Author Topic: CPC6128 white screen on startup  (Read 10444 times)

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Offline robcfg

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Re: CPC6128 white screen on startup
« Reply #25 on: 11:54, 20 June 13 »
Gerald, Thank you very much!


The scans are perfect and I already uploaded them to the Mainboard Versions page.


Cheers!

Offline dragon

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Re: CPC6128 white screen on startup
« Reply #26 on: 17:24, 20 June 13 »
I guess it is an accessibility reason. The moved component were under the internal floppy cable.

Anymone has time to check differences between the 10 revisions of MC00020  :o


At simple view they change ram provider and logics chips provider. A new motherboard revision can be made when a internals component change, not necesarily circuit changue. So when you pick up service manual, yo search the exact spare part remplace.
« Last Edit: 17:26, 20 June 13 by dragon »

Offline Bryce

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Re: CPC6128 white screen on startup
« Reply #27 on: 17:35, 20 June 13 »
You don't up the revision number when you change component suppliers!! That would be crazy. A PCB revision number always means that something of the PCB (component position, routing or schematic) has changed.

Bryce.

Offline Devilmarkus

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Re: CPC6128 white screen on startup
« Reply #28 on: 18:14, 20 June 13 »
Shouldn't we get back to topic?  ;D
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Offline Bryce

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Re: CPC6128 white screen on startup
« Reply #29 on: 10:31, 21 June 13 »
Probably....

The values you've measured seems to indicate that more than one RAM could be damaged, but as I said, the method isn't exactly 100%, it could also indicate that a logic IC has failed. With having the PCB here, there's not much more I can suggest.

Bryce.

Offline JonB

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Re: CPC6128 white screen on startup
« Reply #30 on: 10:58, 21 June 13 »
OK, I'll fit replacement RAM and see where it takes me.

Offline JonB

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Progress..
« Reply #31 on: 15:29, 22 June 13 »
Well, I tried to desolder the RAM and managed to pull a track on the first attempt. There's just not enough space to get the iron and sucker in close enough to get the solder out, the chips are too close together. This isn't going well. I used a heat gun to get the other RAM off the board, much easier, and the RAM chips are intact (all but the first).


I connected the board to the TV and power and thankfully it is still showing the black border / white or grey screen, so I think (fingers crossed) I'm OK so far. Nothing new roasted, maybe. I've put a set of sockets in the left hand bank and plugged 8 of the removed chips in. Same screen.. so now I will repair the broken track and fit the next lot of sockets (to the right hand bank).


Will a bare board to boot to BASIC with nothing connected (so no keyboard, disk)? Reason I ask is that once the new RAM turns up I am going to plug it in and test without ancillaries.

Offline Bryce

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Re: CPC6128 white screen on startup
« Reply #32 on: 16:48, 22 June 13 »
Yes, the CPC should boot normally when the keyboard and drive aren't attached.

Bryce.

Offline endangermice

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Re: CPC6128 white screen on startup
« Reply #33 on: 17:23, 22 June 13 »
Well done for getting them all out. De-soldering chips isn't really much fun. I rarely use a solder sucker there days, often it's very difficult to get a decent seal so you can't guarantee to move all of the solder anyway. I think a hot air gun is probably the best solution if you want to remove chips quickly. I still haven't bought myself one of those yet (I really should do) so I'm still resorting to using de-solder wick to remove solder from the legs. It's not a fast process and requires anything up to four passes to make sure everything is gone.


If you're at all worried about damaged tracks, you can reassure yourself by checking continuity using the schematics as a guide. I found this invaluable when I was repairing a CPC 6128 a few months ago. Someone had botched removal of the AMSDOS ROM leading to all sorts of display weirdness on boot up. I nearly lost my mind with frustration but Bryce guided me through and eventually I found that one of the data lines wasn't connected to the Gate Array. Sorting that sorted the machine - hooray!


