Friday, May 31, 2013

PolyMoog Repair

This polymoog was one of the most challenging units I ever worked on. The circuit design is complicated and not at all intuitive or familiar. There are no circuit descriptions, and the flow chart could be pondered and studied for months, before the depths of its complexity could be plumbed .

 I never really believed those stories about technology being provided to us by alien beings, but the polymoog made me reconsider.

There were issues with stuck notes. This turned out to be relatively simple, the Polycom IC's, which are installed on little cards, were at fault in this instance. This was relatively easy to repair.This unit came to me with little repair cards, as shown below, so I was able to plug them in and fix this issue somewhat quickly.

These cards contain the polycom IC's, one per note. They create an envelope and mix the waveforms. The resistor values for the resistor shown in yellow change for various octaves up and down the piano.

Another issue was that all of the Bb's (except for the highest 2) of a certain waveshape (pulse or saw, I can't remember) were dead.

This was the result of a dead divider IC, shown below. Since these dividers divide down from the top note, and then go down from there, if one is dead everything below it will be dead as well.

This IC and it's neighbors divide down each of the notes and provide them to the polycom IC's. When one section of one IC goes bad, all the notes below it go bad too. Eg: all but the top 2 Bb's were silent on this keyboard.

Another issue was with the CD4007 IC's. These are used to control the preset voltages going to the VCF, resonators, and pretty much all of the various "top" boards. They are used to rout the particular CV's (generated through resistor networks) to those boards when the unit is in preset mode, or they rout the output of the various faders to ththe top boards when the unit is in variable mode. When they have burnt out, either the Variable mode of a control doesn't work, or the preset mode doesn't work. They are readily available from and socketed so as to be easy to replaced. Note that each some are oriented with pin one facing upwards, and some are reversed!

Finally, issues with the VCF came down to bad transistors at the VCF current source (q11 and q12,) as well as some bad CA3080's throughout the VCF section.

When testing these CA3080s, remember that they have a very tiny range of input voltages (a volt and a half or so at max), and fairly low output voltage swings as well, so set your scope appropriately!

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