A Guide to Over-voltage on a CPC

[Bryce]

Or a small expansion to guarantee a "safe supply" ? ie: Regulator and reverse voltage protection.

Bryce.

[dragon]

it looks so peaceful. RIP another cpc.

Oh not!.is overvoltage two decades ago,and repaired from amstrad.its working!.But you can see the componentes killed view the colour diferent of the soldiers in the back.A big diferencie factory soldiers vs manual soldiers .

[00WReX]

While looking at the mainboards, I noticed this one.

http://www.cpcwiki.eu/index.php/File:CPC6128_PCB_Top_(Z80330_MC0100A).jpg

A chunk of the RAM IC "casing" is missing above pin 8 (Vcc) on three of the RAM chips on the left bank.

Would this have been caused by an overvoltage situation ?

Cheers,
shane

[Munchausen]

Oh not!.is overvoltage two decades ago,and repaired from amstrad.its working!.But you can see the componentes killed view the colour diferent of the soldiers in the back.A big diferencie factory soldiers vs manual soldiers .

How much voltage did you put in? With so many dead components it makes me worry for the chances of my CPC! Will be hard to find another CRTC, never mind the gate array

[Bryce]

Yes, I've seen that before. I assume that board got a decent whack of some serous over-voltage.

Bryce.

Edit: When multiple components fail, like on Dragons CPC, it usually isn't the value of voltage that's the problem (Probably only 12V). Rather the length of time the voltage was connected.

[dragon]

How much voltage did you put in? With so many dead components it makes me worry for the chances of my CPC! Will be hard to find another CRTC, never mind the gate array

.

well,as i remember vaguely.In these time i search connect the cpc to other screen.I view in the back 5v cc.So I pick up the generic 3,3v4,5v ,5v,7,5v 9v 12v etc.. with multiple conectors standard transformer.

I select 5v.Conect the cpc and sound "brooommmmmm" in the speaker.And a wonderful cascade of colours and  garbage appear in the screen and cpc died

So i not sure the voltage the cpc recieve.But I 100% sure that 5v mark are false XD. damm crapy transformer.T

The gate array is not 100% sure if its broken.Is a posibility,because the motherboard  have the other gate array normally not mounted in these type motherboard and a socket(as the other chips broken).

The crtc is very easy to find  I think all are pin compatible.In the other hand can be work in the cpc any CRTC(6845) even if not used by Amstrad?.

[TFM]

... 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.

Well, yes, we got enough commodore computers left

[ralferoo]

I select 5v.Conect the cpc and sound "brooommmmmm" in the speaker.And a wonderful cascade of colours and  garbage appear in the screen and cpc died

This reminds me of when I was about 8. My parents had picked up this really old valve organ in a car boot sale for me to play with. Being a bit of a meddler, I played with it in unintended ways... and at the back there was a voltage selector with about 10 settings in a circle. When I put it all back together, I accidentally selected 60VAC which was right next to 240VAC. Plugged it in and several of the valves exploded instantly. I think valves were pretty hard to get hold of even then in the 80s, I just pretended I'd got bored of playing the keyboard and hid it in the back of my cupboard before helping it get lost in the move when we moved house a few years later...

The valve amplifier that it hung underneath the organ part though was fantastic - I used that as the amplifier for my CPC and later my Amiga for about another 10 years...

[Badstarr]

I have been considering swapping out the standard power connectors on my CPCs  for something a little more unique so I can't accidentally connect one of the many 9volt adapters that populate my work space. A MagSafe adaptor with a battery backed up CPC is my ultimate dream 

[Gryzor]

I'll send you mine to mod after you figure it out

[protek]

I have been considering swapping out the standard power connectors on my CPCs  for something a little more unique so I can't accidentally connect one of the many 9volt adapters that populate my work space. A MagSafe adaptor with a battery backed up CPC is my ultimate dream 

I've made an adapter that connects to a Spectrum +3 PSU and gives you the +12 and +5 Volts.

[dragon]

Will be hard to find another CRTC, never mind the gate array

About find gate array or crtc for remplacement.Probably the shop sell the acid chips(i think a member here buy 100 time ago),maybe can sell you gate arrays.

There are 15328 4010 gate arrays waiting in old almacens since  1990.

[db6128]

CRTCs are pretty easy to find. Ones with the exact same model numbers as those from original CPCs are rarer but not impossible to locate, and apparently, a good proportion of 6845-type chips not on that list should still work, such as the Hitachi HD6845SP, the UMC 6845P (if I remember correctly), etc.

I can't make any promises as I'm far from an expert, but someone like arnoldemu or Bryce are likely to know more. Also, there have been a thread or two here recently by other users who have installed slightly different CRTCs and not had any problems.

I contacted a wholesale supplier in the UK (can't remember who) with a copy-and-paste of the above-linked list of model names, and I was told that they had them all in stock. I imagine some places have huge stockpiles of almost every IC imaginable. So, if you can find a seller like that who is willing to sell in small quantities, no problem.

Alternatively, might the forum organise a group buy and get a nice hoard of spare chips for users?

Granted, a Gate Array will be more difficult to find. Hopefully not impossible, though!

[Munchausen]

My replacement RAM ICs arrived yesterday. When I get the chance I'll try swapping them into my dead 6128 and report back here.

[Munchausen]

I spent a few hours last night removing the old RAM chips, soldering in the new IC sockets, and putting in the replacement chips.


The good news: The CPC now starts up! Everything looks fine (correct colours, ROM starts up ok etc). When I first tried it, it was in sort-of black and white. I resoldered all the joints and it came up in the right colours!