With the knowledge of the guys on this forum along with their everlasting patience you'll get it working again and when you do the feeling will be amazing :).
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Offline JonB

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Re: CPC6128 white screen on startup
« Reply #34 on: 17:44, 22 June 13 »
Got the sockets in - phew! - now just waiting for the new RAM to arrive.

I know that feeling when you get an old computer to work, it's great isn't it? Sadly I don't get to feel it very often... But fingers crossed with this one. I'm not at all sure how much more I want to spend on it though, it's cost me over £60 so far!

Offline endangermice

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Re: CPC6128 white screen on startup
« Reply #35 on: 15:23, 23 June 13 »
Quote
it's cost me over £60 so far!

Lol, repairing quite a lot of these old machines can sometimes prove to be less than cost effective, it all depends on what's wrong.  I prefer to look at it as an educational experience. The C64 I've just fixed cost me about £50 in the end, madness you might say when there are working examples around on eBay for about £20 - but that's not the point, the point is you learn something from figuring out whats wrong then repairing it. This knowledge will no doubt come very handy in the future when perhaps there aren't so many machines knocking around on eBay plus it's fun!
Good luck with the rest of your repair, I'm looking forward to hearing how you get on. I'll be publishing, my C64 repair story soon, just in the process of writing it up along with some suitable pictures.
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Offline JonB

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Re: CPC6128 white screen on startup
« Reply #36 on: 17:20, 23 June 13 »
Ah yes, C64 repairs. I learned a lesson there: Don't buy "for spares or repair" machines! Seems you can't troubleshoot a C64 unless you have a known set of working custom chips (so swap until it is working). I sold mine in the end, but I list £20. The 6128 is the same, I bought it "spares or repair" but hoped it would be something easy to fix. Another £30 plus postage gone. Sigh... Got one more on its way, this one is "untested" but being sold by a recycler so might be OK. Hmm, "might". I'll give up if that one is dud!

Offline endangermice

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Re: CPC6128 white screen on startup
« Reply #37 on: 23:27, 23 June 13 »
C64s can indeed be quite frustrating there's a pretty heavy chip count on the older machines and unlike the CPC it uses several custom chips which are often difficult to get hold of cheaply, particularly the SID.

I quite enjoy fixing them because they often pose a reasonably big challenge. If you know how the C64 works they're usually not that difficult to troubleshoot but the problem often results in the replacement of a custom chip - most notably the PLA. Fortunately there's a company now making a replacement that should be 99% compatible (I've never had any problems running various games etc.) and as a bonus it runs completely cool!

Don't give up on that 6128, most that are broken usually aren't that bad. The same goes for most retro computers you'll find on eBay. The C64 I've just fixed had 12V DC down the 9V AC line. In the end surprisingly little bought it, the RAM was shot as was the SID and two CIAs but other than that the rest of the machine survived and I'd call that an extreme case. The CPC really only has one custom chip, the gate array and these are pretty robust. Everything else is just glue logic chips, the readily available CRTC and AY sound chip and of course 4164 RAM. As a bonus compared to the C64, the CPC runs a lot cooler meaning they should survive significantly longer!

My advice is to take your time and if you have any problems just ask on this forum. We're very lucky to have some really talented guys on here who enjoy seeking previously broken CPCs spring back to life :).

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Offline Bryce

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Re: CPC6128 white screen on startup
« Reply #38 on: 11:45, 24 June 13 »
C64s can be a hassle due to the custom ICs, but when they break, the signs they give usually point quickly to what needs to be changed. As well as that, there are 4 or 5 common faults that it usually ends up being.

