CPC464 Keyboard issue

Started by wyldcat34, 13:32, 07 June 13

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wyldcat34

Hi Guys,
I'm a noobie here, but thanks for having a great community!  I have poured over some threads here about keyboard issues and have yet to find anything correlating to my issue, so from scratch and quickly here goes.


Turn on the 464 and immediately the following pops up:
98]PIYTWQ followed by reoccuring arrows (right) til I hit break, the weird thing is, all keys apart from these ones displayed work, if i press the keys displayed...nothing!
So am i looking at a chip problem?  I have taken the membrane out and cleaned it, and have also tried another keyboard (I picked up 2x464 and 4x6128 today) and same issue.


I have noticed people talking about the AY and joyport, but so far I have not found anything to clear up exactly what this means! :)


Cheers guys, and if this has been answered before, my apologies!


Regards
Garry


robcfg

Hi Garry, and welcome to the forum!


I suggest you to start with the most basic troubleshooting, that is, opening your CPC464, and checking that the keyboard membrane is correctly attached to the motherboard, and opening the keyboard itself, and checking that there is no dust between the two plastic sheets.


If that doesn't work, then we'd continue trying the peripheral controller and the AY chip.


By the way, some pictures of your board and the are where the mentioned chips are, would be very helpful to try to diagnose the problem.


Regards,
Rob

Bryce

#2
Hi wyldcat34,
          There are several parts other than the membrane that are involved with keyboard input. IC107 (8255 also known as the PIO) produces a four bit scan code, IC101 (74LS145) decodes this into 9 scan lines. The return signals are then read by the AY-3-8912 which is also used to produce sound on the CPC and sends the results back to the CPU via the 8255 again. There are no pull-up resistors involved, so the problem can only be one of these three ICs.

You've said you've already cleaned the membrane and tried other keyboards without success (@Robcfg = Read his entire post before suggesting he does stuff he has done already :D ). The next thing to test is what happens when you turn on the CPC with no keyboard attached? If it is still producing the series of characters, then the AY is usually damaged. However, the characters you've listed aren't all on the same row or column as most keyboard failures tend to display which is quite strange. So I have the feeling that the 74LS145 might be the issue. The other strange thing is that the series stops when you press break. This would suggest that an IC is unstable.

So here's my suggestions:

1 - Check what happens when you turn on the CPC with no keyboard attached.
2 - If the problem still occurs with the keyboard disconnected, swap the AY-3-8912.
3 - Check that there's no loose wire / screw or anything under the PCB that is causing a short circuit.
4 - Check the capacitors associated with the three ICs. These are the small brownish discs at the end of each IC:
     C128 for the 74LS145
     C124 for the AY-3-8912
     C119 for the 8255
     If any of these are chipped, broken or damaged, this could be the problem.
5 - If you still haven't found the problem, swap the 74LS145.

Bryce.

gerald

Quote from: wyldcat34 on 13:32, 07 June 13
Turn on the 464 and immediately the following pops up:
98]PIYTWQ followed by reoccuring arrows (right) til I hit break

Quote from: Bryce on 14:34, 07 June 13
However, the characters you've listed aren't all on the same row or column as most keyboard failures tend to display which is quite strange.

98]PIYTWQ and right arrow are all on bit 3 of AY input port.  (9 and 8 are numpad f9/f8)  ;)

The problem is between the keyboard connector and the 8255. Likely the AY-3-8912.

wyldcat34

Thanks for your input guys, steps so far.


Yes, problem persists without Keyboard membranes plugged in.
The spare 464 boots to a blank solid colour screen.  Thought that it may be the Amstrad or Z80 chips, swapped these over, no difference for either 464.
Have re-soldered all connections from 9 pin port to AY chip (just to check for dry solder joint) no difference.  My soldering skill and chip removal skill leaves a lot to be desired but i will try to swap the AY chips over today and let you know how I go. 


And just a note - is it a mem chip that causes the solid background to display on the other one as opposed to the basic post screen?

