951 keyboard Relay question

System 55 • System 35 • Model 15
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Alberto58
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951 keyboard Relay question

Post by Alberto58 » Tue Aug 30, 2016 2:48 pm

Hi !
About 951 keyboard I would like to know which is the function of the relay switch connected between the pitch bar and the capacitor C1 delegate to S&H. The discharge of the capacitor is not however possible to happen without any key pressed, infact in this condition left connection of C1 results unconnected thus, also with the the contact of relay shorted, S&H in any case whould preserved !
Apparently Relay would seem unnecessary!
(see Moog Modular Service Manual - 951 keyboard schematic Figure 28)
thanks
Alberto - Italy

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analogmonster
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Re: 951 keyboard Relay question

Post by analogmonster » Sun Sep 11, 2016 10:38 am

When a key is pressed the relay is switched on. The relay then charges the capacitor C1 according to the voltage value of the resistance chain interrupted by the pressed key. If portamento is switched on, this charging process is becoming slower corresponding to the portamento pot position. Portamento Off shortens the portamento pot. When the key is released, the relay falls off, but the voltage is kept by C1, which can't discharge due to the FET transistor Q1 which has near infinite input resistance and which provides the keyboard voltage of C1 to the following processing parts of the circuit. So without the relay the keyboard output voltage always would be zero.

See also my 950 clone description for more circuit details:

http://www.analog-monster.de/mmt950_en.html

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Alberto58
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Re: 951 keyboard Relay question

Post by Alberto58 » Sun Sep 11, 2016 12:14 pm

Thanks for reply!
From schematic the left connection of contacts of relay is connected at Glide pot and relative on/off switch, follow the connection to the Picth busbar of keyboard. When no key pressed (or released) the Pich busbar, see schematics, appears unconnected, so also entire left part of the relay's, then resistance should be infinite. C1 in this condiction can't not discarge!
I think the relay is necessary for a kind of delay on the trigger/sample sequence maybe to prevent inaccurate sampling.
But also in this case the priority appear opposed, considering the slow pace of the relay contact closure, the trigger pulse would be generated before sampling of CV in C1. It should be the opposite!
My doubts persist ;-)

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analogmonster
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Re: 951 keyboard Relay question

Post by analogmonster » Mon Sep 12, 2016 4:03 am

Ah, no, it provides the sample circuit with the keyboard voltage.

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Alberto58
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Re: 951 keyboard Relay question

Post by Alberto58 » Mon Sep 12, 2016 4:26 pm

when no key is pressed all wiring/components marked in red on the schematic are unconnected, no resistor of the resistive divider closes on the pitch bar! So also with relay contact shorted, please tell me where c1 could lose its charge?!
In this condition the relay appears absolutely useless to preserve charge of C1 :?
Image

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MC
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Re: 951 keyboard Relay question

Post by MC » Mon Sep 12, 2016 6:51 pm

analogmonster is correct - the relay serves as a sample & hold switch. The missing detail is that the trigger and pitch busses are designed such that they do not make contact at the same time. The pitch buss must make contact first and break contact last. When pressing a key, pitch buss has the correct CV by the time the trigger contact closes. When releasing a key, trigger must open first while pitch contact is still closed - this action ensures that the pitch CV is retained when all keys are released.

If the pitch buss were to break contact while trigger is still connected, the pitch CV would drop to zero for a brief unmusical moment. This was a problem on the prototype modular.

In fact, the early 950 modular keyboards included a "hold/no-hold" switch that kept the relay closed for those times that behavior was desired. See here:

http://www.matrixsynth.com/2016/03/rare ... l-950.html
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Alberto58
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Re: 951 keyboard Relay question

Post by Alberto58 » Tue Sep 13, 2016 2:55 pm

MC very thanks for the full explanation, now it's all clear! :-)

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