Author Topic: [N00b] CPC 464 Blank Screen  (Read 1106 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 »
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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.
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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?
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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.
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Offline matburton

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Re: [N00b] CPC 464 Blank Screen
« Reply #5 on: 21:18, 11 October 21 »
It's been a year, but it now boots  :D

Admittedly I had to buy better tools to not damage the board, and have some practise first.

Next-up is get the tape deck working (which at the very least needs a new counter reset button)

Sorry for resurrecting an old thread, but I was feeling chuffed  :P

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Offline Audronic

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Re: [N00b] CPC 464 Blank Screen
« Reply #6 on: 01:01, 12 October 21 »
@@matburton


Well done
Can you tell me what was faulty Please


Thanks
Keep Safe
Ray
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Procrastinators Unite,
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Offline matburton

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Re: [N00b] CPC 464 Blank Screen
« Reply #7 on: 15:39, 12 October 21 »
Can you tell me what was faulty Please

Yes sorry, it was indeed a RAM problem.
I assumed it'd be a single chip, but in the end 6 of the 8 were bad.
I scavenged RAM from a second, somewhat rusty, CPC I bought from eBay.
(In time I hope to look at that one as well)

Some n00b questions...

Is it normal for the audio levels to be somewhat low? Both from the internal speaker and the headphone jack?
They're not inaudible, but both seems pretty quiet, and the internal speaker at it's loudest has a little hiss.

Also are there any tape images that would put the tape drive through its paces?
I used a head alignment tape, and the heads are aligned, but I couldn't load the "breaking baud" demo.
Maybe that demo isn't intended to run on real hardware? Just idealized hardware like emulators?

Edit: Seems Im not alone with that demo
« Last Edit: 16:56, 12 October 21 by matburton »
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Offline Devlin

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Re: [N00b] CPC 464 Blank Screen
« Reply #8 on: 15:32, 13 October 21 »
Yes sorry, it was indeed a RAM problem.
I assumed it'd be a single chip, but in the end 6 of the 8 were bad.
I scavenged RAM from a second, somewhat rusty, CPC I bought from eBay.
(In time I hope to look at that one as well)

Some n00b questions...

Is it normal for the audio levels to be somewhat low? Both from the internal speaker and the headphone jack?
They're not inaudible, but both seems pretty quiet, and the internal speaker at it's loudest has a little hiss.

Also are there any tape images that would put the tape drive through its paces?
I used a head alignment tape, and the heads are aligned, but I couldn't load the "breaking baud" demo.
Maybe that demo isn't intended to run on real hardware? Just idealized hardware like emulators?

Edit: Seems Im not alone with that demo


Try something like Mercenary or that has a fancy loader
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