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General Category => Amstrad CPC hardware => Topic started by: JonB on 14:04, 18 June 13

Title: CPC6128 white screen on startup
Post by: JonB on 14:04, 18 June 13
Hi All
 
I'm new here! I just acquired a 6128 from ebay (uh-oh..) sold as untested -to be fair the seller listed it as "for parts or not working". I thought I'd try it as he claimed not to have any power supply to test it with (I guess this is a common problem with these machines what with people chucking CRTs away).
Anyway, once I'd got it out of its inadequate wrapping, I connected it all up using a power supply that is identical to the one sold by RetroComputerShack. It's a cheap Chinese HDD supply - gives +12v and +5v at 2A each. I've wired the 5v as +ve tip and the 12v as -ve tip per all the reading I've done (and the RetroComputerShack picture which shows these polarities too). The 6128 has a dicky power switch that needs a fiddle before it will close, but on successful connection I see the power light come on but it is not booting. The screen is white with a black border. It's connected to an LCD TV via a SCART lead; another RetroComputerShack product.
So... what does this indicate and how can I fix it? I had a look at the WIKI's service manuals page and there doesn't seem to be one for a 6128 - the link takes you to a 664 manual and this has a flowchart that suggests Z80, ULA or PCB bad.
Can anyone advise please?
 
Thanks
 
JonB
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 14:22, 18 June 13
Try swapping the Z80. It likely that it's broken and it's socketed, so easy to swap.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 14:51, 18 June 13
Thank you for your quick reply. I tried it just now with a Z80A out of my Memotech MTX512 (sacrilege but it's the only machine with a socketed Z80 I have). I still have a white screen with no boot.
 
There is also a slight whine (interference?) coming from the loudspeaker which responds to the volume wheel.


One thing - the power supply I was using just blew up and took the mains supply down, so I am only giving it 5v. As I understand it, that should allow me to boot into BASIC but obviously the floppy is not going to work. That's a bit moot because I have no discs and it needs a drive belt anyway!

With the Z80 out of its socket I get no video signal at all - it must be doing something. How can I tell if it is the ULA and if so, can replacements be bought?

Regards

JonB
Title: Re: CPC6128 white screen on startup
Post by: ralferoo on 15:46, 18 June 13
My feeling is that if you have a border at all, the Z80, ROM and gate array must be working, so it's probably just the RAM.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 16:03, 18 June 13
Hmm, that's the hardest bit to fix, all soldered straight to the board like that... 2 banks of 4164s? Can they be tested at all?
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 10:40, 19 June 13
As mentioned in other threads... Stick your finger on each one and see if any are getting hot.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 11:23, 19 June 13
Thanks Bryce, good point.


This morning I have left it running for an hour or so and checked the chips. Those that are warm are all roughly the same temperature and none of them are so hot I couldn't touch them. How hot would a broken chip generally be? As in "too hot to touch"?


Another thing occurred to me - is it a real black border or is my 16:9 TV set to the wrong aspect ratio? So I checked, by forcing it to 4:3. There's a definite border here and I would say the screen is white but one might say grey if the brightness / contrast were set lower. There is another thread where the OP talks about a grey screen and black border that is currently active. Perhaps we have the same problem...


Regards


JonB
Title: Re: CPC6128 white screen on startup
Post by: JonB on 11:38, 19 June 13
I have done a small test on the CPU by pulling the reset line low. It is resetting back to the white screen, but I think this is showing the CPU is working. Quick check of the address lines with a logic probe show them pulsing, so the CPU is getting a clock signal and trying to do something.


Interestingly, the two ROMs are cold and so are the four 74LS153 chips. Is this normal?
Title: Re: CPC6128 white screen on startup
Post by: JonB on 12:14, 19 June 13
There's a modification on the PCB, is it normal? See picture. The white wire is connected to the floppy drive cable pins, and a track has been cut (circled in orange).
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 12:19, 19 June 13
Looks like an ABBA switch mod, but you've zoomed in a bit too close, which IC is that?

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 13:28, 19 June 13
Try this..


It's the underside of IC206, a 74LS38 logic chip. Pin 6.. Actually, looking at the PCB layout in the service manual, the link is restoring the broken track. Odd..


Oh, by the way, the CPCWiki's link for the 6128 service manual is incorrect. Following it loads the 664 manual.


Regards


JonB
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 13:41, 19 June 13
Looks like someone had an ABBA switch there at some stage and reverted back to the original configuration later.

