CPC 464 keyboard mute, monitor shows grid of rectangles red and yellow

[Guiomar]

My CPC bought in 1984 was working fine in 1995 when I stored it. I opened it again recently: the red light goes on the keyboard, but the screen does show any message, but a grid of yellow end red rectangles. Pressing any key of the keyboard does not change anything.
It seems that an electrical component is no longer working, a condenser perhaps?
I would appreciate knowing if any body else had the same problem and it it was resolved.
Thank you

[gerald]

My CPC bought in 1984 was working fine in 1995 when I stored it. I opened it again recently: the red light goes on the keyboard, but the screen does show any message, but a grid of yellow end red rectangles. Pressing any key of the keyboard does not change anything.
It seems that an electrical component is no longer working, a condenser perhaps?
I would appreciate knowing if any body else had the same problem and it it was resolved.
Thank you

Can you post a picture of what you see and tell us which version of the main board (http://www.cpcwiki.eu/index.php/Mainboard_Versions) you have ?
Your problem can range from a defective component to a dirty power on switch.
For the dirty power switch.
- did you try to switch it on and off multiple time, it may get better after a while
- if you have multimeter, can you check the voltage on the main board. An easy place to check it is between pin 20 and 10 of IC108, near the printer port.

[Guiomar]

Thank you for your interest.
Attached are 2 photos of the screen.
Multiple on- and of the switch did not help.
Voltage at pins 10 and 20 of IC 108 is 3.3
Ref number are: K31-45 and PTNO Z70100 without any " MCnnnn" code
Regards

[Bryce]

Hi Guiomar,
      I think gerald mis-typed that voltage measurement advice. You should be measuring the voltage between pins 1 and 20 of IC108, not pin 10 and it should be about 5V. The pictures you post suggest that one or more RAM ICs are damaged. Luckily they seem to be all socketed, so just source some new ones and swap them out.

Bryce.

[gerald]

I think gerald mis-typed that voltage measurement advice. You should be measuring the voltage between pins 1 and 20 of IC108, not pin 10 and it should be about 5V.

Typo indeed, but I was thinking of IC106 

The pictures you post suggest that one or more RAM ICs are damaged. Luckily they seem to be all socketed, so just source some new ones and swap them out.

May just be a socket issue. I would first try re-seating them.

Looks like one of the early CPC made.

[Guiomar]

Thank you Gerald and Brice,
My previous measurement was on IC 108 between pins 10 and 20= 3.3 V
I checked IC 106 and I found the following:
Between pin 1 and pins 3, 4, 7, 11, 20 = 0 V
Between  pin 1 and other pins= varying between 4 and 5 V depending on which pin.


Regarding the RAM ICs, which are presumably those close to the Z8000, they are indeed socketed and they are well in their sockets. Before I go buying those ICs why would they be the cause of the malfunction? There are 8 of those ICs and surely not all of them have a defect? Wouldn't there be some response with only a few defectuous.

I had some sort of graphic response when I was touching pin 1 of IC 106, either with the negative or the positive probe. From a screen without any graphics, just by touching pin 1 I had the blue and yellow rectangles grid, then by further touching the pin, they turned red and yellow. By touching again they disappeared and I had a "rolling" screen at the frequency of about 10 Hz. Does this mean that IC 106 could be the problem?


Your responses will be much appreciated.

[gerald]

Thank you Gerald and Brice,
My previous measurement was on IC 108 between pins 10 and 20= 3.3 V
I checked IC 106 and I found the following:
Between pin 1 and pins 3, 4, 7, 11, 20 = 0 V
Between  pin 1 and other pins= varying between 4 and 5 V depending on which pin.

In fact we just need one measure of the power supply on board.
either between 1 and 20 on IC108 (HD6846DP)
or 10 and 20 on IC106 (M74LS243P)
or between any of the decoupling capacitor which are behind every IC 

Since you're getting 5V on some means that the CPC is powered properly.

Regarding the RAM ICs, which are presumably those close to the Z8000, they are indeed socketed and they are well in their sockets. Before I go buying those ICs why would they be the cause of the malfunction? There are 8 of those ICs and surely not all of them have a defect? Wouldn't there be some response with only a few defectuous.

Each device handle on bit of the whole 64k range. So one failing and all memory space is failing.

I had some sort of graphic response when I was touching pin 1 of IC 106, either with the negative or the positive probe. From a screen without any graphics, just by touching pin 1 I had the blue and yellow rectangles grid, then by further touching the pin, they turned red and yellow. By touching again they disappeared and I had a "rolling" screen at the frequency of about 10 Hz. Does this mean that IC 106 could be the problem?

Half of IC106 are connected to the Z80 data bus. If you made a short while doing the measure, it's expected that you get some 'response'.
IC1060 is not on the usual suspect list. Also, the fact that you have proper screen setup (visible border and screen area) mean that the data bus is not corrupted.

As I said, since your ram are on socket and your CPC stayed stored for a while it might just be a oxidation issue on the socket.
Just getting the ram out and plug them back could solve the problem.

[Guiomar]

I have taken out all RAM ICs, cleaned their pins and sockets also, and done the same for all Ics with sockets.
Once everything reinstalled things are going better but there is still some way to go. You can see the pictures of the screen which are different every time you switch the computer on and off.
We are getting there...but pressing any key of the keyboard still does not generate any signal to the screen.
My next step is now to source 8 new RAM ICs. It should probably fix the image on the screen but
...What about the mute keyboard?
Thank you in advance for your thoughts.

[arnoldemu]

I have taken out all RAM ICs, cleaned their pins and sockets also, and done the same for all Ics with sockets.
Once everything reinstalled things are going better but there is still some way to go. You can see the pictures of the screen which are different every time you switch the computer on and off.
We are getting there...but pressing any key of the keyboard still does not generate any signal to the screen.
My next step is now to source 8 new RAM ICs. It should probably fix the image on the screen but
...What about the mute keyboard?
Thank you in advance for your thoughts.

In one picture the firmware has booted and has displayed a message, it is the correct mode and colour. BASIC has not started correctly. I think you will not hear the beep when you press a key until basic is running.

[Guiomar]

Thank you arnoldemu. It's good to have some support!
The RAM have been sourced and I should get them by next week...
... I will let  know.

[Bryce]

The fact that the computer has got that far would suggest that the RAM is all fine. I would continue by cleaning the socket pins and the power switch. I don't think any chip is dead at the moment.

Bryce.

[Guiomar]

Thank you Arnoldemu, Bryce and Gerald for your advices. My CPC 464 is running fine. I bought 8 RAM CIs ( at a very good price on EBay Bulgaria) to replace the original ones. When I switched on what a pleasure to see that everything was running fine and the prompt sign blinking. Pressing a  few keys and the characters were on the screen.
Next step was reading a tape. After running it forward and backward to get the stiffness out of the joints, run" and press any key and I could enjoy the galactic sounds of the computer reading the basic language. What a pleasure! Two views of the screen for sharing.
Thanks again,


Bernard

[CraigsBar]

Thank you Arnoldemu, Bryce and Gerald for your advices. My CPC 464 is running fine. I bought 8 RAM CIs ( at a very good price on EBay Bulgaria) to replace the original ones. When I switched on what a pleasure to see that everything was running fine and the prompt sign blinking. Pressing a  few keys and the characters were on the screen.
Next step was reading a tape. After running it forward and backward to get the stiffness out of the joints, run" and press any key and I could enjoy the galactic sounds of the computer reading the basic language. What a pleasure! Two views of the screen for sharing.
Thanks again,


Bernard

Great news. Another rescued cpc.

Sent from my ONEPLUS 3t using Tapatalk