CPC6128+ repair job

[steve]

I think I read here about a test cartridge for the plus models, does someone have one?, or any more details about it?

[ralferoo]

I'm still learning about SMT.  I've learned enough to know I need a reworking station though!

I bought myself one of these http://www.ebay.co.uk/itm/260896529440 a couple of months ago. It's definitely a "budget" rework station, but it does the job remarkably well. The "automatic switching off when a magnet in the handle is detected by the stand" isn't very reliable, but that's not a massive problem. Dave Jones at EEVblog gave it good reviews.

[ralferoo]

I think I read here about a test cartridge for the plus models, does someone have one?, or any more details about it?

On here: http://www.winape.net/downloads.jsp at the bottom - Arnold Diagnostic cartridge. I put the link to the downloads page because there's other useful stuff there too...

[beaker]

As luck would have it, there was this item on eBay:
http://www.ebay.co.uk/itm/250999218764

Nice find. If you don't want all the Burnin Rubber Carts I'll buy one off you (being an opportunist, sorry).
Good luck with the 6128 plus!

[Bryce]

Thanks for the compliment!
If you ever need a ZXVid module (a little board I make that fits inside the ZX81 modulator case, but gives a properly compliant composite video signal so you can connect to modern TFT TVs) then let me know and I'll give you a discount as you've been so generous with your help!
I would list all the machines I have at home and at work but it'd take a while. In terms of Amstrads, though, I don't have much - just two CPC464s at work, a CPC6128 of my own at home, and the 6128+ at work. And of course my GX4000 at home.

Actually my ZX81 has the cheap and nasty BC337 / 100R Resistor to ULA pin 16 solution, which is far from compliant, but works perfectly on my TV. I don't know the ZXvid mod, I'm off to check that now...

I forgot the link to the "patient" reference above: http://www.cpcwiki.eu/forum/other-retro/anatomy-of-an-8-bit-over-voltage/msg26308/#msg26308

Why do you have 464's and a 6128+ at work? I need to talk to my boss

Bryce.

[Spirantho]

Because I am the boss. I run a (very) small company which  currently writes bespoke Android apps, but also has a growing sideline in retro computers (hence my shop, http://www.mutant-caterpillar.co.uk/shop/).
Part of what I do is repair old machines - I'm the guy who fixes Spectrums for people mostly, but I've also done Atari STs, an Archimedes A3010, and other oddments. Hence it is my job to collect as much retro-related stuff as possible. I have my wife part-time working for me testing all the games I get in, while I fix up the hardware, and another fellow here is working on a completely seperate contract.
This 6128+, by the way, is (hopefully) going to be my "test rig". I have to have each bit of hardware so I can test software and the like, and "junkers" like this are just what I need - it's unsaleable because the disk drive is missing, but I can probably rig up a standard 3" drive in there (with a 12V cable connected to my CPC6128 modulator). That way I can repair other machines as they come in - not that I'm likely to see many other pluses!


Anyway, back on topic, and enough of me droning on.
There's something missing in this 6128+. It's ... er... kind of important.
It's the /NCAS0 line. It's flat. Flat as a pancake. The RAM chips are activated on the column strobe (CAS) which never comes, so the RAM does nothing. On my GX4000 the CAS0 line immediately kicks into life, but not on the 6128+. I've removed the RAM chips, still no signal on it. If the RAM is never being accessed, goodness knows what the 6845 core of the ASIC thinks it's doing, probably causing no sync.
So... I'm thinking after all that it is the ASIC.
I've asked the bloke with the GX4000s to send them first class instead of second.


@beaker:
Sure, give me a quid for postage and it's yours. They're not worth anything really and I'm guessing you don't want it for playing Burnin' Rubber so much as hacking apart for your own EPROM or something...?

[Bryce]

Hmm, no /NCAS0 is not good and (unless R156 has been removed or shorted to ground) would suggest that the black square you are looking at is indeed an ex-ASIC. Just out of interest, is /NCAS1 doing anything?

Sounds like a nice little business you've started, there are many UK based users here that might like to make use of your services. But why bother with a Plus if you only want to test CPC software? As far as I know, all disc based CPC software will work on a standard 6128 too and with a simple lead, you can test CPC tapes too.

Btw. If you are looking for hard to find retro components, there are several people here with good sources, so just post a "I'm look for..." message. The user "Eliot" also has an online shop for CPC parts (no ASICs though).

Bryce.