Gryzor has just updated the service manuals lately because they were corrupted, looks like he's mixed something up.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 14:08, 19 June 13
Thanks Bryce. What about the temperature test of the chips? Should they be too hot to touch if blown?
Title: Re: CPC6128 white screen on startup
Post by: Devilmarkus on 14:29, 19 June 13
If they are already blown, they perhaps don't even get warm.

Another step is to measure all IC's input voltages.

Perhaps somewhere is a power missing?
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 16:10, 19 June 13
Yes, they would be noticably hotter than other the others, possibly too hot to touch.

Regarding the ROMs and other ICs, they will always be cold, the ROMs aren't accessed enough to get them even slightly warm, same with most of the 74xx logic.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: gerald on 17:45, 19 June 13
Actually, looking at the PCB layout in the service manual, the link is restoring the broken track. Odd..

That's an AMSTRAD original patch  :)
I have one 6128 with the same patch (MC0009B). That version of the PCB has two flaws
  - the drive select for internal drive is connected on Pin 6 instead of Pin 4. The track cut and wire fix this
  - the ready signal from internal drive is unconnected. There is a wire that goes from floppy centronics footprint pin 2 to the internal drive connector pin 26

The funny thing is that the service manual shows a MC0009A (older ?) PCB, which is correct.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 17:48, 19 June 13
Ah, good... that's a relief. Now just need to get some new RAM for it..
Title: Re: CPC6128 white screen on startup
Post by: robcfg on 18:41, 19 June 13
Guys, could any of you take high quality pictures/scans of both board's sides?


The MC0009B model is missing from our motherboards wiki page (http://www.cpcwiki.eu/index.php/Mainboard_Versions#CPC6128_version_1_.288pin_data_separator.2C_solder_points_for_old_and_new_gate_array.29).


Your help is appreciated!  :D
Title: Re: CPC6128 white screen on startup
Post by: JonB on 19:15, 19 June 13
@robcfg : I will try and get round to it.


Meanwhile, following some of Bryce's other advice, I tested the RAM chips with a multimeter using the method in this post (I tried to paste the URL into my post but it does not work (due to the brackets, I suspect) - please cut & paste into your browser URL bar):-


Code: [Select]
http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/(hw-problem)-cpc464-with-'poked'-screen/msg50859/#msg50859

The results are a bit confusing. These measurements are for the left hand bank and all are in Mega Ohms. There doesn't seem to be many patterns there, perhaps they are all zapped? Has anyone got a set of measurements from a working 6128 for comparison?




PIN| IC126 | IC125 | IC124 | IC123 | IC122 | IC121 | IC120 | IC119 |
1  | 0     | 0     | 0     | 0     | 0     | 0     | 0     | 0     |
2  | .8    | 0     | 0     | 0     | 0     | 0     | 0     | 0     |
3  | 0     | .47   | .78   | 1.37  | 1.52  | 1.68  | 4.45  | 1.42  |
4  | 0     | 5.13  | .526  | 0     | 1.62  | 0     | 0     | 0     |
5  | 0     | 0     | 0     | 0     | 1.62  | 0     | 0     | 0     |
6  | 1.5   | 0     | 0     | 7.43  | 6.76  | 6.79  | 6.55  | 0     |
7  | .454  | 0     | 0     | .462  | .462  | .462  | .465  | .460  |
8  | 0     | 0     | 0     | 0     | 0     | 0     | 0     | 0     |
Title: Re: CPC6128 white screen on startup
Post by: gerald on 19:56, 19 June 13
Guys, could any of you take high quality pictures/scans of both board's sides?

Best picture I can  :D
My scanner is not big enough !!!
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 22:37, 19 June 13
@robcfg : I will try and get round to it.


Meanwhile, following some of Bryce's other advice, I tested the RAM chips with a multimeter using the method in this post (I tried to paste the URL into my post but it does not work (due to the brackets, I suspect) - please cut & paste into your browser URL bar):-


Code: [Select]
http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/(hw-problem)-cpc464-with-'poked'-screen/msg50859/#msg50859



The results are a bit confusing. These measurements are for the left hand bank and all are in Mega Ohms. There doesn't seem to be many patterns there, perhaps they are all zapped? Has anyone got a set of measurements from a working 6128 for comparison?




