Author Topic: CPC 6128 stays black  (Read 1306 times)

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Offline msmthng

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CPC 6128 stays black
« on: 15:50, 02 November 17 »
The Monitor stays black if i turn on my 6128. The Power Led red, so power is available (Monitor etc. is working because iam able to use it with another CPC).


I have tried to exchange the Z80 (after disambling I thought yes this could be the issues, some pins were broken) but it is also not working with the exchanged one.


What else can should be checked for (and how) functionality.


thx




Offline Audronic

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Re: CPC 6128 stays black
« Reply #1 on: 23:48, 02 November 17 »
Can you Take a Photo of the Screen and Post it here Please.
Thanks   Ray
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Offline Token

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Re: CPC 6128 stays black
« Reply #2 on: 23:51, 02 November 17 »
Do you have sound when you press DEL?

Offline Tai

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Re: CPC 6128 stays black
« Reply #3 on: 12:56, 12 November 17 »

The Monitor stays black if i turn on my 6128. The Power Led red, so power is available (Monitor etc. is working because iam able to use it with another CPC).


I have tried to exchange the Z80 (after disambling I thought yes this could be the issues, some pins were broken) but it is also not working with the exchanged one.


What else can should be checked for (and how) functionality.


thx





Have you been able to fix it?


One of my 6128 has the same behaviour. Black screen on power on and no sound when you press DEL. The Z80 was dead but swapping it with a working one didn't solve the problem. I also tested its 40010 on a working 6128 without problems. The Vcc lines on the main ICs look ok (4.8V).


What else should I check?

Offline Bryce

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Re: CPC 6128 stays black
« Reply #4 on: 09:32, 13 November 17 »
I've repaired msmthng's 6128 and it was just the Z80. Possibly his attempt didn't work because the socket contacts were too oxidised, or maybe the other Z80 was the one from his 464 which also seems to be broken.

Bryce.

Offline Tai

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Re: CPC 6128 stays black
« Reply #5 on: 00:27, 15 November 17 »
Nice tip Bryce.


I did a continuity check on all pins from the Z80 socket to the backside of the PCB and all is good. I also tested three working Z80s with no luck...


┬┐Any other ideas?


Thanks!

Offline Bryce

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Re: CPC 6128 stays black
« Reply #6 on: 09:35, 15 November 17 »
Is it giving any flicker or sign of life on the screen? Did the CPC suffer an over voltage?

What other test equipment do you have available?

Bryce.

Offline Tai

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Re: CPC 6128 stays black
« Reply #7 on: 21:24, 15 November 17 »
There is no sign of life on the screen. It's similar to turning on the CPC with the Z80 removed.
I don't think that the CPC suffered an over voltage. I'm not sure but maybe the problem was due to a short in the back of the MX4 board.


What other test equipment do you have available?
 
I only have a multimeter  :(


Thanks!!

Offline Bryce

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Re: CPC 6128 stays black
« Reply #8 on: 09:30, 16 November 17 »
Ok. Then test pins 39 and 40 (voltage between each pin to gnd) of the CRTC and tell me what you get => This is H-Sync and V-Sync - If either are 0V or 5V it means that the CRTC is most likely damaged.

Also test the voltage on pin 6 of the CPU => This should be around 2.4V which will tell you that the CPU is getting a clock signal.

Bryce.

Offline Tai

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Re: CPC 6128 stays black
« Reply #9 on: 21:22, 16 November 17 »
These are the voltages readings on those pins:
  • CPU pin 6 -> 2.39V
  • CRTC pin 40 -> 3.5V
  • CRTC pin 39 -> 0.07V
What do you think?


Thanks!

Offline msmthng

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Re: CPC 6128 stays black
« Reply #10 on: 21:49, 16 November 17 »
I've repaired msmthng's 6128 and it was just the Z80. Possibly his attempt didn't work because the socket contacts were too oxidised, or maybe the other Z80 was the one from his 464 which also seems to be broken.

Bryce.


It was one working z80 i had on spare, maybe the socket. What do you to remove oxidation on such parts. And thx for repairing.

Offline Bryce

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Re: CPC 6128 stays black
« Reply #11 on: 09:56, 17 November 17 »
These are the voltages readings on those pins:
  • CPU pin 6 -> 2.39V
  • CRTC pin 40 -> 3.5V
  • CRTC pin 39 -> 0.07V
What do you think?


Thanks!

Pin 6 = Good, you have a 4Mhz Clock which means there's a good chance that the GA is good.
Pin 40 = V-Sync seems good too.
Pin 39 = Hmmm, suspect. Without H-Sync your screen will obviously stay black. My suspicion would be that the CRTC has failed, but I can't be 100% certain, it may still be something else.

I can't remember for sure, so a question to our CRTC coders = Does the CRTC send out a H-Sync and V-Sync signal before the CRTC has been fully set up, or do these ouputs NEED to be initialised before they start sending anything? I'm pretty sure they default to something as soon as the CRTC has power, but I can't remember.

Bryce.

@msmthng: Contact cleaner such as this one: https://www.reichelt.de/Kontaktreiniger/KONTAKT-60-PLUS/3/index.html?ACTION=3&LA=446&ARTICLE=98070&GROUPID=4069&artnr=KONTAKT+60+PLUS
I always clean all IC sockets before I start looking for any further issues. Quite often, especially on older computers when they were stored outside, it's all that was needed.

Bryce.

Offline Tai

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Re: CPC 6128 stays black
« Reply #12 on: 21:01, 20 November 17 »
Pin 6 = Good, you have a 4Mhz Clock which means there's a good chance that the GA is good.
Pin 40 = V-Sync seems good too.
Pin 39 = Hmmm, suspect. Without H-Sync your screen will obviously stay black. My suspicion would be that the CRTC has failed, but I can't be 100% certain, it may still be something else.
 
The GA is good. I tested it on a working PCB without problems.
To test the CRTC I have to desolder both chips (working PCB and no working PCB), buy some 40-pin sockets and solder them. It's going to take a while  ;)


Thanks!

Offline arnoldemu

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Re: CPC 6128 stays black
« Reply #13 on: 21:34, 20 November 17 »
I can't remember for sure, so a question to our CRTC coders = Does the CRTC send out a H-Sync and V-Sync signal before the CRTC has been fully set up, or do these ouputs NEED to be initialised before they start sending anything? I'm pretty sure they default to something as soon as the CRTC has power, but I can't remember.

At reset all the crtc registers are 0.

If they are not setup by the firmware then there will be no hsync, or maybe a continuous hsync with no transitions, the same is true of the vsync.

I guess it'll be border, but the border is also defaulted to black on CPC.

Increasing the brightness may see the hsync vs border colour, but likely that it'll just be black because the monitor/tv will not sync to it.
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Offline Bryce

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Re: CPC 6128 stays black
« Reply #14 on: 22:19, 20 November 17 »
I've had systems where the H-Sync is missing completely and the V-Sync was outputing a random sawtooth looking signal. In both cases the CRTC had failed. However, I've just booted a CPC with a known bad GA and both the H and V-Sync were steady state outputs.

My bet is still that the CRTC is dead, however, I wouldn't try removing a good CRTC from a CPC, the chances of damaging the CRTC and/or the PCB is pretty high. Only remove the suspect CRTC and replace it (with socket) with a new one (readily available on ebay).

Bryce.