CPC 6128 Plus - keyboard key problem (really weird)

[willow3411]

Hello everyone !
Here is my problem:
I have a CPC 6128 Plus. Everything works very well except: the 5,6,R,T,F,G,V and B keys.
during the test with my diagrom these keys activate the joystick directions and shot 1 and shot 2.
Under basic they write anything.
My tests:
- Keyboard tested on another CPC 6128 Plus: Test ok
Another keyboard on My CPC: Test KO does not work, same problem.
- I changed the AY to an 8912 (removing the 2.2Kohms resistance network): no changes
- I cleaned the keyboard connectors (I read this on the forum): No changes.
So I put back the original AY 8912 /P with the resistance network.
I also just read on the forum that it would also be necessary to remove the resistance network which is original on it. What do you think?
I admit I'm a little lost here.
A little help would be welcome.

I'll give you photos of the results, key by key.

thanks in advance
Willow

[willow3411]

the rest of the photos

thanks
willow

[Kris]

Hi,

A good cleaning of the membrane and:  Réparation du clavier des CPC+ [Quasar Net]

[eto]

I also just read on the forum that it would also be necessary to remove the resistance network which is original on it. What do you think?

Might help. I had to do that on my 6128 Plus. It worked fine but suddenly had some issues. Removing the network resolved it.

[willow3411]

I also just read on the forum that it would also be necessary to remove the resistance network which is original on it. What do you think?

Might help. I had to do that on my 6128 Plus. It worked fine but suddenly had some issues. Removing the network resolved it.

Hi eto

I'll test this tomorrow morning. 
THANKS

Willow

[McArti0]

You can check with an oscilloscope whether there is a 50Hz signal on the Y1-Y10 lines (second keyboard ribbon). You connect the oscilloscope probe between +5V and the next pins of the Y connector.

[willow3411]

I also just read on the forum that it would also be necessary to remove the resistance network which is original on it. What do you think?

Might help. I had to do that on my 6128 Plus. It worked fine but suddenly had some issues. Removing the network resolved it.

hello,

Well I removed the 2.2Ko resistor network.
That didn't change anything .

I took the opportunity to put back a new AY: Same result...
I'm going to take measurements with an oscilloscope but that won't solve my problem....

willow

[ZorrO]

Are you sure nothing is connected to pin 8 on second joystick port?

[willow3411]

Are you sure nothing is connected to pin 8 on second joystick port?

hello,
no I don't have anything connected to it.
Could it be defective?

thanks
willow

[Bryce]

Are you sure nothing is connected to pin 8 on second joystick port?

hello,
no I don't have anything connected to it.
Could it be defective?

thanks
willow

Hi Willow,
      there's a bunch of small diodes between the AY and the ports. Check that these are all still working and aren't shorted anywhere.

Bryce.

[willow3411]

Are you sure nothing is connected to pin 8 on second joystick port?

hello,
no I don't have anything connected to it.
Could it be defective?
thanks
willow

Hi Willow,
      there's a bunch of small diodes between the AY and the ports. Check that these are all still working and aren't shorted anywhere.
Bryce.

hi Bryce,
I just tested them all. 
they are ok

so still the same 

thanks 
willow

[willow3411]

Are you sure nothing is connected to pin 8 on second joystick port?

hello,
no I don't have anything connected to it.
Could it be defective?
thanks
willow

Hi Willow,
      there's a bunch of small diodes between the AY and the ports. Check that these are all still working and aren't shorted anywhere.
Bryce.

hi Bryce,

just one question:
Couldn't this come from IC108?
the 74HC4051N?

[Bryce]

Are you sure nothing is connected to pin 8 on second joystick port?

hello,
no I don't have anything connected to it.
Could it be defective?
thanks
willow

Hi Willow,
      there's a bunch of small diodes between the AY and the ports. Check that these are all still working and aren't shorted anywhere.
Bryce.

hi Bryce,

just one question:
Couldn't this come from IC108?
the 74HC4051N?

I don't have access to the service manual at the moment. Are the non-working keys all in a row or all part of a column? That will tell you whether it's IC108 or the AY that's the cause.

Bryce.

[willow3411]

Are you sure nothing is connected to pin 8 on second joystick port?

hello,
no I don't have anything connected to it.
Could it be defective?
thanks
willow

Hi Willow,
      there's a bunch of small diodes between the AY and the ports. Check that these are all still working and aren't shorted anywhere.
Bryce.

hi Bryce,

just one question:
Couldn't this come from IC108?
the 74HC4051N?

I don't have access to the service manual at the moment. Are the non-working keys all in a row or all part of a column? That will tell you whether it's IC108 or the AY that's the cause.
Bryce.

@Bryce
Thank you for the always very good advice.
these are the keys 5 6 R T F G V B which do not work
so I would say a column?

Willow

[GUNHED]

All these keys are in keyboard line 6, there seems to be an interruption of he connection.