The bad news: Tried starting with FW3.1 in Bryce's LowerROM... and it either doesn't start (with weird patterns on the screen), loops on startup, or starts and reports only 64K RAM. It pretty much always freezes after starting too (but not always).


So I think I'm almost successful, but probably didn't do a great job with all the soldering.


I'm going to write a RAM test program, hopefully some evening this week, to see if I can figure out which RAM IC has a bad connection. Then hopefully I can fix it and be back in business! This is dependent on my old laptop working well enough for me to make floppies of course (haven't tried it yet, it hasn't been booted up for maybe 10 years)...

[Bryce]

Did you clean the contacts of the edge connector before you connected it? It doesn't use any additional hardware inside the CPC, so if the CPC works as much as you've described, then the LowerROM board should work too.

Bryce.

[Munchausen]

I think the LowerROM is fine, and guess the connection to it is too. I think that it's just the RAM is badly connected somewhere maybe. If my logic isn't flawed, this follows from FW3.1 not being able to count all 128K. Having said that, I'll try cleaning the contacts later to see if it helps.


I'm a little worried that the PAL could have fried, and that is why 64K of RAM isn't working. I tried swapping the PAL with the one from my other 6128, but then the machine wouldn't work at all (had strange colours/patterns everywhere). My theory at the moment is that maybe the PALs are different because they have different gate arrays - the one I've just replaced the RAM on has a 40007, while the other (the one with edge connector issues) has a 40010. But that's just a guess.


I also picked up my oscilloscope today, so I'll be able to have a go at finding what's wrong with the edge connector on my other 6128 some time soon.


I'm planning to write a little RAM test app with sdcc (because then I can use the rand functions from libc). I figure I'll create a random buffer, copy it to RAM, and then read it back, testing 8KB at a time. Hopefully from that I can see what's going wrong...


Get this; I tried starting it up again a minute ago, and I was back to the colours being slightly off (white instead of yellow, on very dark blue). Restarted a few times, and on boot it would have the correct colours for a split second, then go wrong again. Wiggled the video connector around, and the the whole screen corrupts and CPC crashes. Wiggle it again and it wont start (strange colours/patterns again). Wiggle it some more and it works correctly with the right colours. I removed my fried S-Video modulator (a home made one I've made a while ago, not a Bryce model) when I was changing the RAM ICs so I could have bodged the solder joints doing that. But why would that make the CPC crash? Seems very odd to me 

[Bryce]

The PAL should be the same whether you have a 40007 or a 40010, there was only one version of PAL. It does sound like the upper 64K is still having problems though and possibly a fried PAL too.
The crash when wiggling the monitor connector sounds like you have a short-circuit inside the connector or wire. If you've added this connector yourself, make sure the connector shielding isn't making contact with any of the pins, and that no stray wires in the connector are touching.

Bryce.

[Munchausen]

I finished a big piece of work this week and had a few hours this afternoon to play.


I used the multimeter to test the connections of the RAM ICs - and found that one of the data lines wasn't connected on one chip. Soldered that up, and voila - 128K found on firmware 3.12!


However, I am getting a black and white picture. I checked the joints on the video connector (they came out fine) and resoldered them to be safe, but no joy.


The cable (a SCART cable) is fine, as I tried my 464 with it and that works perfectly.

What's weird is the colours were coming and going before I fixed the RAM, but now it's all black and white all the time.

Any ideas? Could it be the CRTC?

[Munchausen]

I've double checked, and the problem is not that the tv is getting luminance and just giving a black and white picture because of that (disconnected luminance and the picture remained the same).

I'm wondering if the signal could be attenuated somehow, but I've no idea how. When I remove the blue connection, the whole screen goes much darker, so it seems some colour information is getting through. I tried it with a Bryce s-video regulator, and it gives a really fuzzy, shaky picture that can barely be made out at all (and again, the converter works great with my other CPCs).


I'm going to try a scope on the pins next time I look at this. Does anyone know what voltage the signals should be?



Also, I think this might not be related to over volting. Before I connected the 12V and fried the RAM, I was having a problem with red not being displayed, but I thought it was something wrong with my home made s-video converter. Though, I don't remember problems with any other colours.

[Munchausen]

So today I finally hooked up my VGA board, and the CPC gives full colour through it!


I don't know what is going on here, I get no picture now with the S-video adapter, black and white with the SCART lead, but the VGA board works fine. I'm thinking that the VGA board amplifies the signals?


My other CPCs work great through the S-video adapter and the SCART lead, so something seems a bit screwy with the CPC, but if the expansion port works too (got to test that tomorrow) then I am good to go (my other 6128 still has unresolved issues with the expansion port).

[VincentGR]

One friend gave me his 6128 cause it wasn't working due 12V input on the wrong connection. He put it on the switch connector 
So I felt that the two ram chips were extremely hot. I haven't much time to cook something on it and I replaced them with some spare ram chips from a ZX. Now the CPC works but there are some keys pressed every time I power him up even though without a keyboard attached 

Oh well, a step at a time.

[Bryce]

Then it's very likely that the AY-3-8912 is also damaged. The fact that it boots at all is very good news, it means that the CPU, CRTC and Gate array have all survived!

Bryce.

[VincentGR]

Yes, that is what I thought so. I found this chip. Do you think it is compatible?



Because mine is GI 8638CDA   

Thanks for your fast reply!!!! 

[Bryce]

I can't say with absolute certainty, but that looks like a possible fake to me. The Logo looks wrong and I can't find any record of Microchip ever having produced the AY-3-8912. The GI version should be easy enough to find anyway.

Bryce.

Correction: I just pointed my friend from Microchip to the picture and he says it probably is real. They did produce the AY-3-8912 during that time (and in Taiwan) and the Logo did look like that at the time. It is 100% compatible too.