Need some help on II single board keyboard misbehaving

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Joined: May 31 2022 - 18:18
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Need some help on II single board keyboard misbehaving

Tyring to repair a nearly working keyboard but ran into a problem where the semi colon key does a double press when pressed.

I've looked at the C4 which is the one that controls the debounce and found a charge time that's 50% of what's specificed in the Apple II Circuit description where it's stated the charge is 8ms, but this charge is only 4ms which may be causing the problem as a second poll happens too quickly. Right?

 

I don't have a schematic, but did notice there doesn't appear to be any resistor in series to restrict current, instead it's tied directly to 5V at whatever current is available.  My guess is there's something else upstream which should be consuming more current but isn't. Would anyone have suggestions on what to check? Strange as it's only the semi-colon key that does this too. I've replaced the switch and found the same response so that cap response is where I'm looking. The C4 measures at just about 1.32nF which and I believe it's a1nF cap so if anything it should take slightly longer to charge.  Or many the II circuit description book has the wrong timing? I should note the mask disable voltage for me is -5V (-4.7V specifically) and not 10V so maybe the timing is off too? And because a second press is detected a second disable period occurs, in some cases I see 3 or 4 of the mask periods in quick succession on the press and then again on the release.

 

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Last seen: 11 hours 52 min ago
Joined: May 31 2022 - 18:18
Posts: 268
Sorry please ignore, somehow

Sorry please ignore, somehow I failed to post this the the Apple II forum!! 

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