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Hi guys,
I've managed to salvage my keyboard somewhat. I discovered the 7400 near the space bar was toast, so I did a very careful replacement (socketed). I didn't have any more 7400s so I used a 74SL00 as it seems to be close enough from what I can figure out. Anyway, immediately there was life again. I think the encoder chip is ok. But a lot of the keys produced the wrong outputs. Thinking it might be my bad hash job of replacing the 555, I removed the 555 chip completely (leaving the socket in) just to see what'd happen. The entire top row started working again. But as you can see by the attached pic, we still have problems in the lower rows. In that pic, I run through each key in the four rows starting with the numbers. I'm trying hard to understand the schematic but it's still kind of greek to me.
I suppose it could be the chips I'm using, since they're SL rather than just 7400s. But I don't think so. During the desoldering of the 7400 at U1 and the 555 the keyboard pads flaked off in a few places (age, it was in bad shape there) and I think my attempt to re-link the correct traces is where I've fallen down possibly. I'm wondering if anyone has a good hi-res shot of the back and front of the PCB (with traces visible if possible) that I could use as a reference to see where I might have gone wrong. I feel like there are some traces passing through/around the 555 that might be broken or that I might have wired wrong. If I can solve this, then all I need are a few replacement keyswitches and I have a fully functional A2S1.
Much appreciated if anyone has.
Using a table given to me in another forum, I looked at each non working key and it appears output 5 is stuck high. That one bit being high changes each key pressed into the one that actually comes up.
I wish I could read schematics better.. I've rotated chips around without getting any change.. now mind you, these are LS chips so maybe something is different there. I'm thinking maybe a short somewhere in my effort to repair the keyboard?
I figured it out!
Finally somewhat figuring out how to read the schematic, thanks to help from a fellow on another forum, I found that output 5 ran through pin 2 of the 7404. I then looked at pin 2 and discovered it was one of the ones where the pad had broken off. It had no connection to the trace that led up to the output cable socket. So I exposed a bit of trace and then extended a 'string' of solder out from the pin to it. Voila! It worked!
Now I just need a few keyswitches and this thing is fully functional (except for Repeat key.. I'm not sure if I want to go to the trouble of solving that one. That, I would probably need the pictures for if anyone has them.
Oh and by the way, the 74LSXX chips used in the II+ keyboard DO work as replacements for the 74XX chips in the original Apple II keyboard. That's what is in there now.
Thanks!!
IIRC, the LS is for Low power Schottky.. It should be fine, unless Slower Speeds is a necessity..
I kind of assumed there wouldn't be too much difference, given that the II+ keyboard uses them.
Now I just need to replace a few more keyswitches, and this II is done!
All the the repeat key does, is enable the 555 timer circuit to start clocking the repeat circuit. Besides a bad keyswitch, connection or 555 timer, I have seen the capacitor in the 555 timing circuit fail, which stops it from working. It is labeled C2 in at least one schematic that I have seen. It is an ordinary .1uF cap.
regards,
Mike W.