See here:
http://cpctech.cpcwiki.de/docs/keyboard.html


Edit: I use this tool to take a look the keyboard matrix:

[willow3411]

All these keys are in keyboard line 6, there seems to be an interruption of he connection.

See here:
http://cpctech.cpcwiki.de/docs/keyboard.html


Edit: I use this tool to take a look the keyboard matrix:

Hi,
so if I understand correctly this can come from NEC 8255?

Willow

[eto]

so if I understand correctly this can come from NEC 8255?

Afaik the Plus does not have a 8255. If I am not mistaken, the ASIC produces the row signals without further support. 

Have you checked the membrane if it's dirty or if there are any cracks in one of the lanes? 

[willow3411]

so if I understand correctly this can come from NEC 8255?

Afaik the Plus does not have a 8255. If I am not mistaken, the ASIC produces the row signals without further support.
Have you checked the membrane if it's dirty or if there are any cracks in one of the lanes?

Hi eto,
yes I think he confused it with a CPC 6128 Old...
Yes, I tested my keyboard on another CPC: it works very well (as indicated in my first post).
I'll check with the IC108 (as indicated by Bryce).
Otherwise it's ASIC  no?
willow

[willow3411]

Are you sure nothing is connected to pin 8 on second joystick port?

hello,
no I don't have anything connected to it.
Could it be defective?
thanks
willow

Hi Willow,
      there's a bunch of small diodes between the AY and the ports. Check that these are all still working and aren't shorted anywhere.
Bryce.

hi Bryce,

just one question:
Couldn't this come from IC108?
the 74HC4051N?

I don't have access to the service manual at the moment. Are the non-working keys all in a row or all part of a column? That will tell you whether it's IC108 or the AY that's the cause.

Bryce.

@Bryce 

I already changed the AY. 
I will test the IC108 (I received the part today)
willow

[eto]

That will tell you whether it's IC108

Just for curiosity and learning: How can IC108 cause this? I don't see any connection to the keyboard pins but only to the MUX pins of the ASIC. 

[eto]

Otherwise it's ASIC no?

Might be... but I have no personal experience with that kind of fault on a Plus, Bryce is definitely the most competent person based on his experience. 

I would probably have done those checks in addition to the checks you already did: 

• I would probably first check if there is a proper connection between the ASIC pins and the keyboard connector.
• If a single pin is not properly connected, somewhere must be a physical issue.
  
• If all pins are properly connected I would carefully check if I get a signal on pin 75 on the ASIC (row 6) and compare it to e.g. pin 74. Very careful to avoid shorts. Not sure how problematic a short is.
• If there is no signal only on pin 75, then I would expect it's the ASIC - and ask here/Bryce for confirmation or further help ;-)  

[willow3411]

Otherwise it's ASIC no?

Might be... but I have no personal experience with that kind of fault on a Plus, Bryce is definitely the most competent person based on his experience.

I would probably have done those checks in addition to the checks you already did:

• I would probably first check if there is a proper connection between the ASIC pins and the keyboard connector.
• If a single pin is not properly connected, somewhere must be a physical issue.
  
• If all pins are properly connected I would carefully check if I get a signal on pin 75 on the ASIC (row 6) and compare it to e.g. pin 74. Very careful to avoid shorts. Not sure how problematic a short is.
• If there is no signal only on pin 75, then I would expect it's the ASIC - and ask here/Bryce for confirmation or further help ;-) 

Testing with oscillo ?

[GUNHED]

All these keys are in keyboard line 6, there seems to be an interruption of he connection.

See here:
http://cpctech.cpcwiki.de/docs/keyboard.html


Edit: I use this tool to take a look the keyboard matrix:

Hi,
so if I understand correctly this can come from NEC 8255?

Willow

Well, since the keyboard itself works, the problem seems to be "before" the keyboard connector of the 6128plus.

Maybe a broken trace between ASIC and the keyboard connector. You could take a look using a looking glass or a good near-distance photo.

Good luck! 

[Bryce]

so if I understand correctly this can come from NEC 8255?

Afaik the Plus does not have a 8255. If I am not mistaken, the ASIC produces the row signals without further support.
Have you checked the membrane if it's dirty or if there are any cracks in one of the lanes?

Hi eto,
yes I think he confused it with a CPC 6128 Old...
Yes, I tested my keyboard on another CPC: it works very well (as indicated in my first post).
I'll check with the IC108 (as indicated by Bryce).
Otherwise it's ASIC  no?
willow

Yes, sorry, I confused it with the Classic CPC. I was answering from my phone and without schematics. If it's a Plus, then you need to probe the ASIC pins that create the keyboard scan. However, if one of those pins has failed (which is something I've seen before) then there's not much you can do to fix it.

Bryce.

[McArti0]

You can check with an oscilloscope whether there is a 50Hz signal on the Y1-Y10 lines (second keyboard ribbon). You connect the oscilloscope probe between +5V and the next pins of the Y connector.

y1-y10 are wires from asic.