Hi,
I was suggesting that you set the other pots all to 0volts, which would be the
middle position, not the -5V position. But it sounds like the mux may be working. I would try setting all of them to 0V instead of -5, then turn up just one to +5, then two, then three, etc, and you should the the average DC increase by the same amount with each pot that is set to +5. Try that for all of the pots and make sure each one has a similar effect. Then try the same test but set them all to 0V (middle) first, then turn just one to -5, then two, then three, etc, and observe the average voltage change.
The CPU scans the front panel by selecting a particular pot's voltage, then trying to match it with the DAC. Both voltages are fed to comparator IC6, and its output is then fed into the CPU pin 1. You could check the average voltage on that pin with your meter and see if it's switching. Also I guess the logic probe would tell you that.
Are any of the front panel controls working? I think some of them are probably not scanned, such as the main volume control.
Trying to work on a processor-based anything without an oscilloscope is pretty tough. If no analog were involved, you could get a cheap USB logic analyzer, but since there is analog here, you need some sort of 'scope.
Of course the alternative to troubleshooting is to simply start replacing parts
until the problem goes away. Used scopes with at least 60 MHz bandwidth are available for well under $100 if you get the CRT non-storage kind. Personally, I would spend more and get something with digital storage. I have seen some of them used for $300, even Tektronix ones. If you plan on doing more of this kind of troubleshooting, I would definitely consider it.
Why were you looking at IC9 pin 7? I thought we were focusing on the DAC, the front panel analog mux, and that comparator.
Good luck,
Bob
From: "josh.nursing@gmail.com" <josh.nursing@gmail.com>
To: PolySix@yahoogroups.com
Sent: Monday, January 6, 2014 7:51 AM
Subject: Re: SV: [PolySix] Lots of pots not working, MG, VCF, EG.
Hi Bob,
I have no oscilloscope as yet, but a nice digital voltmeter and a cool Logic probe by Elenco, both very affordable.
Not too sure how I could use the Logic Probe to detect any faulty IC, but looking to try that, so any help with this is much appreciated.
When I check the voltage sweep from the center pin of the resonance pot (all other pots to 0, i.e. Min value), I get 5V -> -5V (Res 0 to Res 10).
The data pin on the board left to it shows a smaller value sweep: -4.37V to -5V, so I guess that means the multiplexing is working?
When I check the voltage on KLM-367 - IC9-P7 I get the voltage stuck on 0.35V in normal mode, and stuck on 10V in Manual mode.
Yash