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Hi all, i need a bit of help with a fault diagnosis on a new korg klm367a board inside a P6 to be exact.
i seem to have some wierd happenings going on.
i have the wierd scenario of the release pot current going to the wrong 4051 chip, which never happened before i replaced the damaged board with a new jed jorgensen board.
i have repopulated everything meticulously and done all the mods im aware of.
Prior to this board refurbishment the memory wouldnt work ie: wouldnt not write or read to memory, and all leds were constantly lit on the white programmer buttons, but even though the board was battery damaged my LFO MG fully worked and so did my ADSR section, since doing the transplant i still have the full set of red leds across all the white buttons on the programmer section yet im able to write, and recall patches properly which i couldnt do before changing over boards, i now also have a problem in that the release envelope from the ADSR section has gone awol..
I have done some detective work with my bench DMM and my oscilloscope and checking the schematics and have found that upon investigating two of the working NEW old stock 4051B's there are issues with pin 4 on the chip located at (IC18) which shows a tiny square wave on the osciloscope with a voltage ranging from -1.4mv to +2.4mv when i rotate the release pot on the control panel.
Added to this (Pin1) Attack, (Pin 5) Decay, (Pin 2) Sustain all show the correct voltage ranges from -4.98 volts to +4.98 volts, and they control the voice board as they should.
So somewhere something is not right but does this point to a problem in the circuit or a problem with the CPU ?
If i further investigate the second 4051B (IC19) pins 2 and 4 are located on the circuit diagram but listed as not connected however these pins show voltages ranging from -264mv to +166mv when the release pot is turned. This also shows something odd on the oscilloscope, a strange sawtooth type ramp up and down which follows the direction of the release pot + and -.
Should i be getting anything coming through those pins on the second IC ?
According to the circuit diagram, i shouldnt..
Any help on this really odd problem would be great.
Need the weight of your experience from you guys if you would be so kind.