Robotron killing MC14049 on interface board

Xyla

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A few months ago my Robotron started shooting continuously to the left, and I isolated it to the interface board. I had someone repair it and they fixed it by replacing one of the MC14049s. Today it started doing it again, so I assume the same chip has gone bad again. I just ordered a replacement, but it seems like something else must be wrong for this to keep happening. Does anyone have any thoughts on this?
 
go into the input test and bounce the ribbon cable around. if it bleeps and bloops then it's the ribbon.
 
go into the input test and bounce the ribbon cable around. if it bleeps and bloops then it's the ribbon.

Already tried that, the ribbon cable is not intermittent... nor was it the first time that chip was replaced. I suppose it could be that the ribbon cable has completely lost continuity for left fire, but I doubt it. I guess I'll find out after I replace that chip again, because I'd rather not replace the ribbon cable unless I'm sure it's bad.
 
Hmmmm... Just thinking out loud... The 4049 inverter takes the input from the control panel and is held normally open by a pullup resistor pack. A debounce capacitor is tied to ground on the input. The output goes to a 74LS257 which multiplexes the inputs to the 6821. Can pretty much rule out the 6821 since it isn't directly connected. I would look for the input to the LS257 to be shorting out the 4049 causing it to fail or the pullup on the input is bad causing it to float and that's not good for CMOS. Check for +5vdc on the 4049 input pin that corresponds to the button in error.

If all else fails, find another interface board. They are plentiful and usually cheap.

Kirk S.
 
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