[sdiy] MonoPoly Dogwhistle Help

[mark verbos]

Make sure the solder is not cracked (reflow all joints around those ICs) and that there are no microscopic cracks in PCB traces. The PCBs are terrible quality and could do that. Buzz out the whole path from where the signals come into the board all the way to the components and between components. That seems like the + rail isn’t getting to the opamp or one pin is floating.

Mark




> On Feb 9, 2023, at 3:15 PM, Randel Osborne <randel at redshift-consulting.com> wrote:
> 
> Hello-
> 
> I have a KORG MonoPoly that I would love to get off of the repair bench and into the studio.
> 
> It operates normally, except that all 4 oscillators operate at an ultrasonic frequency and do not track the keyboard.
> 
> My Great Dane is not amused.
> 
> The obvious culprit is the KLM-398 board which takes the place of the summing amp in the SSM2033.
> 
> On my instrument, the input voltages to the board track the keyboard in a linear fashion, but the outputs are fixed around -13v.
> 
> The board has power on both rails, the resistors are in spec, and the op amps have been replaced.
> 
> When I unplug the board from the input to the SSM the oscillators return to audio range and are functional, but without pitch control.
> 
> It should be relatively simple to identify a fault that affects all voices in such an obvious way, but I’m stumped.
> 
> I’d appreciate any ideas, or comments about the theory of operation of the summing amp, which does not appear to have an obvious feedback loop.
> 
> Also, I’d appreciate a link to a higher resolution scan of the oscillator board schematic - I can barely read the component values on mine,
> 
> Thanks!
> 
> -Randel
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