Apple II keyboard problem with the Ctrl key

2 replies [Last post]
Joined: Aug 4 2015
Posts: 31


I need help with the keyboard on my Apple II europlus.
It behaves as if the Crtl key is always pressed.
When I run the keyboard test program from an Apple Diagnostic diskette all keys work as they should, except for the letter keys A-Z.
The A-Z keys work as if Ctrl is always pressed.
With the ] prompt pressing G sounds the bell as if I pressed Ctrl + G.

The keyboard is the same model in this picture with a seperate encoder board.
(I googled this picture so you could see it.)

I only have a basic knowledge about electronics, so please bear with me.

The keyboard itself is okay I think.
I found this this schematic where I can see that pin 3 and 23 is connected when Ctrl is pressed.
I used a multimeter set to read resistance and measured pin 3 and 23 on the keyboard.
When pressing Ctrl key the resistance between 3 and 23 drops to zero and when releasing Ctrl key resistance goes to infinite again.
So I think the Ctrl key itself is working, right?

When I measure pin 3 and 23 on the encoder board there seems to some connection, the multimeter reads some resistance.
This is the same if the encoder board is attached to the keyboard or is removed.

Is the encoder board faulty or could it be a problem on the main board?
Where is the problem?

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speedyG's picture
Joined: Nov 16 2011
Posts: 2376
Re: Apple II keyboard problem with the Ctrl key

Hello tolderlund,
it sounds like the chip U2 ( refering to the circuit plan ) is faulty.
That 7400 chip ( on the decoder board ) should become replaced.

sincerely speedyG


In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

Joined: Aug 4 2015
Posts: 31
Re: Apple II keyboard problem with the Ctrl key

Thanks speedy, I will order a couple of chips and see what happens.