PIN| IC126 | IC125 | IC124 | IC123 | IC122 | IC121 | IC120 | IC119 |
1  | 0     | 0     | 0     | 0     | 0     | 0     | 0     | 0     |
2  | .8    | 0     | 0     | 0     | 0     | 0     | 0     | 0     |
3  | 0     | .47   | .78   | 1.37  | 1.52  | 1.68  | 4.45  | 1.42  |
4  | 0     | 5.13  | .526  | 0     | 1.62  | 0     | 0     | 0     |
5  | 0     | 0     | 0     | 0     | 1.62  | 0     | 0     | 0     |
6  | 1.5   | 0     | 0     | 7.43  | 6.76  | 6.79  | 6.55  | 0     |
7  | .454  | 0     | 0     | .462  | .462  | .462  | .465  | .460  |
8  | 0     | 0     | 0     | 0     | 0     | 0     | 0     | 0     |

You measured from pin to ground?

@Gerald: Very interesting, I didn't know there were production corrections on any CPC board (Spectrum was famous for them).

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: gerald on 23:35, 19 June 13
@Gerald: Very interesting, I didn't know there were production corrections on any CPC board (Spectrum was famous for them).

When I first saw this, I had the same reaction you had, ABBA switch gone wrong ;)
But I the soldering looked too clean and the ready signal fix involved an additional drill in the PCB.

It seems the layout engineer messed up when moving the diode and resistor from bellow to above the internal floppy connector.
These with a re-routing of the 12V input seems to be the only changes from MC0009A to MC0009B
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 23:50, 19 June 13
What was wrong with the previous layout? Are there known issues that justify a layout revision?

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 10:58, 20 June 13
No, from pin to opposite per your advice in the linked-to thread I mentioned above.
 
Quote from: Bryce
Ok, with a multimeter you can do a quick test to check the RAMs without having to remove them:

With the power turned off, measure the resistance across oposite pins of each IC. ie: Resistance between pins 1 and 16, resistance between pins 2 and 15, 3 and 14, and so on. Write the value down for each IC in a table. If an IC is damaged, it will be obvious from the table, because the value of one of the measurements will be a lot different from the others.

If no RAM IC seems to be damaged, then I am guessing that it's one of the RAM related logic ICs that's broken, but this could be any of the following: IC104, IC105, IC109, IC113, IC114 or IC115 and this could be difficult to diagnose without further equipment (unless you spot one that's getting hot.

Bryce.

Perhaps my measurements indicate they are all blown? But of course, as they are in situ, we are also measuring the resistance of the board and all other connected components.
Title: Re: CPC6128 white screen on startup
Post by: gerald on 11:17, 20 June 13
What was wrong with the previous layout? Are there known issues that justify a layout revision?

Bryce.
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
Title: Re: CPC6128 white screen on startup
Post by: robcfg 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!
Title: Re: CPC6128 white screen on startup
Post by: dragon 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.
Title: Re: CPC6128 white screen on startup
Post by: Bryce 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.
Title: Re: CPC6128 white screen on startup
Post by: Devilmarkus on 18:14, 20 June 13
Shouldn't we get back to topic?  ;D
Title: Re: CPC6128 white screen on startup
Post by: Bryce 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.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 10:58, 21 June 13
OK, I'll fit replacement RAM and see where it takes me.
Title: Progress..
Post by: JonB 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.
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 16:48, 22 June 13
Yes, the CPC should boot normally when the keyboard and drive aren't attached.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: endangermice 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 :).
Title: Re: CPC6128 white screen on startup
Post by: JonB 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!
Title: Re: CPC6128 white screen on startup
Post by: endangermice 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.
Title: Re: CPC6128 white screen on startup
Post by: JonB 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!
Title: Re: CPC6128 white screen on startup
Post by: endangermice 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 :).

Title: Re: CPC6128 white screen on startup
Post by: Bryce 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.
Title: Re: CPC6128 white screen on startup
Post by: endangermice 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!
Title: Re: CPC6128 white screen on startup
Post by: Bryce 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.
Title: Re: CPC6128 white screen on startup
Post by: endangermice 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!
Title: Re: CPC6128 white screen on startup
Post by: JonB 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...
Title: Re: CPC6128 white screen on startup
Post by: endangermice 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.
Title: Re: CPC6128 white screen on startup
Post by: Bryce 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 (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.


