Thanks
At 02:00 PM 12/18/2012, you wrote:
So here are a few more informations about the CV generation in the ARP Pro
Soloist and Pro DGX; this is coming from the service manual (theoretically
I could look things up in my Pro Soloist, too).
As I said those two ARP models have some digital functionality at their
core - which is somewhat amazing since they don't use a processor at all!
Since we are dealing with a keyboard problem I'll focus on this.
When looking at the keyboard for those models we find something strange -
there is just a single key contact for each key. Most other synths use at
least two (one for CV, one for Gate). Here everything is generated from one
contact.
Keys are transmitted as a 6 bit value, with 4 bit reserved for the keys
(within an octave) and two bits for the octave code. 4 bit can actually
encode 16 values while we only have 12 different keys within an octave;
this means a number of combinations are unused.
To give an example: Each "C" on the keyboard is represented by the code
"0000" (all four bits are off) plus the appropriate octave modifier.
For the lowest octave the octave modifier bits are 00, for the next 01, for
the next 10 and for the highest C both bits are active (11).
The digital nature of this approach means that the data lines for the
octaves must be ok (otherwise the lower octaves would make problems as
well) and the way they are decoded basically correctly.
So the question is if the problem happens in the DAC or afterwards.
The actual DAC functionality is very straightforward: Essentially the bits
and the modifiers are summed - which again means that if things are
correctly summed for the lower keys it is unlikely that the summing would
fail for a single higher key.
My suspicions go like this: The voltage coming from the highest "C" is
naturally also the highest voltage here. Perhaps some part has aged so far
that it saturates or breaks down when supplied with this voltage??
Possible test scenario: Activate pitch bend for the pressure sensor, play
hte highest "B" and apply pressure. Do you experience similar symptoms then
as well?
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