[sdiy] One of the greatest synth puzzles I've ever seen..

[Bob Weigel]

Man...this one is kicking my butt.  A while back I thought I'd fixed 
this DK600 by filtering a couple of the non-dynamic lines (ie. I slowed 
the knob movement response down basically by .1 second) after the 
sample/hold buffer.  I ran it for a couple days playing it here and 
there and never heard the effect again I was hearing so assumed it was 
ok to sell.  Then in testing it after ebay sell to make sure it's ok to 
ship...guess what?  Oh it's back..with a vengeance.   If anyone says 
"learn to fix" in response to this...all I can say is you have no idea 
what you are talking about :-)
      Ok here's the low down

1) If you aren't familiar with the DK600 read the ditty on my web page 
cu.imt.net/~sounddoctorin in the global synth help thing.  The unit has 
the typical interface between knobs and cpu and analog circuitry.

2) All works well right now until I hit a key hard.  Now., you hopefully 
know that you can see the analog voltages coming off the DAC going to 
not only the comparator that compares actual knob settings with 
previously stored ones coming off the DAC from the SRAM (as the 
comparator is read by the cpu so it can quickly adjust the comparator to 
go the other direction by stepping up/down the value the SRAM was 
providing to the data buss) , but goes also to the line that hits the 
common of all the analog switches (4051's) here.  Then as the 
appropriate address opens the appropriate analog switch the sample and 
hold cell is charged with that voltage and that buffer amp feeds the 
actual analog circuitry of the synth engine.  Anyway....when I hit a key 
hard, these voltages begin to all have a 'pit' or 'bump' in them.  Hot 
one note hard...some effect. Hit several keys hard LOTS of effect.  The 
sound quality goes to garbage.  Sounds like a wierd sample and hold 
effect but nothing you'd want to use musically I don't htink really.  
Garbled.

3) While this signal appears on ALL the Sample/hold buffer amps 
outputs...it diminishes as you hit the keys softer again and you have 
anice normal sound if you just play lightly..

4) If you view the analog voltages coming off the dac, you will see that 
the 6 voltage 'segments' which represent the value of the actuals keys 
you have hit most recently, and the ones which represent their 
corresponding velocities are side by side in the flow of things.  One 
MIGHT think that somehow these voltages are just..leaking through the 
4051's somehow and affecting the other signals. However I submit that 
hitting six high C's creates exactly the same long section of high 
voltage values as hitting six low C's in a row very hard.  
However.....hitting the high C's doesn't affect the sound of the next 
note any differently than hitting six low C's or middle C's in a row for 
that matter..(or D's etc. :-) )  However..hitting ONE of them 
hard...starts to garble the signal..even with the dynamics turned off in 
the patch. 

5) You can clean up the signal by opening the filter all the way quite a 
bit.  However there is still noise on the other parameters as well.  
Just doesn't affect the sound quite so badly as that one it seems.

       I could see a tiny amount of the 'noise' I'll call it...down at 
the 10mV level on one of the power supply legs. However putting filters 
locally real quick..seemed to actually..make the problem worse. Oh one 
thing I forgot to mention.  Since it seemed heat related (problem 
reappeared after playing sort of near a wood stove for a bit.) I did 
some checking and if I put the soldering iron near the 4051 that 
switches for the key values....the problem gets way crazy.  New 
chip...no help.  Same effect.    I hate having spent so much time on 
this problem but...then again I've learned a lot from it.  This is the 
same synth that taught me really the details I've learened about how the 
whole thing I just described is done in so many synths.  It was I think 
the fourth or so analog I got ahold of....(Let's see..six 
trak...polaris...a broken multi-trak...forgot about that...and MKS-70 
were first I think..).  I'll kind of miss it in a way....every time I go 
out to do batting practise :-).  hehe. -Bob