CPC6128 faulty keyboard, suspect bad AY-3-8912 input BIT2

[richud]

Hi,
I have an intermittent keyboard failure on my CPC6128, often the Escape, E, R, O, U  etc. (everything on BIT2) keys fail to work.
From a cold start (as in unused overnight) the keyboard usually is ok for maybe 10 mins, but then the above usually, but not always happens . 
Even then powering off for hours , so its back to room temp, those keys still won't work. A bit strange.
(It used to also turn on sometimes with multiple key sequences flying across the screen with no keyboard presses, although hasn't done this since getting it out again this year to try and fix it.)

Anyway at the point of the O,U,R etc. NOT working - 
 - scope on IC101 (75LS145)  pins 5,6,7  - O, U and R all register on the scope ok so assume 'lines' are good
 - scope on IC102 (AY-3-8192)  pin 12, O,U,R register on the scope ok  , so assume BIT2 is good to this point - and thus assume keyboard is physically ok. 
      However the output lines, pin 21 to 28 ; R , O , U etc, the scope shows nothing at all when they are pressed - whereas any other key being pressed shows, on any of those pins, as a signal.

I presume therefore my AY internally is somehow 'ignoring' any pin12(BIT2) signal and the AY is defective and the cause of the problem - but as they seem to be expensive and hard to come by, could anyone concur with me on this? - as in there isn't some strange fault of the Intel 8255 (or something else) that could somehow making the AY appear broken?

Many thanks for any advice - I'd rather have a second opinion before I have to scrap a 464 for its AY to fix this one
Cheers,
Rich

[Richard_Lloyd]

Take a look at this - it might be your problem.

http://quasar.cpcscene.net/doku.php?id=electronique:reparation_clavier_cpc_plus

Good luck. Richard.

[richud]

Hi, 
Merci for your reply  - sadly there is no resistor network with my non CPC+ AY to remove

[GUNHED]

Well, maybe this one could be of help...


https://www.youtube.com/watch?v=uiF6vFeZHdk

[eto]

Just to be sure it's not a simple fix: did you clean the membrane?

There's also an adapter to fit a AY-3-8910 into the CPC6128. If you have a socket and/or pin headers laying around, this might be an option. 

If you are in the UK, there is an offer on Ebay for a AY-3-8910A for just £15,00 + p/p. Compared to any other solution, this is a good deal. However since it's an A, you need to also add the resistor array to the board. 

[richud]

I swapped AY-3-8912 with one from CPC464 and 6128 keyboard then fine....

If this helps someone in future....

Symptoms:
Occasionally on first power on, garbage characters across screen, almost always went away after one power cycle.
99.9% of the time , all keys work fine for ~10 mins.
After ~10 mins the keys in Bit2 line fail to generate anything onscreen (or cause a beep if the buffers full).
In all circumstances, resistance test between the two keyboard chips show 100 - 130 Ohm resistance for any random key. 
In all circumstances, oscilloscope shows all key-presses show activity on chips.
Freezer spray on AY didn't effect anything once problem manifested. (seems odd?)
Leaving it off for a few hours resulted in keyboard working again.

Conclusion:
Bad AY-3-8912 chip (heat sensitive? on Bit2)