Author Topic: [N00b] CPC 464 Blank Screen  (Read 674 times)

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Offline matburton

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[N00b] CPC 464 Blank Screen
« on: 23:12, 15 October 20 »
Hey all,

A while back I bought a "for parts" CPC 464 on eBay thinking it might be fun to try and fix it.
(The only other Amstrad I own is a ZX Spectrum +2A, so I'm a CPC n00b)

I've fashioned a power supply and a video connection but I now have a solid coloured square displayed.
I can't feel any of the ICs getting hot, in fact I can't really feel them getting warm.
I'm guessing at this point I should check voltages, especially to the RAM, but I think I need someone to spell out exactly what to look for.

The power supply is a USB charger, 5.2V, 2.4A.
I checked the polarity before connecting it for the first time, and it is correct.
However I didn't spot that it was 5.2V until later, the writing was tiny and I thought USB was always 5V.
Is that voltage a bad idea? Could I have done the damage?

For a video signal I've connected the luminance and ground pins to an RCA jack and plugged that in to the composite input on a TV.
I assume that the luminance includes a c-sync signal?
The TV can display the mono-out from an Amiga 500 I have (15kHz h-sync) so I assume it should be OK with this as well?

Anyway, any hints on how to get started with a diagnosis appreciated.
My (de)soldering skills a somewhat poor, so I'd like to try to avoid just desoldering and replacing all the ICs until I've tried everything else.
The board is a Z70200 MC0002D
« Last Edit: 23:16, 15 October 20 by matburton »

Offline Skunkfish

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Re: [N00b] CPC 464 Blank Screen
« Reply #1 on: 17:49, 20 October 20 »
I assume the power LED on the 464 is lit and the tape drive appears to be getting power?
I don't think a 5.2V supply would be an issue, perhaps a good thing if anything...
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Offline Bryce

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Re: [N00b] CPC 464 Blank Screen
« Reply #2 on: 20:08, 20 October 20 »
5.2V should be ok, there'll be voltage drops across the power switch and PCB anyway. As for what's wrong with it, it's most likely a RAM issue, but if your soldering skills aren't good you should give it to someone who can repair it without causing further damage.

Bryce.

Offline matburton

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Re: [N00b] CPC 464 Blank Screen
« Reply #3 on: 00:20, 21 October 20 »
I assume the power LED on the 464 is lit and the tape drive appears to be getting power?

Yes the LED lights-up and I can fast forward and rewind a tape without issue.

I don't think a 5.2V supply would be an issue, perhaps a good thing if anything...

It's good to know I haven't done more harm that good already!

it's most likely a RAM issue, but if your soldering skills aren't good you should give it to someone who can repair it without causing further damage.

Yes I might yet have to pass it on, but I was hoping to try and learn something first if possible.
I have been getting better with an iron, but I'm not sure I can take on all those pins without risking a mistake yet.


Is there any hope of making an educated guess as to which RAM chips are faulty without de-soldering them all?
It'd be a lot less daunting if I could avoid an en-mass de/re-soldering.


Anything to do with voltage perhaps?
Could I try piggy-backing a new IC over a partially disconnected existing one maybe?

Offline Bryce

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Re: [N00b] CPC 464 Blank Screen
« Reply #4 on: 10:59, 21 October 20 »
Piggy-backing will work with certain types of RAM failure, but not all. RAM don't always get hot or drop more voltage when they fail either. You can test the Din and Dout of each chip with a scope to see if they are functioning properly, or you can do a RAM test if you have a device to replace the lower ROM.

Soldering the new chips isn't where most people go wrong. The usual mistake is damaging traces while removing the old chips. If you don't want to chance damaging the board you can send it to me and I will swap the broken RAMs.


Bryce.