[sdiy] six-trak another problem!

[Bob Weigel]

Ok, first of all, if it's octave jumping....there is no programming 
facility for that.  The six trak is only edited in coarse (half steps) 
and fine frequencies.  So it 'plays a couple octaves higher'...seems 
odd.  Or...does it?  Because hey..maybe it's banked out as high as it 
can go.  I seem to recall it sweeps from 0 to 48 which is 2 octaves up 
and down.  So...that could be what's going on for sure.  The unit is 
multi-timbral and I seem to recall that you can set each pitch 
individually on each oscillator..so....
     Look on the diagram and find the S+H cell that holds the data you 
are concerned with. Measure the output of the op amp buffer that feeds 
that circuitry witha  scope and you should be able to see the jitter.   
Now this is where it's nice to have dual trace scope.  But if you don't 
you can still manage to figure it out usually.   If you do then you can 
just hook one lead, having established that the problem is sometime 
before that op amp output test point, to the signal (with x10 selected 
if need be to reduce the probes effect on the signal..) common leg of 
the MUX chip.  Put the other one on the OUTPUT of the MUX chip going to 
the sample and hold cell for the octave function.  Now edit the 
function!  You will hopefully be able to see some correlation if you 
have triggered and set up the scope properly.....and quickly you'll see 
"ahh..there is the section of this garbled mess of data that is 
concerning me at the moment".   When you aren't moving the knob...does 
the signal still jump all over there? 
     Anyway in the processor sweeps through all the functions of the 
machine in sequence so the idea is, if you trigger on one of them that 
is NOT jumping all around...the whole sequence should be laid out on the 
screen of your scope!  So trigger on the probe that is monitoring the 
common input of the mux.  If it is a consistent signal at the S+H cell 
but the output of the op amp is jumping all around...hmm..well try 
swapping the op amps or..check the solder on it.  Oh and "learn to fix" 
of course. :-)  That's always the best advice. Hehe.   -Bob

Robert Leiner wrote:

>Hello list members,
>
>I have a problem with one of the voices on a Sequential six-track (voice 2),
>it plays a couple of octaves higher and occasionally it's sounds like fast
>random pitches.
>
>I had a similar problem with my Oberheim OB-8 and after changing some
>multiplexers the problem disepeared.
>
>Any one have some advise on what it can be and where to start to look?
>The multiplexer IC's are solered straight on the board so it's not as easy
>to change as on the OB-8.
>
>
>Any help is wellcome!
>
>All The Best - 
>Robert Leiner
>
>http://www.leinermedia.com
>
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