Author Topic: A Guide to Over-voltage on a CPC  (Read 6872 times)

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Offline Bryce

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A Guide to Over-voltage on a CPC
« on: 11:45, 07 February 13 »
Hi All,
    due to several threads in the past and present discussing over-voltage and reversed voltage on a CPC, I thought I'd put a short guide here. This list in only for over-voltage, not reversed voltage.

Raising the voltage slowly:

5V - All is fine.

5.5V - Still pretty much ok, but the 74LSxx, AY, CRTC, Z80 and FDC are starting to feel the pain.

6V - The 74HCTxx are also starting to get hot, but nothing has failed yet.

7V - The 8255 and PAL are at their limit, the parts listed in 5.5V above are starting to fail.

7.5V -  The RAM ICs have reached their limit, but most of the parts in the 5.5V list will have already died.

12V - Everything is failing, just the analogue ICs are still holding out.

15V - The LA4140 in the tape deck finally gives up.

30V - There's probably smoke coming out somewhere and even some of the capacitors are starting to pop. The only surviving chip is the LA6324

36V - The LA6324 fails.


Instant Voltage:
Although the list above seems to suggest that the Logic / Z80 / CRTC etc would die before the RAM, this isn't the case if you were to connect 9V to the CPC. Althought they can survive 7.5V and the Logic only manages about 5.5 to 6V, they tend to fail faster. So 9V would probably kill the RAM first and (if you're really lucky) the failed RAM might even protect the other ICs by dropping enough voltage.

Either way, connecting more than 5V to the CPC is generally a bad idea. Reversing the voltage (even at 5V) also isn't advised. This will also kill the ICs pretty quickly, but again, the RAM ICs seem to be most sensitive to this and will most likely fail first.

Bryce.
« Last Edit: 11:49, 07 February 13 by Bryce »

Offline Munchausen

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Offline Munchausen

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Re: A Guide to Over-voltage on a CPC
« Reply #2 on: 13:15, 07 February 13 »
So maybe someone can help diagnose my over-volted CPC. The display shows a normal blue border but the centre looks like this picture:




(This image taken from the fpga amstrad page)

But without any text, just that random multi-coloured pattern all over.


I'll take a picture on the weekend.


Any clues what this might be caused by/what's dead?!
« Last Edit: 13:17, 07 February 13 by Munchausen »

Offline Bryce

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Re: A Guide to Over-voltage on a CPC
« Reply #3 on: 14:10, 07 February 13 »
Blue border - Good - CPU, ROM, CRTC, etc probably still fine.
Coloured stuff - Bad - RAM probably fried.

Does the computer beep when you press the Del key or when you've held down a letter key for a minute or so?

Is the coloured stuff static, or changing constantly?

Bryce.

Offline ralferoo

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Re: A Guide to Over-voltage on a CPC
« Reply #4 on: 15:25, 07 February 13 »
So maybe someone can help diagnose my over-volted CPC. The display shows a normal blue border but the centre looks like this picture:

But without any text, just that random multi-coloured pattern all over.
Any clues what this might be caused by/what's dead?!
So, the picture you linked looked like ROM was being read from #c000 instead of RAM, so probably (as it was from an emulator) the gate array ROMDIS not being decoded correctly.

If you're seeing it without any text at all, I'd say it's probably just the random contents of RAM on power-up and it looks like the WE pin on the RAM is broken. Actually, as it affects all the RAM chips, it's more likely to be the pin on the gate array or a broken trace.

The good news if you're getting all those colours are that your ROM, CPU, CRTC and most of the gate array are all functioning correctly! :)

Offline morley27

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Re: A Guide to Over-voltage on a CPC
« Reply #5 on: 16:33, 07 February 13 »
I have a picture of the motherboard MC0044A infront of me, I have located the areas on the board from which that particular voltage affects the chips,
 
There are 3 chips left on the board which I havn't been able to mark due to my lack of knowledge
 
UM6845R
AMSTRAD 40010
AMSTRAD 40009
 
From what Bryce has posted, it seems I need to replace the whole board as I was using a 8.4 voltage!!! hope not though, I will get hold of 8 x 4164 chips and see if that fixes the problem.

Offline ralferoo

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Re: A Guide to Over-voltage on a CPC
« Reply #6 on: 16:46, 07 February 13 »
UM6845R is the CRTC.
40010 is the gate array.
40009 is the ROM.

Offline Bryce

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Re: A Guide to Over-voltage on a CPC
« Reply #7 on: 16:57, 07 February 13 »
From what Bryce has posted, it seems I need to replace the whole board as I was using a 8.4 voltage!!! hope not though, I will get hold of 8 x 4164 chips and see if that fixes the problem.

Not necessarily. When one or two chips have failed, the internal short circuit pulls all the current and stops the other chips from being damaged. But any chip that's getting hot is most likely dead.

Bryce.

Offline morley27

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Re: A Guide to Over-voltage on a CPC
« Reply #8 on: 17:03, 07 February 13 »
I owe you a cyper drink Bryce for all the online help  :D
 
Right, the 1464s have been purchased, and i will see how they do when fitted. Thanks

Offline morley27

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Re: A Guide to Over-voltage on a CPC
« Reply #9 on: 17:04, 07 February 13 »
4164 not 1464