 
Title: Re: CPC6128 white screen on startup
Post by: endangermice 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 ;).
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 10:37, 25 June 13
I'll PM you with further information, so as not to take this thread completely off topic,

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: JonB 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?
Title: Re: CPC6128 white screen on startup
Post by: Munchausen 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?
Title: Re: CPC6128 white screen on startup
Post by: JonB on 23:09, 25 June 13
Yep. Already tried the Z80, off to give the gate array a whirl...
Title: Re: CPC6128 white screen on startup
Post by: gerald on 23:58, 25 June 13
The CPC only need the 1st bank (IC127-134) populated to run.
The 1st bank is the right one when you have the CPC in front of you.
The second bank is not used by FW, and you do not need it right now.

You can also check the PAL (IC118), which is already on socket. It is dealing with the memory access.
You can even try without it by closing LK5/LK6/LK8.

Next on memory/cpu data path are the bi-dir buffer made by IC115 and IC114. If IC115 is dead, write to memory and GA will not work. if IC114 is dead, read from memory will not work.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 00:04, 26 June 13
Alas, swapping out the gate array or the other socketed chip (IC118 - 40031) hasn't fixed it, neither has swapping out the Z80.
 
 :(
 
I suppose I will have to swap out those buffer chips next. I tried piggybacking an LS244 over the one on the board but it had no effect. Don't have an LS373 right now.. any way to test these in situ?
Title: Re: CPC6128 white screen on startup
Post by: Munchausen on 02:41, 26 June 13
Hmmm. I guess you could test them in situ, but it will be a bit of a pain.


Here's the pin out:


(http://circuits.datasheetdir.com/105/DM54LS373-pinout.jpg)


The output control pin enables/disables the outputs (Q pins), I think it is active low, so to enable the outputs this should be low. Then you should see that whatever you put on any of the D pins (a high or low level) appears at the corresponding Q pin whenever you transition the enable pin from low to high.


So you could test it in place by connecting the power lines and control line up, putting inputs on some of the pins and toggling G, and checking the outputs with a multimeter but it seems a bit tricky to me (you'd probably need a couple more pair of hands to get it done without soldering!)


I'm quite surprised you haven't managed to coax some kind of life out of it yet, TBH... I have problems I haven't figured out yet on both my 6128s, but they worked to varying extents without this much trouble. You'll get there if you keep at it though!
Title: Re: CPC6128 white screen on startup
Post by: Munchausen on 02:42, 26 June 13
Alas, swapping out the gate array or the other socketed chip (IC118 - 40031) hasn't fixed it, neither has swapping out the Z80.


Also, just in case: tried both at the same time, and the PAL (40031)?
Title: Re: CPC6128 white screen on startup
Post by: gerald on 10:14, 26 June 13
Have you tried the other way : testing IC from defective CPC in a working one, one by one ?
I would also re-check the RAM path with a multimeter for shorted or open track
Title: Re: CPC6128 white screen on startup
Post by: JonB on 11:28, 26 June 13
@gerald: It's a good idea. I'll see to it. As to the ram path, yes, it's possible there is a problem there of course. I noted the 6128 PCB has particularly thin tracks in places, which makes it fragile. A close examination under a magnifying glass reveals nothing obvious, but you're right, the meter is the only way to be certain...
Title: Re: CPC6128 white screen on startup
Post by: endangermice on 16:15, 26 June 13
Checking continuity with the schematics is definitely the way to go. I spent ages trying to get my 6128 board to work but in the end I did manage to track a single broken trace on the data bus. It's time consuming but if there is a break somewhere this will find it!


I think if you have a border the gate array is probably working. Are you able to check the address bus for pulses, assuming you haven't already done so. This will tell you whether the CPU is addressing anything. It's possible that it starts then crashes which is what was happening to me with the broken data bus.
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 16:40, 26 June 13
You should also keep an eye out for dry joints. These will usually look dull compared to the other joint, or sometimes you will see a tiny crack around the pin in the solder.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 17:04, 26 June 13
@endangermice: Yes, pulsing. I checked with a logic probe.. :)
 
@Bryce: Good advice, thanks. As I said, I inspected the board very carefully using a magnifying anglepoise type lamp (the sort with a ring shaped flourescent bulb with a huge magnifying glass in the middle) and could not see anything in particular... except that all joints look a little dull, which is probably down to the age of the thing.
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 21:54, 26 June 13
except that all joints look a little dull, which is probably down to the age of the thing.

