[sdiy] Ray Wilson's Single Bus Keyboard

[Ray Wilson]

Hi Pat

You should be seeing a change in voltage every time you press a new key 
whether you are playing legato or stacato at point Z in the circuit. That 
point is the raw voltage that appears on the keyboard bus as you press keys. 
When you are playing legato you get changes of 83 mV per half step. When you 
play stacato you will get changes from  about -0.6 volts to the voltage of 
whatever key you press. If you are not seeing changes in voltage then check 
that the constant current source (IC2-A, Q1 and associated components) is 
providing 833 uA by putting your DVM set to Amps between the collector of Q1 
and the the keyboard resistor stack. I say AMPs first then decrease the 
range until you are sure that you have the necessary 833 uAs flowing into 
the resistor stack. If all o that is good then process to the differentiator 
(IC5-D and associated components). Every time you hit a key or play legato 
you will see a spike at the output of IC5-D. It is differentiating the 
changes in voltage from point Z. They go positive if the change is positive 
and negative if the change is negative (relative to the last note's 
voltage). The positive and negative spikes are rectified by IC5-A and 
associated components (CR5 and CR6). Op amp IC5-B is used as a comparator 
and you should see a ground to V+ going spike at the kathode of CR4 every 
time you change notes. If you don't then something is not connected 
correctly in the differentiator/rectifier/comparator section. The ground to 
+V spikes charge C14 via CR4 and effectively stretch the pulse a bit. The 
rising edge of the stretched cleaned up pulse is dropped across R47 via C12. 
The pulse is inverted by IC6-D and the rising edge of the inverted pulse is 
dropped across R48 via C13. IC6-B inverts that pulse which is anded with 
inverted point B (effectively the keydown logic). The upshot of thewhole 
thing is that the negative going pulse at IC6-B pin 4 is slightly delayed 
from the edge of the inverted point B signal. This insures that samples (and 
triggers) are only generated when a key is first pressed, when a note is 
changed with the key down (legato) and NOT when the key is released. The 
last key down's voltage is the last sampled voltage and not the no-key-down 
bus voltage (which is about -0.6V). The point T pulses are fed to pin 13 of 
IC3-A and the current bus voltage is sampled onto C2 which is buffered by 
high impedance buffer IC4-B. The output of IC4-B is fed via R18 to the 
non-inverting input of  IC4-A whose output feeds the keyboard's sampled and 
held control voltage to the world via 100 ohm output protection resistors. 
(I know the outputs are probably short proof but hey why not be belt and 
suspenders safe). I am going to update the page with this info and I 
replaced the abysmally out of date timing chart (my sincere apologies).

I hope this helps

Ray

----- Original Message ----- 
From: "Pat Kammerer" <spivkurl at wearerecords.com>
To: "sdiy" <synth-diy at dropmix.xs4all.nl>
Sent: Monday, March 07, 2005 12:14 AM
Subject: [sdiy] Ray Wilson's Single Bus Keyboard


> Hi everyone!
>
> I just finished all of the stuffing and wiring of Ray Wilsons Single Bus
> Synth Keyboard.  I have a partial problem with it.
>
> If anyone has built this circuit, then I'm curious.  What should I expect 
> as
> an interaction between the 40106 and the 4066?  Am I right in assuming 
> that
> part of the 40106's job is to cause the switch to close when a key is
> pressed. I'm not getting CV output or trigger where I should.  I can trace
> the keyboard reaction in the circuit, and I seem to run into a dead end
> somewhere around the 40106 and the 4066.
>
> I've never use the 40106 before, so I'm a bit above my head.  If any one 
> can
> help me understand what's going on in the CMOS part of this circuit, then 
> I
> would be grateful for help in troubleshooting.
>
> Take care,
> Pat
>