Apple //c repeating character issue

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CWJ_Wilko's picture
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Apple //c repeating character issue

I haven't used my //c in a few weeks, and tonight I've booted up and immediately hit an issue. While it will boot normally, a space character will keep repeating over and over, and the computer will not accept any other key strokes.

I have taken the entire thing apart, tried two different external power supplies, reseated all socketed chips etc. I looked inside the internal power supply and couldn't find anything immediately obvious (i.e. caps). I have tested the keyboard itself and the switch for the space bar is working as intended.

At first it was repeating the '1' character. Hitting shift or control would change this to another number, but I couldn't insert any other characters, nor would any other key presses cause a change. I also tested the '1' key switch on the keyboard and it too worked properly.

All disks will boot correctly, but it's stuck with this damned repeating character issue, which seems to be present after booting a disk (i.e. I can't use the keyboard).

Obviously with no keyboard attached the //c doesn't start at all, just brings up garbage on the screen.

Is this problem likely to be the keyboard or something else? I heavily cleaned the keyboard with electronic cleaner spray and dry lubricant months ago, it's the only thing that is currently hanging in my mind at this point. Otherwise the computer has had little work done to it. It was working well a few weeks ago, even following a bumpy move interstate!

EDIT: CTRL Reset and CTRL OpenApple Reset works, as does OpenApple+CloseApple system diagnostics (System OK message appears).

EDIT 2: With seemingly nothing to lose I'm soaking the keyboard in the sink at the moment, soon I'll give it a rinse and let it drip dry for 24 hours before attempting another go.

gsmcten's picture
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Re: Apple //c repeating character issue

CWJ_Wilko,

Character Generator ROM maybe?

Steven Smile

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Re: Apple //c repeating character issue

Do you have a meter, logic probe, or scope? With a meter, you could check the keyboard encoder chip B16 at pin 16 which is the strobe. It should only pulse high when a key is pressed. If no key held down, should be zero volts. Any DC on that pin will cause a key echo.

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Re: Apple //c repeating character issue

Hello CWJ_Wilko,

well after the cleaning procedure also test test the Return key, the arrow up key and the apostroph-key.
These 3 keys also have connection to the same line like the space-key.
Make sure that none of the 4 keys keeps stuck in pressed position.
Check the ceramic disk capacitor C46. It´s value should be 47 pF.
And also check ceramic disk capacitor C45. Its value should be 22nF ( 0.022 or altzernate notation 223 ).
both capacitors are located close to the AY-5-3600-PRO keyboard decoder chip ( 40 pin ).

sincerely
speedyG

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Re: Apple //c repeating character issue

Thanks all.

Based on the above:

Pin 16 on the keyboard encoder is constantly pulled high to 5v (tested against ground on pin 15). If I short these pins out with a screwdriver, the character changes to a '9' for the duration of the short.

If I test continuity using a multimeter between these two pins, the entire board dies into a series of lines on the screen, for the duration of the test (extra volts from the multimeter?). AFTER this, there will be no more repeating characters, but still cannot input characters. I can still use CTRL Reset etc though. This state will remain until power is completely disconnected and left for a period of time, at which point the repeating space character will return.

Tested C46 and C45 and both are holding charge, but not really sure if they are in tolerance or not. As ceramics I think they are probably fine?

I'm strongly suspecting the encoder chip at this point, especially after reading this post from a few years ago: http://www.applefritter.com/content/apple-iie-keyboard-decoder-gone-bad

So it seems like the //c uses the same AY-5-3600 PRO encoder as the //e, is it worth my time swapping these? It's socketed in the //e but not in the //c so seems to be low risk as far as the good chip is concerned.

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Re: Apple //c repeating character issue

Hello CWJ_Wilko,

well the Keystrobe line leaves from pin 16 at the AY-5-3600
and leads to the pin 17 of the 6551 serial chip with labeling of UD9.

Before swapping chips its recommended to examine that line .....

Does it change status if key is pressed and released
or does it change behaviour even though no key is pressed
and thereby causing the repeating of the characters ?
If the line changes but the 6551 does not react
- then the 6551 might be damaged and the AY-5-3600 might be O.K.
- so previous testing is required to decide which chip really is to be swapped....

note: The pulses from the keystrobe are rather short... so probably they might not get detected with a simple multimeter and a osciloscope might be required....

second note: That 6551 is also responsible about the internal modem port.
So it´s recomended to disconnect any device from that port while testing.

and if a chip is to be desoldered then it´s rather good idea to instead insert a socket and get it soldered
and then insert chip in the socket.....
that will make future repair more easy...

sincerely
speedyG

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Re: Apple //c repeating character issue

Thanks as always speedy. I was too impatient and went ahead with the surgery, replaced the AY-5-3600 PRO with a known good version from the //e, and that fixed this issue. Back to full functionality!

That's now at least two experiences where the 3600 has gone bad in the exact same way, myself tonight and that link from a few years ago. Perhaps we will see more of this issue in the future?

You'll also be happy to know that I anticipated your advice, and put a socket in before replacing the 3600. This was my first experience desoldering a chip directly on the board, believe it or not. It went alright, although I lost one of the solder pads off the board. Fixed it with an extra wire soldered direct to the pin though. Was pretty happy with that, but the bad 3600 chip was physically broken by the time I got it off the board (I probably could have been more gentle).

Speedy - seriously, thanks again. You're a human Apple ][ Technical Manual and more. Or some kind of Apple ][ repair search engine.

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