[sdiy] Issue with Korg Mono/Poly

[Peter Cooke]

Hi list


I have a problem with my Korg Mono/Poly.  It works fine as long as
VCO4 has its octave switch set to 16'.  When VCO4's octave switch is
set higher, a seemingly random offset goes into VCO4 which also feeds
into the other three oscillators.

>From the service notes, I can see that the octave switch adds an
offset (3v, 2v, 1v or 0v) into each VCO and this is derived from a
potential divider.  So in my case, when VCO4 is connected to anything
but 0v, it must be grounding the voltage in some way which skews the
potential divider and thus affects the other oscillators too.

This offset voltage goes into a standard op amp (TL072) which I guess
is the faulty component.  Is this likely?  (Can op amps fail in this
fashion?)

I'm just checking to see if I may have overlooked something before I
pull my PCB apart!

Many thanks
Peter