One advantage that had a huge effect on the lifetime of the CPC was the monitor. Having this meant that most CPCs were used on a desk. C64s and Spectrum ZXs tended to be put in front of the TV, which meant plastic on carpet = Major static problems. Both the C64 and the Spectrum ZX were known to die just as you had connected an external peripheral. This was because the user slid a piece of plastic over the carpet and it discharged into the computer. On the C64, this usually fried one of the CIAs, on the Spectrum is was usually the HC Logic IC or RAM that took the hit. Just by having a monitor, Amstrad inadvertantly lowered the chance of this happening.
Both the C64 and Spectrum also had dodgy power supplies. The C64 tended to overheat and when it died the voltage went up instead of going down. Luckily the C64 tried to counteract this by fusing the C64 internally (something the CPC could have learnt from). The Spectrum had their crazy 9V to 12V DC/DC convertor, which worked, but relied on components that were running constantly at their limit (TR4/TR5).
Amstrad chose to supply a proper 12V and 5V, but never envisaged people using the CPC without the original monitor or an MP1/2, so there is absolutely no protection inside the CPC (as anyone who has ever connected the worng voltage/polarity will tell you).

Bryce.

Offline endangermice

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Re: CPC6128 white screen on startup
« Reply #39 on: 14:56, 24 June 13 »
That's a very good point about the C64 power supplies they are notorious for over-volting when they go bad. The fuse definitely helps on the C64, I think it's death saved many components on the broken one I received. I think Amstrad's policy is probably fair enough. In all the years I had and still have my CPC 6128 with original monitor, I've never had any problems with the machine at all but it dose mean you have to be extra careful especially with the 12V reversed polarity on the drive's power source.


It nice and easy to get a good quality switcher to make a replacement CPC supply. I use such a device for mine now, it's from a mini ATX machine and is very nicely made. I house it in a custom case that also happens to hold a 3.5" disk drive. It makes a very nice monitor stand too! The C64 is more complex because of the 9V AC which rules switchers out of the equation (well for the 9V line anyway). I'm planning on building a replacement C64 supply soon and will probably use a Switcher for the 5V supply and an ordinary linear transformer for the 9V AC - all protected with appropriate fuses. It's annoying to have to have two transformers but unless you want to tap the 5V off the 9V transformer which IMHO doesn't give you anything better than the original supply, you're a bit stuck.


Interesting about the static buildup from carpets etc. I must admit I never thought of that and indeed my CPC spent all of its life on a desk whereas friends' C64s and speccys were inevitably left on the floor! I have many friends with failed Spectrums and it's nearly always bad RAM or the voltage regulator, definitely running to their limit!
« Last Edit: 14:59, 24 June 13 by endangermice »
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Offline Bryce

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Re: CPC6128 white screen on startup
« Reply #40 on: 16:08, 24 June 13 »
Tapping the 5V from the 9V using a 78L05 would be much better than the original C64 design. The 78L05 can supply 2A on the 5V rail and has overheating and short-circuit protection built in.
I power my CPCs (and quite a few other computers) from a Spectrum +3 PSU. All the amps you could need and fully serviceable with internal fuses :)

Bryce.

Offline endangermice

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Re: CPC6128 white screen on startup
« Reply #41 on: 19:13, 24 June 13 »
Hmmm, now that sounds like an interesting compromise. Do you know that the dropout voltage of the 78L05? Some sites are suggesting it might be as low as 0.5V. Off the top of my head I'd probably try to source a transformer with dual windings so I can be sure to provide enough current at each voltage. 2A for the 5V will be more than enough for a C64.


Anyway this is getting off topic - though such a device could also power a CPC ;) .


JonB, really looking forward to hearing how you get on!
« Last Edit: 19:32, 24 June 13 by endangermice »
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Offline JonB

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Re: CPC6128 white screen on startup
« Reply #42 on: 21:57, 24 June 13 »
Well.. 6128 number 2 arrived and it's working, apart from a few keys (U for example so I can't run any programs or load disks). And with a huge collection of flight simulators and other wartime stuff.


So I can use this to work out what's wrong with the other one, maybe.


Remarkable collection of software. The owner liked F15 Strike Eagle that he bought a cassette and disc version. And 2 versions of Gunship... No Elite, but there's an Elite keyboard map. Odd...

Offline endangermice

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Re: CPC6128 white screen on startup
« Reply #43 on: 22:29, 24 June 13 »
Great stuff, having a working machine is a great place to start. Hopefully the broken keys are just a knackered membrane (happens quite a lot on the 6128s).