That may be your problem. It can't hurt to slightly freshen up those joints. You don't need to remove the old solder, just reheat each one with a tiny bit of new solder and they'll be like new. It's no guarantee, but it may solve your problem.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: endangermice on 22:19, 26 June 13
This is very good advice, at worst a dry joint will render the contact useless, it
 may also make a part connection with a large amount of resistance which will play havok! I had a SID chip recently that I thought was knackered - it was playing very quietly and out of tune. In the end it turned out to be a bad joint on the VCC which caused the 5V to drop to 1.74. I was amazed it worked at all!

Have you checked the RAM address and data lines for pulses, at least the first 64KB not sure if the second bank is addressed until switching has taken place....
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 22:27, 26 June 13
The most annoying dry joint is when they completely break through and then the surfaces of the break oxidise. It creates a perfect very low value capacitor that varies with temperature and creates the most random results possible. :D Especially on data or address lines.

Bryce.
Title: Re: CPC6128 white screen on startup
Post by: endangermice on 03:23, 27 June 13
It's a shame that this "freak" capacitor can't restructure itself into something useful like say an extra bit of RAM ;).
Title: Re: CPC6128 white screen on startup
Post by: JonB on 15:53, 27 June 13
Hmm, how many joints are there on that motherboard ?

I was thinking it least I might wash it with isopropyl achohol to remove any flux resue or other crap; then I could perhaps spot dry joints a bit easier. I am not sure I have the patience to resolder every joint, and I bet if I did it would make no difference.

As you can see I am losing faith. Don't want to spend much more money on it as it will be uneconomical.
Title: Re: CPC6128 white screen on startup
Post by: endangermice on 16:34, 27 June 13
Totally understand, I know how infuriating it can be! How are you getting on with the other CPC, have you managed to get the keyboard up and running yet?

To be honest once you get going with the soldering iron you'll be amazed at how fast you can re-melt all the joints it really doesn't take that long at all.

One thing you could do (and I've done this on a C64 in the past) is assuming the other machine boots to the Basic screen (I think it does from what you've said) you could spend some time with your logic probe measuring the results on the pins of each chip and comparing to your broken board. Start with the major chips i.e. CPU, ROM, RAM. It does take some time and the easiest way is to note down you results which be be along the lines of low, high, pulse, pulse low, pulse high etc.


If you find a chip that's behaving differently to the working machine then you need to identify what that pin does, is it an input or output. If it's an input then maybe the chip supplying that input isn't doing the right thing, so check there - it could all boil down to a broken trace or bad joint but you might be able to identify it a bit more easily using this method.


It's not an exact science but with some knowledge of how the machine operates it should help you towards diagnosing the problem. It will obviously help if the two machines are the same model though by and large you should get similar readings on most chips anyway.


The only thing you need here is time since you already have the testing tool.
Title: Re: CPC6128 white screen on startup
Post by: JonB on 18:43, 27 June 13
OK, thanks; again it's good advice. I think my order of working will be:
 
1. Clean the board with alcohol (heh, I could use some of that myself!) and check for dry joints.
2. Resolder dry joints, or all if there are no obvious ones (  :( )
3. Check the data lines with a meter.
4. Check the IC readings with a meter.
 
Obviously retest between steps..
 
6128 #2 is up and running, with the keyboard and (re-belted) drive of 6128 #1. So at least I can start playing, indeed, I have done. I even ordered a copy of "CP/M The Software Bus"..  :laugh:
 
I also took delivery of a pair of Amigas (A500 and A1200) but there was a problem with transit damage and a slight misdescription and considering I paid a goodly sum (due to the way the lot was described) I am engaging with the seller and hoping for an amicable settlement whereby I can keep them. That said, I've been playing with them (primarily, to ensure they work, which they do) and I am not certain when I'll be back in 6128 land. A few days, for sure...
Title: Re: CPC6128 white screen on startup
Post by: Bryce on 22:55, 27 June 13
Re-soldering really doesn't take that long. You only need to bring the solder to melt and jab it with some fresh solder at the same time (to make sure it gets some flux), which only takes 1 or 2 seconds per joint. I'd need about 30 minutes for a full 6128, but I've had a lot of practice, so you might need a little longer. But it's a well invested 30 Minutes. Make sure you clean the tip every four or five pins too. Set the iron to 260°C, then you're certain to be low enough for even the most sensitive parts on the board.

Bryce.