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CPC 6128 BAD KEYBOARD

Maybe bad sn74ls145 :(
Already we are getting desperate and are going to replace something that is not faulty 😞!

Sometimes you don't get everything set out for you in the manual in a form that is useful. Sometimes this is by accident, other times it is deliberate.

Now you have to put some effort in I am afraid.

Get the existing keyboard membrane and trace out (and document!) which keys each connection goes to. You should be able to do that visually by following the flexible PCB tracks. Each key will sit at the intersection between two (2) lines.

For each key on the keyboard, work out which two connections it goes to.

For example, let's pick the letter 'A'. Follow the two sides of the 'A' switch until you find the two connections it goes to, then apply your resistor to the corresponding points on the PCB connectors and you should get an 'A'. My guess would be (if you don't) that the two cables are probably plugged in the wrong way round...

Dave
 
And from the shift key it will go to somewhere else. Don't forget that the flexible PCB may be double sided...

If in doubt, use your friend the multimeter set to read conductivity...

Dave
 
Ok keyboard repaired! This is a double foil membrane, i opened this two foil and i cleaned with a contact cleaner inside... Now it works fine!
 

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I have found that the A key should be connected to the D, F, J, K, :, SHIFT, and numeric pad keys 1 and 3 in one direction (as well as one pin of the connector).

In the other direction, the A key should be connected to the Z, CAPS LOCK, TAB, Q, ESC and the number keys 1 and 2 (as well as another pin of the connector).

This forms one row and one column of the keyboard. One of the connector pins should be connected to the output of the '145 decoder/driver and the other connector should go to the input port of the sound generator chip.

Dave
 
I have found that the A key should be connected to the D, F, J, K, :, SHIFT, and numeric pad keys 1 and 3 in one direction (as well as one pin of the connector).

In the other direction, the A key should be connected to the Z, CAPS LOCK, TAB, Q, ESC and the number keys 1 and 2 (as well as another pin of the connector).

This forms one row and one column of the keyboard. One of the connector pins should be connected to the output of the '145 decoder/driver and the other connector should go to the input port of the sound generator chip.

Dave
unfortunately I have already closed everything, I had to insert plastic shims in the connectors where the membrane is inserted. They were too loose and sometimes didn't make good contact.
 
As a bit of homework, can you download the datasheet from Texas Instruments for the SN74LS145 (IC101) and tell me how it works?

When you have done that, we will devise a test for it and we can check it out with your oscilloscope to see if it behaves correctly...

The same (or a similar device) exists on the PET (and numerous other computers), so this knowledge is transferable.

Dave
 
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