[sdiy] Resolution(?) of Siel DK600 trouble..interrresting

[Bob Weigel]

Ahh you guys give up too easy. :-)  One hint that I didn't mention...the 
'bumps' in apparently all the S+H signals that were introduced when you 
began hitting keys with max velocity...were at around 60 cycles in 
frequency it appeared.  That would have just sown more confusion 
though.. as it turns out the refresh frequency for the cells is actually 
around 20msec=50cps.  But..recall with me the 'bumps' only appear when 
keys are hit hard..not when anything else like a bunch of high notes 
gets hit.
      You'd think they'd all contribute equally.   But the hint was that 
the top of the waveform was showing a little sawtooth when we'd hit a 
max velocity on the sample/hold cell that stores that voltage. BUT NOT 
when it was at the voltage below that. (Turns out there are 8 different 
velocity voltages on the Siel...if anyone had ever wondered :-) )  
Anyway the 5V supply sagged to 4.8V as it warmed up consistently over 
about the same time frame as our little noise appeared.  The 470 ohm 
resistor in the bias leg of the regulator (which has no adjustment on 
the 5V ONLY on DK600..) had gone low..to 420 or so.  Putting in a 
trimmer and getting the voltage up a hair over 5V fixed the problem.  It 
was consistently warming up and doing it in the room temp I had and that 
quick change, reboot and play for hours without that scrambled sound.   
      So...apparently the problem was that the DAC was generating a 
signal that pushed a little beyond the rail on the 4051 chip and..they 
don't like that...it forward biases the switch junction or something as 
I recall and I couldn't see the tiny difference in voltage between the 
peaks on the velocity and the peaks on say the key value..but it was 
there just enough to cause the problem it seems.  
       This is a great example of a problem where you think you've found 
a cure and it gets you thinking that it must be something related to 
something that seemed to bring some success before when it resurfaces.  
But it was a great study for me. Ahh..I learn to fix one more time...:) -Bob