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Apple II+ sudden keyboard death

TMA-1

TMA-1

Experienced Member
Joined
May 3, 2013
Messages
67
Location
Ontario Canada
I have an Apple II+ that has a keyboard which is very nice mechanically and never given me issues on any key. I was using the machine last night, and suddenly the keyboard became unresponsive. No key produces a character on the screen or response from the computer, with the exception of Control-Reset, which still works.

I have tried rebooting of course, removed all the daughter cards, and re-seated all the ROM chips. No change. In reading for answers I finally discovered what that little toggle switch inside is for; it still performs as advertised--controlling whether "Reset" alone performs a reset, or "Control-Reset" is required.

I'm guessing that some character-generation hardware has suddenly gone south on me, but I don't know where to look for it. Are there any likely suspects?
 
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I would suspect the keyboard encoder IC since the failure was so instantaneous. The reset key bypasses the encoder.
 
Thanks retrogear. That was what I was afraid of. It seems these encoder chips are as rare as hens' teeth. I gather others are in the same boat.
 
Briel computers used to make a PS/2 encoder replacement board ("Super Encoder") for the II+ keyboards. I have one, it works great. But it looks like he's out of stock and not likely to produce more.
 
Well, I thought I had lucked out when I read that an AY-5-3600 would replace the encoder chip. There was one rattling around on eBay so I bought it. Alas, it has not restored function.

Now I don't know if the new (old) chip is also bad, (it came in an anti-static sponge, but not in an anti-static bag or shell), an inappropriate replacement, or if something else is amiss on the encoder board. I replaced the only electrolytic capacitor, and the 555 timer. No dice.

There are three other logic chips that I'd prefer to diagnose rather than replace wholesale, but I'm not used to debugging logic, and don't know really where to start, other than poking around with an oscilloscope. Any guidance from others who have looked at this encoder board would be appreciated.

P.S. That Briel board looks ideal; too bad they're not making them anymore.
 

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I don't know too much about the keyboard itself, but you could also check chip B10 on the motherboard. Some of the logic signals for the keyboard go through that chip. I don't know if that chip could cause the issue you describe but its another chip to check besides the encoder.
 
I have a scope on my A2+ standalone motherboard for comparison. B10 pin 11 should pulse high 5VDC with each keypress, turn your sweep rate down slow to 20ms to see it. It's a very narrow tick pulse.
 
That's awesome guys. Thank-you for the bread crumb trail.

B10 pin 11 appears to be 5 VDC high all the time, regardless of whether a key is pressed or not. What does that mean?

I checked again for stuck keys; there are none.
 
I think it's pointing the problem back to the encoder board. Unfortunately, the only II+ encoder board is in my fully assembled II+ which I don't want to tear down. I'm running my out-boarded MB with a PS/2 keyboard converter I purchased.
What is the nnn suffix on your AY-5-3600-nnn ? From what I've researched, if the suffix is PRO it's for the //e and not compatible. Both the AY3600 and KR3600 have internally programmable ROM's and the suffix is the programming.
For anyone reading this, this is pure speculation from reading about the encoder chips. Do you have an oscillator waveform on pin 1 of the encoder IC ?

here's the adaptor I'm using which is on ebay for $38
http://www.ebay.com/itm/PS2-Keyboar...164814?hash=item41a604294e:g:yJgAAOSwh-1W3jvp
 
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Completely understand not wanting to tear down your II+; I appreciate any help you can provide without it, pure speculations included. :)

Interestingly, I don't seem to have a suffix on my AY-5-3600

04 AY-5-3600.jpg

And yes, I do have an oscillator signal on pin 1.

I have come across that eBay item, but haven't ordered it yet. It is my backup plan, as I would much rather restore and use the original keyboard/encoder if possible. The listing says he has a small quantity available at the moment, so I'm holding off for the time being. Nice toy to have though.

Edit: I've ordered the PS/2 adapter, and some 74LS00 and 74LS04 chips. I've also put the original encoder chip back in the circuit to reduce the number of variables. Still hoping its one of the 3 logic chips.

Edit again: Okay, maybe I'm getting the hang of this. Using the oscilloscope as a logic probe, the standing outputs at the 3 logic chips would all seem to be correct,... NAND and NOTs. Darn, I was hoping one or more of those would prove errant. On the encoder IC, I've seen input lines change state when I press a key, but the output lines all seem to be high (except for X9, pin 4, which is low), regardless of key-presses. That points to the encoder itself, doesn't it?
 
