Sequential Split-8 repair

[WeAreAs1 at aol.com]

Hello Barry,

Isn't the pitch bender read by some kind of A/D circuit?  I would supect that 
section before anything else.  How is the synth reacting to movements of the 
parameter knob or mod wheel?  That knob and the mod wheel would also be 
connected to the same A/D.  Check the A/D and any associated multplexers or 
opamps that might be feeding it.  I would not mess with that voltage 
adjustment just yet. (the unexpected voltage you're reading might simply be 
due to the additional load that a bad component is presenting to the supply)

Also, many synths of this type have a pitch bender system that outputs both 
an analog signal for the bender's position, and a digital signal to indicate 
the current polarity of the bender (whether you're bending up or down).  
Typically, the polarity signal is derived by a simple opamp comparator 
circuit which looks at the analog output of the bender.  Additionally, some 
of them have a polarity switching circuit connected to the bender.  This is 
usually in the form of an opamp with a transistor (shunting the + input to 
ground) which can be turned on or off to invert/not invert the output of the 
opamp.  The transistor is switched on and of by the aforementioned comparator 
(which senses the up/down position of the bender).  Thus, the bender's 
typical +15 to -15 volt output is converted to +15 volts (all the way up), 0 
volts (at the center), and +15 volts (all the way down).  This is then 
usually attenuated to +5 volts, then fed into whatever A/D circuit is used.

The reason for this seemingly complicated system is that most of these synths 
like to see 0 to 5 volts at their A/D's, and it would be more difficult for 
them to read +2.5 volts as the pitch bender's center position (what with 
needing a center "null" range, not to mention the difficulty of getting full 
7-bit resolution from the smaller voltage), than it is to simply read the 0 
to +5 volt range in either direction, and use the derived "polarity" signal 
to determine direction of bend.

Why am I telling you this?  Because it's very possible that your Split-Eight 
has this type of pitch bend system (or something like it), and the problem 
may lie somewhere in the aforementioned bender polarity circuitry.  Your 
descrition of the symptoms lead me to this hunch.  If you have schematics for 
this section (I don't, unfortunately...), then look for something like what I 
described.  You might have a bad opamp or comparator in there, or that 
polarity-switching transistor (if one is implemented) might be bad.

Good luck,
Michael Bacich