Author Topic: CPC464 grey border black screen etc.  (Read 3376 times)

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Offline pledg

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CPC464 grey border black screen etc.
« on: 20:46, 01 December 19 »
 
I have a revision MC0002 D board with the heat-sinked 70008 IC chip in the 40007 slot.
The board is in excellent condition with no sign of any heat damage or any visible broken or corroded traces.
I have dismantled and cleaned the main power switch which was full of oil but for the purposes of testing the board I'm bypassing this switch.
When I first powered up I was getting a black screen with a grey border. On subsequent power ups this changed to a full grey screen and then to just a black screen with some flickering white interference like lines. Then onto no connection at all!
On power up the 7008 chip gets warm and ALL the RAM chips get very warm (HOT) very quickly all at the same time. The Z80 and video chip are warm to touch.
I have swapped in a working Z80 and this has makes no difference.
I have checked the voltage throughout the board and this is correct.
At first I suspected a failing RAM chip but now I'm wondering if the video chip is failing. I have checked the video chip with an oscilliscope and some pins are giving a negative reading leading me to suspect this chip has failed? I have swapped in a working Z80 and this has made no difference.
Am I on the right track?
Any suggestions appreciated.
 
Pledg
« Last Edit: 22:47, 01 December 19 by pledg »

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #1 on: 09:40, 02 December 19 »
Don't worry about the video chip for now. If the RAMs are getting hot, then they are shorting the 5V supply rail which would cause the CRTC to do random things. Swapping the RAM will most likely bring the CPC back to life.

Bryce.

Offline pledg

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Re: CPC464 grey border black screen etc.
« Reply #2 on: 11:21, 02 December 19 »

Hi Bryce, Thanks for your speedy reply. I suppose I was fearing a RAM issue. I think my best bet as I cannot easily diagnose which chip is defective is to remove them all and socket them. I have the sockets and I also have a donor board but probably better to go with a new replacement set from a far away place! Although it won't hurt to swap the one that seems to get really hot the quickest first? Can you recommend any suppliers of these RAM chips. Is there a newer equivalent... TMM4164AP-15???
Thanks again for your help!

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #3 on: 11:31, 02 December 19 »
There's no new equivalent. Ebay is probably the best place to find replacements.

Bryce.

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Re: CPC464 grey border black screen etc.
« Reply #4 on: 12:00, 02 December 19 »
So as long as a chip is labelled 4264-15 it will be compatible? Please excuse my naivety...

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Re: CPC464 grey border black screen etc.
« Reply #5 on: 12:22, 02 December 19 »
4164-15. There are a few other compatible parts, but the 4164 is easily found.


Bryce.

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Re: CPC464 grey border black screen etc.
« Reply #6 on: 11:30, 04 December 19 »

Thanks Bryce...


Before I socket all the RAM in the CPC464, can I piggy back test the RAM if I suspect a certain chip?


Pledg

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #7 on: 11:47, 04 December 19 »
For certain failures (possibly bad, but not getting hot) yes, but if the suspect chip is getting very hot it has a short inside and will "steal" the current that the piggybacked chip needs and the computer still won't boot.

Bryce.

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Re: CPC464 grey border black screen etc.
« Reply #8 on: 11:56, 04 December 19 »

Thanks, I'll remove them all...


Pledg

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #9 on: 12:31, 04 December 19 »
It's known that Amstrad (and other computer manufacturers of the time) had no problem using parts from the "B Bin", so it's not a bad idea to swap all of them while you have the iron heated up.

Bryce.

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Re: CPC464 grey border black screen etc.
« Reply #10 on: 11:48, 06 December 19 »

Progress report:
I socketed and replaced all the RAM chips and now I have gone back to a full black screen no borders with the odd flickering white horizontal interference lines. This feels like a step in the right direction from the previous state which was no video signal at all!
I think I'll re-check the input/outputs from the video chip, although I would have thought it would have booted without an issue elsewhere like the Z80 or the gate array?


Pledg

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #11 on: 12:12, 06 December 19 »
Check pins 39 and 40 on the CRTC Chip.

Bryce.

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Re: CPC464 grey border black screen etc.
« Reply #12 on: 16:17, 06 December 19 »
I'm reading .475v on pin 40 and .250v on pin 39

Offline pledg

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Re: CPC464 grey border black screen etc.
« Reply #13 on: 16:29, 06 December 19 »
I just swapped the Z80 and that now gives me a blank grey screen with no interference?

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Re: CPC464 grey border black screen etc.
« Reply #14 on: 18:47, 06 December 19 »
I am getting steady oscilloscope readings from pins 6, 7, 18, 39 and 40 but nothing from pin 21 which I believe is the clock signal. So work back from there to IC125?
« Last Edit: 22:30, 06 December 19 by pledg »

Offline pledg

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Re: CPC464 grey border black screen etc.
« Reply #15 on: 12:05, 08 December 19 »
Okay so I'm not getting a clock signal going to look at the caps associated with the crystal X101.


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Re: CPC464 grey border black screen etc.
« Reply #16 on: 09:22, 09 December 19 »
Where are you measuring the clock from?

Bryce.

P.s. When I say "measure", that means with the scope unless it's a power pin/rail. Pins 39 and 40 of the CRTC should be the H and Vsync signals.

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Re: CPC464 grey border black screen etc.
« Reply #17 on: 11:25, 09 December 19 »
Yes I am getting the signals from pin 39 and 40 of the CRTC chip with the scope.
« Last Edit: 11:44, 09 December 19 by pledg »

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #18 on: 11:43, 09 December 19 »
If you are getting signals on pins 39 and 40, then the clock is working. Which is why I asked where you are measuring the clock from.

Bryce.

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Re: CPC464 grey border black screen etc.
« Reply #19 on: 11:57, 09 December 19 »
Okay! Thank you, so where/what should I check next?

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #20 on: 12:03, 09 December 19 »
Well you still need to confirm that the clock is available where it should be. You still didn't say where you were measuring it from when you said it was missing?

My next check would be the 74LS153's

Bryce.

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Re: CPC464 grey border black screen etc.
« Reply #21 on: 12:34, 09 December 19 »
Okay so which pins do I need to check on those 4 chips?

Offline Bryce

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Re: CPC464 grey border black screen etc.
« Reply #22 on: 14:03, 09 December 19 »
These chips are multiplexers, that allow the address bus to be shared by both the CPU and the CRTC. The pins you need to check are pins 7 and 9 of each chip, to ensure that all are switching and none are stuck either high or low.

Bryce.
 

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Re: CPC464 grey border black screen etc.
« Reply #23 on: 16:16, 09 December 19 »
pins 7 & 9 are giving good readings on all 74LS153 chips.

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Re: CPC464 grey border black screen etc.
« Reply #24 on: 10:11, 10 December 19 »
They may be giving signals, but are the signals coming from the correct source? You will need to use a logic analyser to confirm that the source switches when pins 2 and 14 are enabled/disabled.

Bryce.