CPC acts as if CONTROL pressed.

[m_dr_m]

On my naked CPC 6128 with CTRC mod:
When I press A i get the ASCII code 01 (same as CONTROL A)
When I press B i get the ASCII code 02 (same as CONTROL B)
...

If I try with another keyboard same result, it suggests the problem come from the motherboard.
How can I diagnostic that, or even better, fix it?

May your day be filled with joy and learning.

[Bryce]

Hi,
     It could be just a stuck key, which you can solve by cleaning the membrane. If that doesn't solve it, either the AY or IC101 (74LS145) has failed. The AY is the most likely culprit.

Bryce.

[GUNHED]

The problem is probably located between the keyboard and the PSG.
Maybe some short is on the PCB in that region. Worst case: New PSG needed. But having a looking glass ready to look at the PCB could help - I hope.

Shall the light of CPC help to fix the thing.

[SerErris]

You can test the PSG. 

Remove the keyboard cable from the connector. Then just pull the correct pin of the PSG to +5V and you should see chars popping up on your screen.

You will see all chars of the corresponding input line (actually 10) because you do not simulate IC101 as well. But that is good. 

If you can see "CTRL" chars, then you know it is the PSG - not the keyboard. 

[GUNHED]

It's not the keyboard, see 1st post.

[SerErris]

Yes, that is true - still IC101 is in play.