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951 keyboard circuit

Posted: Thu Feb 22, 2018 1:44 pm
by Rarecomponentadsr
I'm considering making a 951 keyboard clone but before I commit I need to know if the 951 circuit requires that the S Trig bar and Key value resistor bar needs to be trIggered in a 'staggered ' way similar to a multi choir harpsichord.
I assumed that the S Trig and Key Value bars were both triggered at the same time but I think I read somewhere that no, there's a sequence as to how they are contacted on the Prat Read keyboard.
If this is so I guess I'll pass on this particular project. Also, there is an enigma regarding the relay in the circuit which I believe prevents false note values ? Totally unsure about this.
I wonder if anyone has successfully cloned the 951 ?

Re: 951 keyboard circuit

Posted: Thu Feb 22, 2018 7:26 pm
by EMwhite
I'd like a 952, 4 octave tolex covered keyboard for my Model 15, but I'll need get one.

To answer your question, I'm 99% sure that the new 953 is a Fatar keyed with the typical 'modern' membrane switches (two per key) and a micro controller that converts this into a key on/off gate and the key voltage.

When the new 953 was first announced, Moog documented plans to offer an expansion card which would provide V-trig, Midi, etc. in addition to the [Moog] standard barrel connector.

Point is that you will not see the P&R busbar mechanism on the inside.

Re: 951 keyboard circuit

Posted: Thu Feb 22, 2018 9:54 pm
by noddyspuncture
If you do 'clone' the 951 keyboard you will need the Pratt Reed system though - and yes, the pitch needs to be established a micro (or milli...) second before the trigger is applied.

Same with the Minimoog (the 'vintage' Minimoog, not the Meh'ringer clone...!)


Re: 951 keyboard circuit

Posted: Fri Feb 23, 2018 7:19 am
by Rarecomponentadsr
Thanks for help with this, as this is a big project I'm not happy about the lack of info out there and that it looks like no one has faithfully cloned the 951 circuit and keyboard ..I could be mistaken but I've found nothing so far.
Added to that the staggered contacts ...I think also that this applies to note off too.

I'll probably pass on this one...a clone too far.

Re: 951 keyboard circuit

Posted: Fri Feb 23, 2018 7:59 am
by noddyspuncture
The 951 keyboard circuit as actually very simple, you could knock that up on strip board quite easily - I cloned the whole 1125 sample & hold unit on strip board and added a few functions not on the original unit so it is possible..!

(video link here: )

I think making the Pratt Reed type chassis would be your biggest problem...!

Re: 951 keyboard circuit

Posted: Sat Feb 24, 2018 10:00 am
by Rarecomponentadsr
It is the Prat Read system that's the problem...otherwise I would go ahead...but modifying a vintage keyboard
to get the required on/off contacts to function in the right order is something I don't want to tackle just yet.
I can imagine doing the work and finding odd keys not working or intermittent ...I may attempt it in the future.

Re: 951 keyboard circuit

Posted: Thu Mar 01, 2018 8:24 pm
by drogoff
Yes - all the early mono synths (Moog Modular, Minimoog, Arps?, etc) used the 2-bus Pratt-Reed keyboard. One bus is the resistor chain to get the voltage. The other just shorts the two contacts whenever any key is pressed. The trick is that the first one must make contact first and release contact last. The is so the key voltage is ready when the gate/sample signal captures it in a sample/hold circuit.

Later mono synths (Micromoog, Prodigy) used a single bus and added a low-voltage, 25KHz triangle wave mixed with the key voltage. There was a circuit that looked for this wave to generate the gate / sample signal. And the triangle wave was filtered out as part of the glide circuit before the sample/hold.

These type of keyboard circuits were a pain for many reasons. They needed a precision resistor chain and current source and sample/hold. They're also expensive and heavy and require regular cleaning and adjustment. And, they could only work for mono or duo-phonic use.

Once synths started going poly - and microprocessors started getting cheap - everything went to digitally scanned keyboards that aren't much different than your computer keyboard. Even the current analog/CV keyboards, like the Moog 953 and unit, are really digital keyboards with built-in DACs to generate the CV outputs.

99.9% of all synth/organ/electronic-piano keybeds are made by Fatar these days so there's also not much choice!