[Spirantho]

Excellent, thanks for that tip. I'll take a look.
R156 is fine - I measured 33 Ohms between pin 93 of the ASIC and pin 16 of the 41464.
As for why I want a plus - partly because they're cool. For me, anyway. More seriously, though, I want to be prepared for any repair job that may come in.... and if I can learn how to repair Amstrad Pluses, maybe I can get more on the cheap and fix them up. This has the benefit of me being able to eat, and also the benefit of there being one more Plus in the world, which is a Good Thing (tm).
It also helps to have such weird machines as pluses in case I get a query from someone asking if a game I'm selling works on a plus, and that sort of thing. I know there are emulators, but there's nothing like the real thing, especially for testing!

[Bryce]

Yeah, I find the Plus cooler too and it takes up less space on the desk. Unfortunately it belongs to the catagory of 8-bit where most of the functionality was done in a difficult to get custom chip, like the Acorn Electron and to a certain extent the ZXs with their ULA, which means many of them aren't repairable anyway. I've seen there's now an Altera based ULA replacement for the ZX81. Maybe we need to start looking at something similar for the Pluses ASIC?

Bryce.

P.s. Who "thinks" that they have trademarked "Good Thing" ?

[Spirantho]

http://boards.straightdope.com/sdmb/archive/index.php/t-211240.html is also a Good Thing (tm)
Spectrums are easily repairable. They were so common, you still get lots of erstwhile technicians stocks come up. Hence I have about 12 ULAs spare! And you can buy a tatty Speccy for a couple of quid anyway.
An ASIC replacement for the Plus would be very cool, but quite complicated. Any takers?

[Bryce]

http://boards.straightdope.com/sdmb/archive/index.php/t-211240.html is also a Good Thing (tm)

Who the fuck is Martha Stewart?

Hence I have about 12 ULAs spare!

Yup, I hoard these type of parts too

An ASIC replacement for the Plus would be very cool, but quite complicated. Any takers?

Not me!

Bryce.

[Spirantho]

http://en.wikipedia.org/wiki/Martha_Stewart


To be honest, though, I don't know, I just know the name. It was something around many years ago at the dawn of time/the internet, though, when companies were trying to trademark everything, so it just became a self-effacing dig at corporate bods.
Personally, I just do it for the heck of it.

[Badstarr]

Drat! Dead ASIC! Few words are as disappointing to a Plus/GX4000 owner! I've been watching this post with some interest as I had my eye on not only the 6128 Plus auction, but also the GX4000's. I actually logged into ebay to buy them after sleeping through the day after being on nightshift and found they had been bought, I wondered who else in the world but me would want two dead GX4000's!??


I guessed that the Plus may be a victim of a dead ASIC as I could see the power switch interlock was missing suggesting the cartridge might have been pulled out with the system powered up.


As for the GX4000's you won, it would be worth checking the Z80, I had picked up a dead GX4000 recently and was greeted with a black screen when I powered it up, just for the sake of daftness I swapped the CPU for a known working one I had lying around, and unbelievably it booted straight up. I'm using it for the final version (read neat and tidy version  ) of the GX4000 to a CPC Plus modification I'm working on. Just wish I could get the disk interface working properly!


If you do manage to find a working ASIC and repair the Plus, please document it so we can see how it was done!


Good Luck!

[Spirantho]

Whoops, sorry about that. For what it's worth they should be very handy! And yes, the Z80 will be the first thing to check as it's simplest, followed by the RAM as they're most likely to be gone.
What were you after two dead GX4000s for? Maybe I can help somehow once I've finished with them.


Incidentally, the power switch is just fine, it was just a bit loose.... you'd have to try pretty hard to take a cart out while it was on! Plus I don't think that would have blown all 4 RAM chips, that smacks more of a major power surge or wrong polarity PSU or something.
For what it's worth, the 6128 case is actually quite good - the biggest problem is that posts to hold the drive seem to be broken, and of course there's no drive - finding a 3" drive that doesn't need 12V is going to be ... entertaining. I've not looked on this site yet, though... First thing is to see if I can get the thing booting up.
Needless to say I've been taking photos for my blog, so if all goes well you can expect it to be up there soon!
Even if I can't get it working, there's nothing quite like a properly dead machine to teach you about how they work. Without this machine I had no idea about the innards of the pluses, and now I know much more about the ASIC, the ACID, the 6845 emulation, the 5V only floppy drive, all that gubbins. Dead machines are great, even if you never get them working.