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Thanks retrogear. I've inquired. (I don't do Facebook though.)

Poking around with this has been a learning experience. I only wish I knew enough to create a substitute encoder chip out of an Arduino or its engine. There seem to be plenty of smart chips about, that could be programmed to do the same thing this chip does? No doubt that is what the Briel Super-Encoder was; too bad it is not in current production.

I will make do with the PS/2 adapter until I can find, or am knowledgeable enough to make, a replacement. Thanks again all for the help.
 
It seems that somewhere in my reading I picked up bad information.

(1) I read that the encoder chip, which Apple marked 331-0931-B, is under the covers an MM5740/AAE.
(2) I read that the AY-5-3600 is a valid substitute for this chip.
One of these, and possibly both, is false.

The datasheet just posted for the AY-5-3600 vs. the datasheet for the MM5740, shows the pinout is not even the same.
 
Here is the datasheet of the IC shown in the Sam's ComputerFact CC1 encoder board that matches your board photo but like I said, the programming suffix is different: KR3600-070BI
All the suffixes in the links listed point to the same sheet.
http://www.datasheet4u.com/share_search.php?sWord=kr3600
(1) MM5740 is shown in the Sam's as on an alternate keyboard with a separate numeric keypad so doesn't apply for you
(2) AY-5-3600 is for the //e with a separate keyboard ROM so won't work in a II+ (KR3600 has an internal programmable ROM. Notice the datasheet even shows the uv window)
 
Unlike a paperback or a movie where you can see how much time is left to wrap things up, I don't know where or when this saga will end.

Thanks to my good friend Ed in South Dakota, who posted an ad in an antique radio forum on my behalf looking for an encoder chip, I now have a replacement 331-0931-B.

Alas, my encoder board still does not seem to work.

The chip is said to be a working one, and the vendor has been so helpful and supportive that I'm sure it is the case. I think I have to go back to the drawing board with my logic probe and revisit those other 74LS chips for some sort of anomaly. Unfortunately I am travelling this weekend, so it will be next week before I get a chance.

I do know that the motherboard is okay, because the PS/2 adapter from South Korea on eBay works like a charm.

The only sure fire way to tell if it is the chip or the board, is by direct substitution. For that I need another working encoder board. I have one of those coming too, actually, if the threatened Canadian Postal strike holds off.

Thanks Ed. You're a life saver. Two hot leads is two more than I thought I would find.

I'll come back and post my results when I have them,... some day.
 
Hmm. Revisiting my gate-by-gate examination of the chips, I found a couple of cases where the "logic" was suspect. So I broke down and replaced the two 74LS00N NAND chips and the 74LS04N inverter chip. Now, with the new encoder chip in place, the computer produces a "?" when the repeat key is held down. Nothing otherwise. I'm not 100% sure that this is a change in behavior--I just might not have noticed the repeat key behavior before. Unfortunately there's not other hardware to address. Still waiting for my complete encoder board,...
 
A double ?? represents all character bits set $FF so somehow your encoder is strobing out data without a keypress
 
Well, I'm kind of stuck, but in a happier place. The good news is I received a complete encoder board in the mail yesterday. (It was from an Apple user who had replaced his board with an aftermarket board, I think called the Enhancer II.) The complete board works. Whew! I'm going to get my Apple II+ back just as soon as I reassemble it.

The perhaps better news is that using the working board to test the encoder chips I have, the chip I bought on eBay is indeed working also. Somewhat surprisingly, so is the original encoder chip. That clearly indicates something else is wrong with my original encoder board.

That's where I'm befuddled. I have socketed and replaced all 4 "support" chips, 2x 74LS00, a 74LS04, and a 555 timer. I have replaced the electrolytic capacitor with a tested good new one. I have tested the resistors in place. I have verified the far end of the ribbon to the solder contacts on the socket. I have re-soldered all the chips and connector joints. There seems to be nothing else to possibly go wrong, but clearly something has. The new board works. The old board, which had died so suddenly, is no longer producing "?"s with the repeat+key, but rather is back to producing nothing at all. I just don't get it.
 
Double check all your work, or have someone put a fresh set of eyes on it. Either you bridged some solder somewhere, broke a trace, or one of the chips or sockets you replaced are bad...

It's hard to find when you are the one that did it, because you will be "sure" it can't be what the problem really is... if you know what I mean. It happens to everyone.
 
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