Faulty Keyboard Decoder on europlus - only CTRL

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Joined: May 13 2015
Posts: 16

Hi,
I'm refurbushing a 1980 II Europlus. Re-capped PSU working fine now, Applesoft coming up. I've spent a long time cleaning the keyboard, using contact cleaner and lots of key presses. I now seem to have a fully functional keyboard... But - CTRL-C, or CTRL-anycharacter doesn't work. CTRL-RESET works fine, so it's not mechanical.

I've traced through the circuit, and the CTRL key feeds into a NAND gate, whose output feeds the AY-5-3600-931-B6 encoder.

So, I'm pretty much convinced the encoder is faulty. Two questions for you, my esteemed technical friends;

1. Has anyone got any other ideas?

2. Does anyone have a AY-5-3600-931-B6 for sale? Or know where I can get one? I'd like to keep machine as original as possible.

Cheers,
Con

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Offline
Joined: Jun 15 2014
Posts: 305
Re: Faulty Keyboard Decoder on europlus - only CTRL

What happens if you press only the reset key? Can you check the voltage/logcal level change at pin 11 of the 7400 or at pin 28 of the AY-5-3600 when you press and release the CTRL key? What about voltage change at the input pins of that NAND (pin 12 and/or pin 13)? It is highly unlikely that the AY-5-3600 encoder is faulty.

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Joined: May 13 2015
Posts: 16
Re: Faulty Keyboard Decoder on europlus - only CTRL

Hi, I did indeed do that. Pins 12 / 13 get pulled down to 0.02V when I press the CTRL key, and the output, pin 11 hits 5.3V. Which is odd, because Vcc is 5.01V. Pin 28 of the AY-5-3600 also hits 5.3V when I press CTRL. Also, Ctrl-Reset works just fine - ie it resets the CPU.

I'm glad to hear you say that it's unlikely to be the Encoder. Looking at the Block diagram for it, it seems that Shift and Ctrl feed into a mode block, before being passed on to the ROM.

I'm not sure what to do next, as I don't have another encoder around. (Yet). I was thinking using a scope triggered by the Strobe output to check each of the 7 output pins when CTRL-C is pressed. Maybe there's a short someplace. But, every other key checks out. This def wouldn't be the case if a line was stuck at 1 or 0.

Any further thoughts? Even on next steps? I appreciate your help.

C.

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Joined: Jun 15 2014
Posts: 305
Re: Faulty Keyboard Decoder on europlus - only CTRL

Would you describe better

Quote:

CTRL-anycharacter doesn't work

?
Do you see the character typed as if no CTRL key is pressed or nothing is typed when you press CTRL-any key combination? You should also check the cable and higher bits from the data bus output, and the 7404 and 7400 inverters of the data bus...

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Joined: May 13 2015
Posts: 16
Re: Faulty Keyboard Decoder on europlus - only CTRL

Hi George, of course. Basically, the CTRL key has no effect on characters typed. So, if I hold CTRL and press C, I just see a C on the screen. Same for all other characters. And, as I mentioned earlier, if I have the toggle switch set to CTRL-RESET, the CTRL key does indeed cause the reset key to work.

I'm keeping a watchful eye on a pot of chilli, but I hope to get back to debugging this within the hour. I will check the cable, and the high bits, as per your suggestion, but the characters "0" .. "9" and "A".."I" all work fine, which as you undoubtedly know are Hex $30..$39 and $41..$49, so the high order bits of the data out are looking good.

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Joined: Jun 15 2014
Posts: 305
Re: Faulty Keyboard Decoder on europlus - only CTRL

I think that with 99% probability the bit6 of the keyboard data is stuck to 1, e.g.:

1. Check wire of pin 7 of the cable.
2. Most likely once upon a time your keyboard cable was incorrectly rotated by 180 degrees in the socoket thus pin 7 went to -12V (pin15) and the NAND of 7400 burned. Check if the corresponding NAND of 7400 is working (pins 1&2 (shorted on pcb) are inputs, pin 3 is the output from that NAND).

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Joined: May 13 2015
Posts: 16
Re: Faulty Keyboard Decoder on europlus - only CTRL

OK, sorry that took so long. I've checked it all thoroughly, end to end. No NAND faults, no cable faults. If I press "C", I get 100 0011 (Hex 43) on the output pins of the AY-5-whatever chip. This correctly works its way along to the output cable. Pressing CTRL-C produces exactly the same result.

UPDATE: I found it, and the reason I persevered was your encouragement that it's unlikely to be the AY-5-XXX chip. The NAND gate feeding the CTRL signal through to the Encoder is faulty. But, but a bizarre coincidence, the value it feeds when CTRL is pressed is .5 V - and my ageing eyes took this to be 5V.

Thank you so much for your kind help George - I'm off to warm up the soldering iron.

Cheers,
Con

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Joined: Jun 15 2014
Posts: 305
Re: Faulty Keyboard Decoder on europlus - only CTRL

You even said

Quote:

Pin 28 of the AY-5-3600 also hits 5.3V when I press CTRL

...Anyway by replacing the 7400 IC you replaced all suspected NANDs Wink So it is good that your keyboard works now.

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Joined: May 13 2015
Posts: 16
Re: Faulty Keyboard Decoder on europlus - only CTRL

Yep, I'm a dope Smile And I haven't desoldered a 74xx chip in over 30 years - I burnt my fingers. Anway, I managed to get a socket in, and all connectivity on the board looks good. Just need to get a replacement 74LS00 tomorrow and test.

Will let you know how it goes, I know you will be worried Wink

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Joined: May 13 2015
Posts: 16
Re: Faulty Keyboard Decoder on europlus - only CTRL

Hey Georgel - I just wanted to thank you for your help and encouragement. I socketed in a 74ls04 and the keyboard is now fully functional. If you ever find yourself in Ireland let me know - I owe you a beer or two.