CPC6128+ repair job

MacDeath · 20:03, 26 February 12

just a question...
What happens if you connect a GX4000 on both a PLUS monitor and a TV... or perhaps a scart/peritel video recorder ?

or both on a monitor and a TV ?

concerning the "replacement" of an ASIC...
Would it be easier to use a socket ?

Also, is it possible to properly Retro engineer it ?

Bryce · 22:16, 26 February 12

GX4000 connected to a monitor AND TV or SCART: Shouldn't be a problem because the CXA would still drive the TV SCART signal and the monitor signal would also be strong enough, so both should work in parallel.

ASIC Socket: The socket for this IC has a different footpirint, so it would be very difficult to solder the socket to the PCB.

Retro engineering: The function of the ASIC has already been simulated in emulators, so the function is known, converting this function to an FPGA or CPLD is not difficult, just a lot of work.

Bryce.

MacDeath · 12:20, 27 February 12

QuoteGX4000 connected to a monitor AND TV or SCART: Shouldn't be a problem because the CXA would still drive the TV SCART signal and the monitor signal would also be strong enough, so both should work in parallel.

So it can be easy to get direct/real video recording from a GX4000...

Bryce · 10:26, 28 February 12

Do you mean - Watch one record the other? If so, yes.

Bryce.

MacDeath · 13:03, 28 February 12

Most video for amstrad stuff are either Cam Recorded from a real monitor or emulated...

Perhaps a moded GX4000 (like one turned into a proper 664PLUS...) would be a good longplay recording station...

This may be interesting and give a true Hardware experience.

Has anyone tried this ?

(Also sorry to spoil the topic.)

Spirantho · 14:36, 28 February 12

If I may briefly go back on topic....

[attach=2]
I got an actual sign of life from my 6128+!
OK, it's not exactly the Burnin' Rubber screen I'm hoping for... but it's something.
This is the third ASIC in this machine - I think the second one works but I took it out because the pins were a bit misaligned so I thought I may want to come back to it later when I've had more experience with SMT stuff.
The screen above only happens with a cartridge inserted! This is good news, as it implies the ACID chip is talking to the ASIC. If I run it with no cartridge, I get the "No Signal" display I always got before.
The problem I'm having now is that after having had an ASIC soldered to it 5 times now, some of the PCB tracks are dry and the solder won't stick to them. I've connected the CPU clock line CLK4 via a wire - I'll probably do something similar with the rest, so it may get a bit cluttered!
Right now though, I have a motherboard with a working display, a working CPU, working RAM, and a working ASIC that just needs 10 connections soldering up somehow.
Phew.
I'm going to take a break for a bit now. Too much looking through a magnifier.... soldering SMT is tricky!

Bryce · 16:21, 28 February 12

Getting close

I'm sure it's just a dodgy connection on the ASIC at this stage. Yeah, SMD soldering is hard on the nerves

@MacDeath: It's also possible to do this with a CPC that has my S-Video adapter connected (effectively the same thing as the CXA1455 does in the GX4000). As far as I know, Markus has already experimented with recording things this way.

Bryce.

MacDeath · 19:26, 28 February 12

Just a basic stuff : have you verified all components and connectics ?

ok, this can be boring, long and delicate...
But a faulty condensator may be trickier too.

Badstarr · 01:58, 29 February 12

The only thing I can think of from my experiences with soldering ASICs is that you should check ASIC pins 127 and 129 (RAMDISABLE/ROMENABLE) for any cross connections from adjacent pins. I had some similar on screen behaviour when there was a slight short on one of these pins. There should also be a 2.2k pull down resistor on each pin. Just out of interest which number pins are currently not connected from the ASIC?

It's great to see some signs of life from the machine! It's getting there!

Spirantho · 17:05, 29 February 12

I AM THE LIZARD KING!!!
Or something.
You won't believe this but....
[attach=2]
Here's the board:
[attach=3]
I noticed that most of the pins at the top right of the IC aren't used except by the printer buffer - which I'm not going to use anyway. All that was left there was the RAMDISABLE/ROMENABLE lines, the CURSOR and the Light Pen. Now I don't care about the Light Pen, and I don't need the CURSOR line for now. That's why there's two wires around the ASIC at the top right - they're pin 127 and 129 lifted (VERY carefully) and wired up directly as the solder pads are too dry now for the solder to stick.
Thank you to everyone who's helped me resurrect this very dead machine! I reckon I can still use the ASIC I took out of the other GX4000 - it should work, it's just a few pins need a touch of straightening. Is it of use to anyone?
Do I get a badge now for doing an ASIC transplant?

CanonMan · 17:21, 29 February 12

Nice work

You have far more patience than I have

Spirantho · 17:55, 29 February 12

I still have a few bits and pieces to do - I think some of the keyboard lines may be dodgy but I'm not sure as I don't have a BASIC cartridge. Also it crashes every now and again and the ASIC needs pushing down again - there's ONE bad connection to the CPU I think! Plus there's a dodgy connection on the video output line so it loses colour sometimes.

But overall, it's working and I'm well chuffed. Not only that but it's my first bit of SMT QFP removal/refitting, so it opens the door to all sorts of new repairs!
Yay!