My first port of call would be to try plugging the keyboard from the broken 6128 into the working one, if you're lucky that will solve your problem if you're unlucky the keys still won't work but if they don't work in a different way you have two broken membranes and everything else is probably ok. If you have a problem with the same keys then it could be a broken board trace or knackered AY chip - there's a logic chip involved too, can't remember which it is off hand but Bryce will be able to advise.

If there's a pattern to the broken keys you can use the schematic in the service manual to track down the broken track, several keys will be connected to the same trace so if one is broken, assuming the break is at the connector end then others on the same trace will also be broken and you'll see a pattern.

It's probably a broken membrane since a broken AY chip often leads to the machine writing crap on the screen.
« Last Edit: 22:40, 24 June 13 by endangermice »
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Offline Bryce

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Re: CPC6128 white screen on startup
« Reply #44 on: 23:15, 24 June 13 »
Hmmm, now that sounds like an interesting compromise. Do you know that the dropout voltage of the 78L05? Some sites are suggesting it might be as low as 0.5V. Off the top of my head I'd probably try to source a transformer with dual windings so I can be sure to provide enough current at each voltage. 2A for the 5V will be more than enough for a C64.


Anyway this is getting off topic - though such a device could also power a CPC ;) .

Sorry for going off topic again for a minute...
First.... Oops, I wrote the wrong part No. above, I meant the 78S05. The Dropout is 8V, so you could easily feed it with the a 2x 9V transformer, taking the 9V from the centre tap. And it will keep the 5V stable even if the 9V dips for any reason. Here's the datasheet: http://www.reichelt.de/index.html?;ACTION=7;LA=3;OPEN=0;INDEX=0;FILENAME=A200%252Fl78s05c.pdf

Back on topic. If it's only single random keys that aren't working, then the membrane probably just needs to be cleaned and dust removed. If it's a row or group of keys, then it's something on the PCB usually.

Bryce.


 
« Last Edit: 23:18, 24 June 13 by Bryce »

Offline endangermice

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Re: CPC6128 white screen on startup
« Reply #45 on: 00:43, 25 June 13 »
Thanks Bryce, really appreciate it, that makes a bit more sense now - the datasheet for that other component suggested it delivered just 100 or so milliamps I assumed there were several versions. I've found a transformer with 2 x 9V taps at 2.5 amps each which should be good. I'll see if reichelt have something similar, would be great to get them both from the same place (along with supporting components etc.).

Good point about the dirty membrane, in a moment's stupidity I forgot the possibility of that which is an all together easier repair.

Anyway Jon, if nothing else you've learnt from this thread how to fix your 6128 and as an aside how to build a C64 power supply ;).
« Last Edit: 00:49, 25 June 13 by endangermice »
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Offline Bryce

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Re: CPC6128 white screen on startup
« Reply #46 on: 10:37, 25 June 13 »
I'll PM you with further information, so as not to take this thread completely off topic,

Bryce.

Offline JonB

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Re: CPC6128 white screen on startup
« Reply #47 on: 21:41, 25 June 13 »
Today I received 16 ram chips bought from someone on eBay. Since I've already socketed the motherboard, I rushed upstairs and whacked them in.

Unfortunately, the new memory hasn't made any difference. I still have black border and white screen. I can swap out the other socketed chips and check I inserted the memory correctly but assuming the RAM is OK, what else should I check, and how?

Can the 6128 be run with only one bank?
« Last Edit: 21:43, 25 June 13 by JonB »

Offline Munchausen

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Re: CPC6128 white screen on startup
« Reply #48 on: 22:14, 25 June 13 »
Yeah, it should still boot with only one bank, though I think the ROM message will still say 128K.

I think on most 6128s the Z80 and gate array (numbered 40010) are socketed, so you could try swapping those out with the ones in your working 6128?
« Last Edit: 22:16, 25 June 13 by Munchausen »

Offline JonB

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Re: CPC6128 white screen on startup
« Reply #49 on: 23:09, 25 June 13 »
Yep. Already tried the Z80, off to give the gate array a whirl...