I'm trying to figure out why a PolySIX I have in my shop has the wrong spread. It's off by a half-step--too high. In other words, the scale is not chromatic. If I play C5 to B5, it's almost dead on one octave space between notes. C6 is too high by about half a step.
I've gone through the alignment procedure, but the octave spread is still wrong.
What could be causing this?
I've gone through the alignment procedure, but the octave spread is still wrong.
What could be causing this?
This is basically a single-note keyboard? But with 6 transposable oscillators "voices".
The keyboard should output 1V per Octave *exactly*. It divides-down a precision 5V or 10V reference with a row of equal resistors. Sounds like that has drifted off of 10.000V.
EDIT - found a service manual. It is 0.5V/Oct, and the keyboard is digital through a chip to R/2R DAC ladder. The reference voltages on this ladder need to be spot-on. Power to IC7 IC8, also bias to the '072 after that. From there on, it gets so murky you would have to put me back on salary to study it.
The keyboard should output 1V per Octave *exactly*. It divides-down a precision 5V or 10V reference with a row of equal resistors. Sounds like that has drifted off of 10.000V.
EDIT - found a service manual. It is 0.5V/Oct, and the keyboard is digital through a chip to R/2R DAC ladder. The reference voltages on this ladder need to be spot-on. Power to IC7 IC8, also bias to the '072 after that. From there on, it gets so murky you would have to put me back on salary to study it.
Not a single note keyboard, if you mean like the old Moog, it has six-note polyphony, hence the name POLY-six.
This is helpful. When the customer proposed to bring it to my shop, due to its age, I thought it was an analog synth with individual oscillators, but alas, it's digital.
A wrong reference voltage on a ladder circuit sounds logical. I'll see if I can hunt down that reference and measure it then work from there.
Thanks for the link to the service manual.
UPDATE:
TP4 (-10V) reads -10.003V. IC 7 & 8 seem to be hidden under the keybed, so more disassembly required.
A wrong reference voltage on a ladder circuit sounds logical. I'll see if I can hunt down that reference and measure it then work from there.
Thanks for the link to the service manual.
UPDATE:
TP4 (-10V) reads -10.003V. IC 7 & 8 seem to be hidden under the keybed, so more disassembly required.
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I realized later that it is more complicated; nevertheless it runs as 0.5V/Oct voltage control. (I guess improved-offset chips allowed a lower scale than the old 1V/Oct machines.)
10.003V for 10.0...V seems awful close (as close as could be held in production and use) but I have not done the math. Hmmmm... maybe (I ran out of thumbs), but 1mV precision is asking a lot of consumer gear, even if very expensive.
A part can fail of course, but I serviced a lot of these back in the 1980s, and I do not recall them being all that sensitive to adjustment. Compare to Moog circuits that relied on super precision resistors of odd values, and lots of factory matched small transistors. After a while a declined to take in Moog repairs other than basic jacks and such.
Yes, your resistor tree is there, just the keyboard is not itself the string.
I agree that 10.003 is close enough, it is about half a percent off value.
Yes, your resistor tree is there, just the keyboard is not itself the string.
I agree that 10.003 is close enough, it is about half a percent off value.
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