[sdiy] Oberheim 4-voice issues (SEM, keyboard)

[Roy J. Tellason, Sr.]

On Wednesday 20 April 2011 01:56:12 am Eric Frampton wrote:
> Keyboard circuit:
> 
> I'm new to troubleshooting CMOS logic, so any rules of thumb would be appreciated here. Basically what's happening (so far) is two things:
> 
> 1. The gate for output #1 (SEM #1) seems to open and close, rather fade in and out, in a very slow and slightly random oscillation; i.e., the keyboard circuit is opening up the gate for #1 on it's own whether a key is pressed or not. I can swap SEM's to that first position and they all do the same thing, so I know it's not the SEM. One particular IC is in charge of the gate output for both voices on that card; on the working side the output swings from 0-10V, but the bad one only swings from 0-4V or thereabouts. Does this sound like the actual chip failing, or something else pulling it down? 
 
Disconnect that output pin and see!  And that "slow and random" bit suggests an open or unterminated input pin someplace -- they all gotta go somewhere!  I'm not familiar with this instrument,  but am with CMOS.  One other thing -- those are fairly low-power parts,  so if any of them are warm to the touch,  they're bad.  That can be caused by blown circuits,  but seems to happen more often with excessive power supply voltage.  Hope this helps,  some...


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