am    Post subject: Help with a Farfisa repeat percussion circuit Reply with quote  Mark this post and the followings unread

Hello,
The farfisa repeat percussion circuit is used in various farfisa organs. Most famously in the compact duo. It was also available as an external pedal called volume/repeat. I have a similar circuit in my professional 110.
However all of these circuits differ in design and components.
The compact duo uses ldr's and light bulbs to create the repeat and tremolo.
I'm not sure what the foot pedal uses.
My 110 seems to use 555 ic chips to create the oscillation.
In the 110 you can press percussion buttons for each of the upper manual drawbar voices which results in a sharp and short percussion of that voice,(the sine wave is cut short). By pressing the repeat button it then continuously repeats (as long as you hold the keys down) and the speed can be controlled with a slider pot. It is nice but somewhat limited and too fast for my taste, especially when compared to the compact duo repeat circuit which seems to be more versatile and can be coupled with a controllable tremolo circuit.
I decided I would look into the schematics of the 110 and maybe find a way to improve the controllability of it. But the circuit is not making any sense to me and I am hoping someone here can explain it a little better.
On the parts list it says ic1 and ic2 of Pa831 on DWG9 are ua555's.
However on DWG9 there is shown a ic12 and an ic14 that seem to be responsible for the repeat circuit. If those are the 555's then they are wired up very strangely as pin 8 is to ground and pin 1 seems to be the supply voltage (-12v). On all 555 timer circuits that I have looked at pin 8 is Vcc and pin 1 is ground. Maybe it's just a special way of wiring it up? If someone could explain this to me I would be most thankful.
Cheers,


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Grumble



Joined: Nov 23, 2015
Posts: 729
Location: Netherlands
Audio files: 21

PostPosted: Wed Nov 29, 2017 5:53 am    Post subject: Reply with quote  Mark this post and the followings unread

When triggered, the 555 wired up this way will provide a pulse at the output (pin 3) coming from -12 volt to GND level.
This pulse is used to have the transistors Qp11 and Qp12 go into saturation (Vce = ~0 volt), what happens next is due to the limited part of the schematics unclear to me.

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Analogman



Joined: Sep 22, 2012
Posts: 7
Location: Vienna

PostPosted: Wed Nov 29, 2017 12:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for that Mr. Grumble.
I could post larger segments of the schematic but then they would be illegable.
The complete schematics are available for free from various sources however.
I would be interested in two things.
1. How to slow down the pulse somewhat more.
2. How to sync or trigger the pulse from an external clock.
Of course if I can achieve the second then the first is not so important.
Cheers.


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Grumble



Joined: Nov 23, 2015
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Audio files: 21

PostPosted: Wed Nov 29, 2017 3:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

I see a potentiometer called Repeat Speed (P03) and if that is what you like to slow down the answer is easy: Increase Cp10 which is now 3,3uF
Try soldering a capacitor in parallel of 3,3uF and see if you get what you want.

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Analogman



Joined: Sep 22, 2012
Posts: 7
Location: Vienna

PostPosted: Thu Nov 30, 2017 3:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Accordingly I could increase the value of the potentiometer, or?
How about syncing to an external clock or pulse?
The reason I want to do this, among other reasons, is because in my farfisa 110 there is a Superpartner Bravissimo drum unit, very cheesy drum patterns and sounds but still fun to play to. However the fact that this unit, which uses a sn74122 monostabe vibrator and a flipflop as a clock, does not synchronize with the percussion rhythm of the keyboard makes the latter feature useless.
Ideally I would like to be able to slave the repeat percussion to the rhythm of the Superpertner, but also be able to sync or slave the Superpartner to an external clock source.
Here is a partial schematic from the Superpartner.
Ic20 is the 74122
Ic D is the sn7473 dual flipflop
Speed is controlled through potentiometer PO1


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