[Badstarr]

To be honest, I was hoping that there would be a good chance that one of them at least could be a quick fix, if that failed then the RAM ICs are always useful to have around as long as they didn't get blixed in the event that killed the console and Z80s are useful to have for future repairs. And its always good to have a junk PCB to check how things are connected should the need arise. As you say, you can learn a lot from a dead machine!


As for what kills these machines (the Gx4000s) I think that its likely due to the 11v power supply being pressed into the 5v socket if its not the cart being ripped out. I know it doesn't fit but it can make enough of a contact to send those volts where they don't belong. I have occasionally made the same mistake when working on or setting up my main GX4000 console. Usually if it happens the console is ok after a power cycle or two, but sometimes is unresponsive, and will not power up even after disconnecting and power cycling several times. Then it just magically comes back to life again after a few minutes. Needless to say I've gotten a lot better at remembering where the PSU needs to be connected!


I suppose an incorrect polarity PSU connected for a while would probably result in a very dead machine so is very probably, as you say the most likely scenario that lead to your machines unfortunate demise.    I guess the switch was a red herring. On the bright side you at least have the hard to find FDC IC and the SED Data Separator in the 6128, assuming they are ok, if you find a 464 plus you can upgrade that to have disk capability! 

[Spirantho]

Exactly! Parts are always useful to have! Not least the case - if I can find a beat up 6128+ in future I can do a switch, as this one is fine apart from being dirty.
If you ever need any Z80s, let me know, I have loads. I buy them about 10 at a time from the Netherlands - there's an eBay seller there who does them for about £1.50 or so each, but postage is higher of course - but if you buy 10 at a time it's not a problem.
I noticed last night the braindeadness of the power supplies. It's almost as braindead as the Sinclair ZX80, which has the power connector at the back of the machine (where you can't see it) right next to the identical cassette lead sockets (where you can't see them). Real masterstroke, there, Clive!

[dragon]

The asic,is very expensive:


http://www.donberg.ie/search?query=40489


I think is best search another cpc + .

[Spirantho]

That price is bonkers. Given I just paid half that for two entire GX4000 consoles, it'd be a bit crazy to pay that much. Admittedly those consoles are non-working but chances are one of the ASICs is good, and even if not it was worth it in ancillaries.... and there's no guarantee that ASIC on that site is working either, unless it's used, in which case you may as well just buy a GX4000 for half the price.
It's kind of like Commodore 64s. You can either buy a SID chip for £25 or an entire C64 (with a socketed SID) for a fiver. Crazy.
Apparently my GX4000s were posted today, but they went second class. I'm hoping they arrive by Friday!

[Bryce]

@Badstarr: Despite it being one of the longest living urban myths, pulling a cartridge out while the machine is running is very unlikely (almost impossible) to damage anything inside the CPC.

@Spirantho: If you're clever, you'll make a small circuit to bring the 5V up to 12V for a standard 3in drive the ZX Spectrum does this too (hint, hint).

@Dragon: I wouldn't believe all that you read on Dönberg, they offer quite a lot of stuff that they don't actually possess.

Bryce.

[Badstarr]

@Dragon; That does seem pricey, however, I'm amazed that one is even available. I think if the suppliers knew the rarity of such an item, its price would climb to ridiculous proportions. The fact is that they are unlikely to sell it unless an enthusiast is in need of one. If I had a broken plus and I knew that a new ASIC would fix it for sure, I would pay €65 rather than wait to pay more for another machine. As long as I was reasonably sure it wouldn't be needing a new ASIC for a fair while!


On the other hand, I would also be quite suspicious that the part might have been removed from a PCB by some poor kid, on a bonfire in a landfill site in a thirdworld country.

[Bryce]

The company is a sourcing company, not a supplier, they don't actually have one, they just (believe) they know where they can get one.

Bryce.

[beaker]

Sure, give me a quid for postage and it's yours. They're not worth anything really and I'm guessing you don't want it for playing Burnin' Rubber so much as hacking apart for your own EPROM or something...?

Fantastic thanks! Yup, you read my mind, I bought an EPROM programmer, EPROMs and a socket a few months back    Also Bryce put a DIY project on this site recently to mount the ACID chip inside the Plus which I fancied having a go at at some point. I'll PM you to get details of where to send the money.

[Spirantho]

A picture speaks a thousand words.
[attach=2]

It would appear that the Aoyue 8208 does, in fact, rock. Especially for £60.

[CanonMan]

Nice job!

That's the easy part though, soldering the replacement ASIC on won't be as much fun 

[Bryce]

Looking forward to seeing the end result. If you clean the solder pads sufficiently then soldering the new one on shouldn't be that difficult.

Bryce.