[sdiy] Re: P5

[Oakley Sound]

> That applies to the scale trims, yes;  but I'm talking about the A & B
> master offset, which slide the tuning of the oscillator sections up &
> down...

Yes, but these are still in circuit with the autotune enabled. So the
autotune will calibrate any summer offsets out. I think these trimmers
are there to trim out any offsets introduced by the wheel modulation VCA
and circuitry.

But the plot thickens. In my last post I said there must be a typo in the
service manual showing that -TUNE controlled the unison CV. I thought it
should be UNI; the unison control line. This got me thinking. What if the
schematic was correct. And indeed it appears that there is no typo on the
schematic. But there is on the master block diagram. There is no UNI line
at all, this is simply a representation of what happens to the '-TUNE',
and 'VCO B KBD S' lines.

So there is another CV applied to the main CV summing busses when not in
auto-tune mode. Its unison CV and while in ordinary polyphonic mode this
line is not turned off but set to zero by the DAC.

So the cause of my offset was the sample/hold circuit in the unison CV
pathway. The TL072 was showing 100mV of unwanted offset and producing the
offset in tuning. Replacing that op-amp fixed my fault.

Phew...

Tony