I posted some years back about reviving an old Apple with keyboard problems. I finally got the thing to stop going crazy when powered on (repeating garbage characters even with keyboard unplugged) by reseating chips!
However, I have two keyboards - one works fine but has some keys that are hard to press/register. The other one I want to fix does nothing at all. The light shines, and the reset button works, but none of the keys work, so I figured it had a blown MM5740AA/C. I found someone selling spare MM5740AA/E chips and bought a few thinking maybe they were compatible, but no luck- same issue.
1) Can the AA/E be used in place of an AA/C?
2) What else on the keyboard itself should I look for that would cause no keys to register?
one of the 74LS04's is frequently blown, if it's plugged in backwards.
It probably was at some point- I will replace them. It appears to have 1x7404 and 1x7400, but there appears to be another chip under the space bar.
As usual, I suggest the excellent Understanding the Apple II. This book contains schematics for the Apple II keyboards.
Concerning your questions :
1) I don't think there are significative differences between these two revisions.
2) Study the schematics carefully.
I also suggest you test the switches on the keyboard with an ohmmeter, just to be sure they work separately from the keyboard controller.
If the controllers match, you can try to swap them.
I have a logic probe, and I can see the signals generated when keys are pressed on the 7400, so it appears the encoder is working. The signals also show up on the 7404. I'll dig into the schematics a little more and make sure the strobe signal is making it across.
Is pin 1 and 2 on the 7400 supposed to be jumped together (GND)? My keyboard looks like original solder mask and these pins are soldered together (blobbed).
Also, when I attach the good keyboard to this board, the keys repeat forever once you press them. I assume from Mike's page that this is possibly a grounding issue. It doesn't happen with the other board, though... so grounding issue is on the main board and not the keyboard.
Can LS or HC TTL parts be subbed in this keyboard?
On Apple II keyboards (except from the very early version), there is no IC on the keyboard itself. All the ICs are on the encoder board.
Concerning the 74LS00 and assuming you have the keyboard shown on page 7-14 of Understanding the Apple II, both pins 1 and 2 are connected together but not to the ground. They are connected to pin 6 of the same chip.
But if you have the second model, on page 7-16, this is a different story.
If you can take pictures of your keyboard, it will help to identify it.
Have you checked if the Repeat key was shortened ?
I can get you a picture later this evening, but this one is an early keyboard - it is all in one with no separate boards. The 5740 is near the bottom right, and above it are a 7400 next to a 7404, and what looks like another 7400 or 7404 under the space bar. It has large old ceramic caps here and there. The keys have a much nicer feel that my other II plus.
You do not have to hold CTRL to reset, and someone has jumped a line from shift key (I think) that goes into the joystick socket (not sure which pin, as it has been disconnected since I bought it).
HelloOkay, I think I see which model it is. It's probably the Shift-Key mod. You will find additional information here.
This might help you: http://macgui.com/kb/article/759
There is a section on the keyboard, page 5.
I replaced the 7404 at U2 but was still missing strobe, so I also replaced 7400 at U1. Now I get something with the new 5740AA/C chips, but the keys are all wrong. I guess it has a different encoding than the Apple keyboard chip. Unfortunately, my original AA/E chip is dead.
Also, with either keyboard on this II, the keys repeat forever after it's on for a minute or so. It's not the REPT key, as I disconnected the 555 and it still does it- it's on the motherboard. Adding extra ground didn't see to help either. What chip heating up/failing would cause key repeat?
just a remark to few points:
the fact that two pins on a 7400 are connected is not everytime a issue to grounding - in earlier days this technic was often used practice so save an extra chip ( 7407 inverter ) by conncting the two pins of a NAND or AND chip just to simulate an INVERTER instead ( with the purpose just to turn the signal around to opposite function in case there where unused gates in a chip ) and putting that to ground you might do a shortcut to a signal.....
and the point of different endings in the mark of the controllerchip relate rather often to be an indicator of the used encoding matrix ( for example us , french, german, japanese or russian keyboard so changing the chip with same id-code but different ending xxxx-e or xxxx-n might just result in using the wrong encoding matrix - for example the difference between
qwertzuiop or instead qwertyuiop and the instead of ; you might get / - so it is a realy important to first examine the related datasheets... they usually explain the difference of the end-markings in a special table...
the repeat might be a result to a timing misbehavior... that might leed up to slowly shift the output of a chip and initiate some kind of swing probaly the same thing might result from a decoupling capacitor.... usualy there are several capacitors put between powerline and ground of the chips to make sure that no noise interference from the powersupply makes the chips not run crazy... and while the powersupply at the apple gets warm it gets more noisy.... and that produces more spikes in the 5 Volt ... this can realy affect the first two rows close to the keybard input - thats the reason that i rather often try to use chips from same manufacturer when replacing a chip.... and taking a close view to the capacitors in the first 2 rows...
Key repeating ended up being the 74LS138 at U12. I swapped it with U11 and the problem went away. Could have just been corrosion or loose, or the problem may be moved now.
Anyway, now I just have wrong keys. I am pretty sure that it's just a wrong encoder, but I think a year ago when I first had the problems with this, the original encoder had wrong keys also, so I may have a stuck key or short. Can you tell from what the keys do?
U=U (hooray I can type that)
Z=Z (I can type this one too!)
Since some of these repeat- it's possible this is not an encoding issue? Maybe there is a short or stuck key?
Thanks for everyone's help. 1978 is almost back.
UPDATE: I checked all the keys and none are shorted. The only oddity I am not sure is normal- the CTRL key does show some connection, not completely open, until it's pressed. All other keys have no connection at all until pressed. Is that right?
that sounds to be correct due to the fact that codes without ctrl-key are to be between 000 and 128 and codes with controlkey are located from 128 to 255. So ctrl-key just adds one upper Bit to all pressed keys. Still rather sure too that the encoder chip is just the wrong version... another possible solution to solve this might be just be to take a hunt for an ancient 3rd party external keyboard for the apple ][ or( europlus with european layout ) in an auction ( like preh or cherry ... ) the hunt for the correct encoding chip might be very much timewaste and very very difficult... unless you have the adress of a former dealershop or you hit a auction like that few weeks ago ( some 5 to 8 weeks ago at ebay ) in which a service technician sold sparepartboxes from the cellar with sparechips included in that boxes...
at least i wish you good luck to solve the problem
That sucks... I really would rather use the original keyboard than use a PS/2 keyboard and a PIC.
First of all - the Apple keyboards that I'm familiar with came with MM5740/AAE - then again, I'm not familiar with any Apple 2 keyboard that has a 74LS138 on it.
Second - littlediode.com sometimes have MM5740s of different varieties.
Third of all - convert your mappings from ASCII to HEX and see if you have a stuck bit, which would indicate a problem after the encoder.
That's a good idea- Thanks Mike.
I don't see a pattern, though. Maybe the last bit is stuck and the inverter is also not inverting the rest? Weird.
D = A
44 = 41
01000100 = 01000001
F = Y
46 = 59
01000110 = 01011001
G = I
47 = 49
01000111 = 01001001
H = U
48 = 55
01001000 = 01010101
J = E
4A = 45
01001010 = 01000101
The keyboard doesn't have an LS138- that problem was on the motherboard (two next to each other in row F, I think, are what I swapped). Also, the replacement chips I found are 5740AA/C, not AA/E, so I guess it is incompatible.
It seems someone tried this before: http://www.applefritter.com/node/7851
I am officially looking for an AA/E chip now- if anyone has any.
Correct... same mystery.... same song... same solution as told here...