Author Topic: A Guide to Over-voltage on a CPC (Read 7393 times)

Bryce · « **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.

Munchausen · « **Reply #1 on:** 12:47, 07 February 13 »

Thanks Bryce!

Relevant threads:

Problem screen border black and paper blue

Grey box black border when cpc464 is switched on

Gray screen of death

And maybe:

CPC6128 - blaues anzeige

Munchausen · « **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?!

Bryce · « **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.

ralferoo · « **Reply #4 on:** 15:25, 07 February 13 »

Quote from: Munchausen 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:

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!

morley27 · « **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.

ralferoo · « **Reply #6 on:** 16:46, 07 February 13 »

UM6845R is the CRTC.
40010 is the gate array.
40009 is the ROM.

Bryce · « **Reply #7 on:** 16:57, 07 February 13 »

Quote from: morley27 on 16:33, 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.

morley27 · « **Reply #8 on:** 17:03, 07 February 13 »

I owe you a cyper drink Bryce for all the online help

Right, the 1464s have been purchased, and i will see how they do when fitted. Thanks

morley27 · « **Reply #9 on:** 17:04, 07 February 13 »

4164 not 1464

Bryce · « **Reply #10 on:** 17:24, 07 February 13 »

I'll have a virtual Guinness then.

Bryce.

Munchausen · « **Reply #11 on:** 19:39, 07 February 13 »

Quote from: Bryce 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.

Ah wait, I was confused. My other 6128, when I boot with symbiface attached slightly wrong and hold down ctrl (to boot symbos), sometimes fades from the "boot screen" to looks as I described.

My broken CPC on the other hand, looks like the pictures below, with a black border and purple and green colours. Sometimes in stripes, generally different every time, sometimes changes a little to begin with but always settles down within a few seconds.

Bryce · « **Reply #12 on:** 19:55, 07 February 13 »

I still suspect a failed RAM, despite the less entertaining screen

Bryce.

Munchausen · « **Reply #13 on:** 20:18, 07 February 13 »

Cool, hopefully repairable then

I didn't want to buy RAM chips before because for all I knew everything was fried!

But now I will buy some next month and see if I can breathe some new life into it!

lynwen · « **Reply #14 on:** 20:26, 07 February 13 »

So Bryce did you test your theory and if so how many cpcs did you get through before writing up your conclusion

Bryce · « **Reply #15 on:** 20:53, 07 February 13 »

No, it was all calculated on paper. No CPCs were harmed to get these results

Bryce.

db6128 · « **Reply #16 on:** 23:28, 07 February 13 »

But it sounded so realistic! It’s like I was there! You can’t just tell me now that it wasn’t real! What will you do to rid me of the nightmares?

TFM · « **Reply #17 on:** 00:23, 08 February 13 »

Quote from: Bryce on 20:53, 07 February 13

No, it was all calculated on paper. No CPCs were harmed to get these results

Bryce.

So you just guessimate using your mystical powers?

TFM · « **Reply #18 on:** 00:25, 08 February 13 »

Quote from: db6128 on 23:28, 07 February 13

But it sounded so realistic! It’s like I was there! You can’t just tell me now that it wasn’t real! What will you do to rid me of the nightmares?

Haha, das nennt man hinters Licht führen.
Sorry have no english sentence to translate that.

Bryce · « **Reply #19 on:** 00:36, 08 February 13 »

Quote from: TFM/FS on 00:23, 08 February 13

So you just guessimate using your mystical powers?

No

I read the Datasheets and read the Maximum operating voltage and Absolute max values. Then I checked what types of circuit was in the IC and graded them by sensitivity, ie: how long an overvoltage would need to be present to damage the chip.

Doesn't sound as exciting as mystical powers, but it's a bit more exact

Bryce.

Edit: "hinters Licht führen" in English is "Pulling the wool over someones eyes". And I wasn't trying to do that. I was just describing it as it would happen. I see these types of tests being carried out quite often (not on CPCs of course), so I have a good idea of how it goes.

morley27 · « **Reply #20 on:** 10:24, 08 February 13 »

They should of printed Bryce's findings on the back of all cpc's, the death rate would of been a whole lot less!!

dragon · « **Reply #21 on:** 10:34, 08 February 13 »

Quote

No I read the Datasheets and read the Maximum operating voltage and Absolute max values. Then I checked what types of circuit was in the IC and graded them by sensitivity, ie: how long an overvoltage would need to be present to damage the chip.

Is more easy,view my old motherboard burned vith overvoltage.

rom killed.
crtc killed.
ram killed.
gate array probably killed.

http://www.cpcwiki.eu/index.php/File:AmstradCPC464_Z70375_MC0044D_GA40010_PCB_Top.jpg
http://www.cpcwiki.eu/index.php/File:AmstradCPC464_Z70375_MC0044D_GA40010_PCB_Bottom.jpg

Bryce · « **Reply #22 on:** 10:37, 08 February 13 »

But you fixed it afterwards?

Bryce.

arnoldemu · « **Reply #23 on:** 11:20, 08 February 13 »

Quote from: dragon on 10:34, 08 February 13

Is more easy,view my old motherboard burned vith overvoltage.

rom killed.
crtc killed.
ram killed.
gate array probably killed.

http://www.cpcwiki.eu/index.php/File:AmstradCPC464_Z70375_MC0044D_GA40010_PCB_Top.jpg
http://www.cpcwiki.eu/index.php/File:AmstradCPC464_Z70375_MC0044D_GA40010_PCB_Bottom.jpg

it looks so peaceful. RIP another cpc.
:'(

arnoldemu · « **Reply #24 on:** 11:21, 08 February 13 »

Quote from: Bryce on 10:37, 08 February 13

But you fixed it afterwards?

Bryce.

so if most problems are overvolt, or wrong polarity, perhaps we need something obvious in the forum and the wiki to tell everyone the correct thing to do to stop these cpcs being given bad volts.

if they're going to do it properly, please wet the sponge

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Author Topic: A Guide to Over-voltage on a CPC (Read 7393 times)