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Apple IIe phantom repeating keypress

zippysticks

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Joined
Nov 21, 2017
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UK, South East
I have seen a few related threads in the forum on this topic but none seem to match my challenge....

When I turn on the machine, it appears to start just fine but immediately starts which phantom repeating keypresses to the screen. If I press a key it will then continue to repeat that key.
I did have a single stuck (Alps long stem) key that has been removed (I'm looking for a replacement) but the problem persists. It is not the keyboard as I have unplugged it (after switching on) and the problem persists. I have also tried a known working keyboard from another machine and also swapped the keyboard controlled IC AY-5-3600 PRO with a known good one to no avail.

I'll get the scope out tomorrow and start probing so am looking for suggestions for testing.

Thanks
 
I don't have the schematics for an Apple IIe handy but I am going to take a stab at this based on the schematics for an Apple IIc. There is an Any Key Down (AKD) signal coming off of pin 5 of the AY-5-3600 chip which is routed to pin 31 of the IOU chip. There's a second signal, Keyboard Stobe (KSTRB), also coming off of pin 16 of the AY-5-3600 chip which is routed to pin 32 of the IOU (it's also routed to pin 17 on the 6551 SER2 chip). If there is no activity, when no key is being pressed, on these two lines then I would suspect the IOU. If there is activity on either line pull the keyboard encoder (AY-5-3600) and check them again.

Keep in mind this is for the Apple IIc, the Apple IIe might be different.

If you're having difficulty locating the keys I would recommend, if overseas works for you, Apple Rescue of Denver.
 
After reviewing the Apple IIe schematics the two signals and related pins I referenced for the Apple IIc schematics are the same for the IIe. The only difference is the IIe does not have the 6651 SER2 chip connection (as it lacks this chip).
 
Not traced the repeating random key fault yet but did manage to find a 3D model of the ALPS internal long stem plastic component for the keyswitch.
Will print it tomorrow and see how I get on.
 
Well after quite a lot of probing around with the scope at some pretty noisy feeds to the keyboard controller I decided to experiment by introducing an additional ground between the keyboard and the mainboard and it seems to have resolved the random repeating key press problem. It was odd that removing the keyboard once powered on still exhibited the phantom keypress problem but that also seems to no longer happen.
 
I just had this issue.
I even tried with another keyboard...which fixed the problem!!!! After some time not finding any issue with the funky keyboard (or the keyboard cable). I removed the keyboard encode IC, sprayed the socket with deoxit and put it back...that fixed it!
Weird!
 
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