Archive of the former Yahoo!Groups mailing list: Discussion about the Korg PolySix synthesizer

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Subject: Re: [PolySix] RE: Lots of pots not working

From: Bob Grieb <bobgrieb@yahoo.com>
Date: 2014-01-11

I sort of see what you are saying, but not sure I agree with your conclusions.
Basically the DAC is used for two things.   It is used with the comparator IC6
to measure the front panel pots.  It is also used with the S&H circuit  IC18 plus
related caps and buffers to feed analog control voltages out.   The comparator is
not used for sending control voltages.  It's only used for reading the front panel.

So it seems there are two ways that the S&H res voltage could be varying.
One way would be that there is something wrong with the DAC or S&H circuit.
Another possibility is that there is something wrong with the front panel voltage
mux that is causing the voltages to be misread.  This would also affect DAC calibration.
A problem with just the S&H circuit would not affect DAC calibration.

Since your DAC calibration is not working, and I think you said that pots other
than the cutoff and Effect intensity/speed affect which LED's are lit when in DAC
calibration mode, I would say that you have a problem with the CPU reading the
front panel controls.   And I would work on that before trying to attack any other
symptoms, as they may be effects of the same problem.

   Bob


From: "josh.nursing@gmail.com" <josh.nursing@gmail.com>
To: PolySix@yahoogroups.com
Sent: Friday, January 10, 2014 7:11 PM
Subject: Re: [PolySix] RE: Lots of pots not working, MG, VCF, EG.

 
I can understand your confusion: there are several simultaneous issues and different posts in this single thread detailing all the issues, including new ones I am finding and fixing along the way.

Yes, the DAC calibration does not seem to work since that LED 5 is still stuck.

However, in Normal mode, changing the front panels 4501s to new ones resolved nothing: I believe the front panel is sending the proper signals.

On the other hand, the Demux troubleshooting shows that IC18-P12 (Res) always has a varying voltage, no matter which pot is moved, so I believe the issue is narrowed to IC6 and the DAC area IMO, possibly IC6 providing erroneous Data for the CPU's T0. This, in combination with two potential issues with the DAC area (lack of proper calibration, Pins 1 & 2) could cause this.

I will be stuck on the IC6 replacement: I checked and do not have the component. I guess I can still socket it for now and test it separately until I get a replacement part. The other thing I will test is the DAC area for sure. It will be an easy mod to test the board with the DAC08.

I will go through the resistance check you mentioned concerning the Px paths between the front panel and the KLM-367. If we can get values to compare, that would be great.

If I can get IC-18-P12 not to pick up voltage when I am using Cutoff that will be a good step as it is currently picking up voltage from all pots!

Yash