Badstarr · 20:22, 29 February 12

Whoo! Hoo! Excellent work! I'm actually quite glad you beat me in those auctions!

So was my suspicion about the ASIC pins 129 and 127 any use? Or had you already got it working by the time I'd posted? I only ask as I would be rather proud of myself if my CPC ASIC skillz are actually becoming useful! lol!

Spirantho · 20:48, 29 February 12

It was of use indirectly - I wasn't going to have a crack at the Plus today, but when I read your post - knowing that those pins weren't connected - I thought I'd give it a go!
You were dead right in your diagnosis though, so well done!

Badstarr · 21:00, 29 February 12

Thanks very much!

I now feel much more useful after a day spent trying to weld my car back together! I've never welded before by the way

I have to admit, I much prefer soldering! As for CPC land I have just received a rather broken 6128 (vanilla flavour) that I will be repairing hope that it goes as well as your repair! I can see where the cause of death originated, the previous owner soldered a 12v PC power supply into it and a trace on the PCB lifted shorting +12v to a NAND GATE IC, I'll wager this will be an easier fix than an ASIC transplant!

Bryce · 10:07, 01 March 12

Congratulations Spirantho a fine job, you have every right to by "well chuffed". It's also a good example why you shouldn't throw away broken CPCs, they can be fixed or at least used for parts to fix another one.

Bryce.

Spirantho · 10:11, 01 March 12

Exactly - never throw out anything like that.....
... which explains the amount of useless stuff I have scattered around.

Never know when it'll come in handy!

endangermice · 22:44, 22 June 12

Absolutely fantastic job, makes my current repair adventures of a 6128 (originial) and Acorn Electron look trivial by comparison!

I have looked into SMT soldering before done a few small things like resistors very difficult - I can't even begin to imagine how you did the ASIC not to mention removing one from another machine first!

Check out this video I got curious and did a YouTube search some very good tips on SMT soldering here though I don't think I'll be attempting any large chips anytime soon!

Spirantho · 10:00, 23 June 12

Thanks for that video, it's great! Not seen it before.

Removal of SMT chips is actually very easy - you just need a desoldering station (mine cost about £40 and works brilliantly, it's an Aoyue 8208). It's the re-soldering that's difficult!

endangermice · 11:26, 24 June 12

No problem! Yeah I thought the video looked great and just gives some really nice tips on how to do this more easily. I don't currently have a desoldering station but having looked at them I can see immediately how tos can make the work of removing the chips much easier!

I can imagine that soldering them back on os a real nightmare made even worse when multiple removals and reattachments begin to damage the solder pads. In the factory of course attaching the chip happens in seconds with a special machine if only we could get those at home!

How is the machine, still working...?

Spirantho · 12:01, 24 June 12

Trust me, get a desoldering station. It's not only useful on SMT chips - I usually use a desolder pump on through-hole, but occasionally you get PCBs that just disintegrate when you touch them with an iron (my bootleg Green Beret arcade board being a good example). Desolder pump (even with a temperature controlled iron) causes damaged tracks because of the low quality PCB, using my desoldering station I removed about 5 chips without a single damaged track.

As for the 6128, it's... well, it's working except I removed the ASIC again.

There was still some dodginess probably caused by the mis-shapen pins on the ASICs I was fitting, so I've bought another dead GX4000 (on its way) and this time now I've had more practice, I'm hoping to get a better fix. Especially now I've seen that video! I've ordered some flux and paste flux specially, as well as a 10x loupe.

I could probably get the two ASICs I have back on it too but I want to get a really good fix on it, and that's safest with a new or cleanly removed chip (and as I've had more practice with the Aoyue now, I should be able to get a much cleaner removal). I'll put the other two ASICs back in the GX4000s - it doesn't matter so much if they're not perfect as GX4000s are so cheap - probably cheaper than the ASIC inside them!

endangermice · 12:02, 24 June 12

You might like this video too:

it shows how to use solder paste and a desoldering station to attach an SMT ic easily and gives a very nice finish! Could be useful for your ASIC transplant!

Spirantho · 12:08, 24 June 12

That does look pretty cool. I wonder how well it would work on such a big chip as the ASIC, though. Maybe I'll try it on my GX4000 boards to practice!

Off I go to eBay once more...

endangermice · 12:33, 24 June 12

Yeah I was thinking the same and was surprised how they just spread the paste across all of the contacts but it does seem to sort itself out! The desoldering station sounds like an excellent investment, I think I'll chase one of those down next month (unfortunately I have reached my quota for tools this month). It explains how you're able to remove ICs so quickly - with a desoldering gun it's a lot slower and as you say tracks often get lifted, caused usually in my experience by solder being left on the other side of the leg i.e topside which then pulls a track from the top of the board when you remove the ic! Fortunately theres nothing that can't be fixed but wire links look ugly (you should see the back of the 6128 I'm repairing previous owner butchered the ROM sockets so I have four wire bypasses to link it all back up to the address bus - not pretty!

Bryce · 12:53, 24 June 12

Solder paste is really good for large ICs like the ASIC, but it's expensive if you only solder on rare occasions. The paste has a shelf life of a only a few months, so it should be bought in very small quantities, just for the job you intend doing. Watching it "melt" under the microscope is really cool

Bryce.

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CPC6128+ repair job