Cheers guys
Garry

wyldcat34

OK swapped the AY chip out, destroyed Two tracks under the AY in the process, repaired the tracks, put the other chip in and Blamo!  it WORKS!  Thanks Guys, your all 6 ft Awesomeness!
So just the last thing, any idea on what the solid coloured screen boot is?


Cheers!

ralferoo

How "solid"? Is it a rectangle with a different colour border? If so, it's probably a RAM issue as to get that far the CPU and CRTC must both be working. If it's all the same colour without a border, then it could be a lot of things.

wyldcat34

OK, you turn on the 464 and the screen just lights up a solid colour with black border.  Nothing printed on the screen at all.


ralferoo

The black border is good. That means the CPU, CRTC, ROM are fine. It's probably also a good sign the gate array is good too.

So, I'd check the RAM chips to see if they're bad or any damage to tracks near them. Bryce is the man to tell you about where to get new RAM chips if you need them - I know he put that info another post recently... :)

wyldcat34

Iahve 2 of the same model with the problem - maybe able to frankenstein a going model?


wyldcat34

OK guys, thanks for your help.  Last question.  How do I test the RAM chips so I can swap out a known dead with a known good?  Cheers all.


Bryce

Known deads will usually (but not always) get hot when they are powered.

Bryce.

wyldcat34

Sweet!  Thanks Bryce, time to go get me some 3rd degree burns ;)


wyldcat34

Well had both on for a while, a few of the chips are warm, but really there is nothing hot to the touch.  Hmm, anything else I can try (apart from becoming an electronics technician, as its well beyond me!)


Bryce

Well there's a quick and dirty comparison you can do. It's not 100% confirmation, but it sometimes points out which chip is definitely not good:

Measure the resistance between opposite pins on each RAM IC. So for a 4164/4864, measure between pins 1 and 16, 2 and 15, 3 and 14 etc... on each IC and take note of all the values. Most of the ICs will give very similar values, but a broken IC will vary on at least some of the values compared to the good ones.

This is the only way I know to test them without removing or piggy backing the chips.

Bryce.

wyldcat34

Cool, ill give it a try, thanks bryce


Rabs

#16
Quote from: Bryce on 14:34, 07 June 13Hi wyldcat34,
          There are several parts other than the membrane that are involved with keyboard input. IC107 (8255 also known as the PIO) produces a four bit scan code, IC101 (74LS145) decodes this into 9 scan lines. The return signals are then read by the AY-3-8912 which is also used to produce sound on the CPC and sends the results back to the CPU via the 8255 again. There are no pull-up resistors involved, so the problem can only be one of these three ICs.

You've said you've already cleaned the membrane and tried other keyboards without success (@Robcfg = Read his entire post before suggesting he does stuff he has done already :D ). The next thing to test is what happens when you turn on the CPC with no keyboard attached? If it is still producing the series of characters, then the AY is usually damaged. However, the characters you've listed aren't all on the same row or column as most keyboard failures tend to display which is quite strange. So I have the feeling that the 74LS145 might be the issue. The other strange thing is that the series stops when you press break. This would suggest that an IC is unstable.

So here's my suggestions:

1 - Check what happens when you turn on the CPC with no keyboard attached.
2 - If the problem still occurs with the keyboard disconnected, swap the AY-3-8912.
3 - Check that there's no loose wire / screw or anything under the PCB that is causing a short circuit.
4 - Check the capacitors associated with the three ICs. These are the small brownish discs at the end of each IC:
    C128 for the 74LS145
    C124 for the AY-3-8912
    C119 for the 8255
    If any of these are chipped, broken or damaged, this could be the problem.
5 - If you still haven't found the problem, swap the 74LS145.

Bryce.
Thanks for the tips. I had what I thought was almost the same problem.

That of:
Power on CPC.
Cursor moves continually right until you press enter.
Diagnostics show right cursor key continually on.

So powered on without the keyboard, as you suggested, and no problem, so keyboard problem. After a lot of prodding and cleaning, I fixed the problem.

Thanks again for the tips.

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