A2S1 Keyboard nightmare

7 posts / 0 new
Last post
Offline
Last seen: 6 days 9 hours ago
Joined: Nov 24 2013 - 13:23
Posts: 216
A2S1 Keyboard nightmare

Guess I had it coming.

I realized I had the materials to make my own keyboard ribbon cable to replace the broken one that came with this A2S1 keyboard. No problem - followed the wiring schematics I had.. etc. But, alas, didn't work. So I switched back to the cable from my II+ and to my horror, none of the keys were working except Ctrl-Reset.

After a bit of playing around and reading schematics, I ended up replacing the 7400 and 7404 nearest the keyboard cable socket. I managed to restore 60% or so functionality.. most keys working but some producing wrong characters. Then I made a fatal mistake - I did the piggyback test (which had helped me nail down the 7400) on the 555 nearby.. and boom. Back to where I was. Now nothing except CTRL-RESET works. Compounding my error, I went to replace the 555 with another of a bunch I had kicking around.. and the pad around two of the chip legs simply fell off! So now I'm not sure if my 555 is a problem, or if it's not getting proper contact. Doing continuity testing. But for all I know, the keyboard controller could have blown.

Anyway, have learned an expensive lesson I guess. Question is -- what are my odds of finding a replacement A2S1 keyboard PCB if all else fails?

Offline
Last seen: 15 hours 8 min ago
Joined: Oct 9 2011 - 12:54
Posts: 1241
Re: A2S1 Keyboard nightmare

First of all, the components on the encoder board are off the shelf parts...

So as long as you have a PCB your cool. As for the encoder being blown. It happens but not as frequently as the TTL chips on the board.

Do you have an oscilloscope, logic probe or logic analyzer. You can test it all that way to make sure when you press a key it appears on the output of the encoder. You can check the 555 and everything else without crazy hacking. If your keyboard cable isn't long enough you may have to take the case top off your apple to be able to plug the keyboard into the motherboard and still press keys and probe the encoder board.

If you don't have a scope, at minimum you should think about acquiring one off eBay. As a novice it may be easier to get one of the scopes you can use your PC with so you can capture long traces.

There are a lot of videos on YouTube to teach you how to use a scope.

Cheers,
Corey

Offline
Last seen: 6 days 9 hours ago
Joined: Nov 24 2013 - 13:23
Posts: 216
Re: A2S1 Keyboard nightmare

Thanks Corey,

I'm hoping it's just a case of accidentally blowing the 555 and because of the problems I had desoldering that I simply haven't been able to get a proper connection. The pads on one side are pretty much gone.. they came up when I used my desoldering tool.. I think just age. I have a whole bunch of 7400s and 7404s and tried the piggyback trick with those to see if I could get a reaction but no dice. Wish I had another keyboard of this type to test the encoder.

I'm investing in a scope.. do you have any you recommend?

Offline
Last seen: 15 hours 8 min ago
Joined: Oct 9 2011 - 12:54
Posts: 1241
Re: A2S1 Keyboard nightmare

I hear good things about Rigol.

I personally have a Chinese knock off analog scope and two identical hand held protek high end Digital storage scopes, long story how I wound up with two for a decent price.

To be honest lately I have been using my 16 channel Saleae Logic16 more than my scopes and using the scopes for more analog type signals or to look for noise on lines or supplies, but I also have been working on non-apples that aren't as strait forward for the past few months as my troubleshooting projects.

Cheers,
Corey

Offline
Last seen: 6 days 9 hours ago
Joined: Nov 24 2013 - 13:23
Posts: 216
Re: A2S1 Keyboard nightmare

Well.. i did my best to repair the connections to the 555. As far as i can tell.. continuity is ok.. voltage is about 5v.. ground is ok.. all thetraces make the correct paths and I know the 555 itself works. But still zero. The only thing I'm unsure of.. on the backside it almost looks like there was a trace between pins 6 and 7? I couldnt tell so i didnt reconnect.

Just so i'm clear.. a 555 is a 555 right? I had signetics ne555n sitting around. The chip that was in there was an lm555.

I'm assuming without a probe or scope there's no further testing I can do to have at least some idea if the other chips are working?

Offline
Last seen: 15 hours 8 min ago
Joined: Oct 9 2011 - 12:54
Posts: 1241
Re: A2S1 Keyboard nightmare

The 555 is only used for the repeat clock. If your keyboard doesn't work at all, it's not the 555.

Cheers,
Corey

Offline
Last seen: 6 days 9 hours ago
Joined: Nov 24 2013 - 13:23
Posts: 216
Re: A2S1 Keyboard nightmare

Ok well that makes me feel a bit better.. i thought it might be the ttl chips again.. but the piggy back trick I used didnt produce any behaviour changes this time. I found a single encoder chip on ebay.. so ive secured that in case. When i did have it sort of working.. it was interpreting most of the middle rows ok but numbers and keys at the fringes were coming up wrong. Then i piggybacked the 555 and that was it. So maybe coincidental.

Log in or register to post comments