Welcome to these forums.
I see you have a related thread at [
here], which I will also quote from.
Thunderbird 98 said:
At first I thought it was the keyboard itself, but now I know that there is something wrong with the keyboard logic on the processor board.
How do you know that the problem is the 'processor' board, and not the keyboard? To determine that (using no test equipment), you would have had to have used a 'known working' keyboard.
Thunderbird 98 said:
I found a schematic of the 16/2 which I've attached below.
Based on that schematic, the keyboard interface logic on the 'processor' board is part of the 8255 chip, plus what you see at [
here], plus a decoupling cap for each chip. Any of those components could be faulty (assuming the 'processor' board contains the fault). A bad solder joint on the connector is also a possibility.
Thunderbird 98 said:
Someone told me it was bad capacitors in the keyboard logic.
Presumably they included the word "might" or "may". If not, how do they know the problem cause?
Thunderbird 98 said:
The capacitors used in the keyboard logic ...
* According to the partial circuit diagram that I pointed to earlier, C6, C7, and C8, are 47 pF. Likely to be of ceramic type.
* According to the partial circuit diagram that I pointed to earlier, C9 is 10 uF. Likely to be of tantalum or aluminium electrolytic type.
* A decoupling cap for each chip.
Thunderbird 98 said:
... are marked "641" and I assume they are 640pf, ...
See how in the photo, each is located at the end of a chip. C31, for example, is the decoupling cap for the pictured 74LS175.
They appear to be type ceramic of value 640 pf.
Thunderbird 98 said:
... which is an odd size that I cannot find anywhere.
I cannot see how an open-circuit failure of a decoupling cap could cause an always-appearing "check keyboard" error. In my opinion, at worst, maybe the occasional "check keyboard" error.
Thunderbird 98 said:
Any other ideas? I really want to get this working!
What test equipment do you have?