Author Topic: CPC 6128: display cutting out, garbled characters - please help  (Read 3051 times)

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Offline Triggerhappy

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Hello there! First of all: Thank you, Mr. Admin, for activating my account. Sorry that my first posting is a cry for help but I am really out of my depth here.


I recently saved a CPC 6128 from the trash. It's the German Schneider variant, board number 94HB R1706 (at least I think that is a version number). Since I do not have a monitor, I bought a SCART cable and modified a power supply with a 12 and 5 volt rail to connect to the respective plugs. I also bought a Gotek drive, since the original was not working anymore and I do not have any disks for the machine anyway (but I have not connected the Gotek yet).


To my delight, the machine powered on and did display a picture on a modern LCD TV. However, the display did cut out every few seconds for a second or two. Since the machine was rather filthy I cleaned it up (including the power switch) and replaced all of the electrolytic capacitors, just as preventive maintenance. While the board does look better now, the display still cuts out every so often (irregularly), additionally, I now have some garbled characters in the middle of the screen that I am pretty sure still looked normal when I first powered it on.


My first guess would be that the DIN connector for the monitor is damaged but I cannot see any dirt or corrosion there. I did read a few threads on this board and watched some Youtube videos (the one from Noel's Retro Lab was really well made) but so far have not found one that describes the problem that I have.


While I can solder, I have next to no knowledge of electronics and only limited tools so I really don't know how to proceed from here. Any help is greatly appreciated! The CPC 6128 was the first computer my family owned, so I would love to have one to play around with.


Also, I was really impressed by the build quality of the machine. I did mess around with a C 64 and a ZX Spectrum in the past and both were built a lot poorer, especially the Spectrum, that had multiple capacitors criss-crossing each other.


Here are two pictures of the display and of the main board, maybe these help.







Offline Bryce

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It could be several things. The first thing I'd check are the solder joints under the monitor connector, they may have dry joints. I'd also try removing and reseating the Gate Array. However, it could also be something more complicated such as a failing CRTC or even failing RAM.

Try the first two things and let us know if the problem persists.

Bryce.

Offline Triggerhappy

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Thank you for your quick reply! I will try these solutions tonight and report back.


Edit: No luck. I cleaned and reseated all socketed chips (the CPU, the gate array and a small one two chips above that). I resoldered the monitor connector. The display still cuts out for a short time every few seconds. In the first trial, the garbeled characters were actually gone, only to reappear the next time I switched on the board.


My next thought was to try a different power supply, since I don't particularly trust my modification. This is the PSU I modified and used originally: https://www.ebay.de/itm/Netzteil-12V-5V-2A-fur-diverse-externe-Festplatte-DVD-CD-Gehause-Case-6Pin-L/312168482731 It is for external hard drives, so it has a 5 volt rail (4.5 amps max) and a 12 volt rail (3.0 amps max). Even if the combined rating is lower, it should still be enough to power the machine. I also tried it with only the 5 volt pin connected but that didn't have any effect.

The other PSU I tried only delivered 5 volts at 1 amp but managed to power up the CPC anyway. Sadly, it showed the same result: The display still cut out intermittently.


Just to see if the machine was rebooting or just showing a black screen I entered some characters using the keyboard membrane and my fat fingers ... but the characters stayed on the screen when it turned back on so I do not think the board is turning off when it turns black.


Any ideas?


(Sorry for my confused English, it is not my native language.)
« Last Edit: 22:26, 20 March 20 by Triggerhappy »

Offline Bryce

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If they don't work, I'm in Germany and you can send the board over to me any time you like.

Bryce.

Offline Triggerhappy

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Thank you for the offer, that is very kind of you. I am still hoping I might be able to fix this myself.


One more thing that might help: The board was really, really sticky, covered with a yellow-brownish goo. Some residue still remained, even after washing the board.

Offline robcfg

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Willkommen!


Would you be so kind as of looking the MCxxxxx code that is located near the headphone jack?


Just checking, your board appears to be the MC0023D, but I'd like to confirm that.


Thanks!

Offline Bryce

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Thank you for the offer, that is very kind of you. I am still hoping I might be able to fix this myself.


One more thing that might help: The board was really, really sticky, covered with a yellow-brownish goo. Some residue still remained, even after washing the board.

My first option is always to see if the owner can fix it themselves. The "send it to me option" is the last resort when the user doesn't have the knowledge, skills or equipment to go any further themselves. However, I always mention the send it to me option early on, so that people don't try things outside their limits that could damage the computer further, or even worse, throw it away because they think the options have ran out (has happened before here).

Sticky yellow/brown goo sounds like the previous owner was a heavy smoker. That shit is hard to remove, you'll probably have to use isopropanol or something similar. If you don't have isopropanol at the moment, don't buy it yet. The corona situation has caused the price of iso to triple in the last month. It was €10 for a litre, now around €30.


Bryce.

Offline Triggerhappy

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The label near the headphone jack says MC0023C, I think. It's a bit hard to read, since it's partially hidden under a metal shield. I did clean the board a second time; I do have various spirits available, including isopropanol. Smoke residue isn't particularly pleasant but it does protect plastics from yellowing. After you've washed off the residue, the stuff looks almost like new. :-)

And don't worry, I don't think I've damaged anything so far. I might leave the occasional scratch mark here or there but it seems to me like a lot of retro electronics came pre-scratched from the factory anyway. The Amstrad board looks pretty good but some of the stuff Sinclair or Commodore produced looks like it was dropped in the factory a few times.;-)

If I cannot get the thing to work, I might take you up on the repair offer. Feel free to quote me your prices in a direct message.
Edit: I do have a cheapo multimeter and a cheapo oscilloscope availabe to measure stuff. (Though I do need to fix the oscilloscope first. It was a DIY kit that was missing one component, which I got recently delivered.)
« Last Edit: 16:03, 21 March 20 by Triggerhappy »

Offline vasilisk

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This chip looks a bit different (kind of burned). Is it the contrast of the image?

Offline Bryce

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This chip looks a bit different (kind of burned). Is it the contrast of the image?

That's just from the PCB cleaning fluid he used.

Bryce.

Offline vasilisk

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ok. just checking  :D

Offline Audronic

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@Triggerhappy


Would it be possible to send a photo of the screen with the Opening Header and the alphabet and numbers below the Ready.
As I am wondering if it is the Rom that has Bit Rot ?(Faulty Rom).


 Just a thought.


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Offline Triggerhappy

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Hello again! Sorry for the radio silence. I tried a few more things mentioned here and in private messages and ordered some replacement parts, which took some time to arrive. Unfortunately, nothing I did fixed either the garbeled characters or the screen cutting out. So I am going to contact Bryce to have a professional look at the board. Thank you, everybody, for your suggestions!

Offline Triggerhappy

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One small addendum: Should anyone look for replacement keyboard springs, these fit perfectly:


https://www.ebay.de/itm/0-3-0-5mm-Druckfeder-Länge-5-50mm-304-Edelstahl-Druck-Kleine-Federn-5-12mm-OD/303319800891


Size: 0.5 x 10 x 15 mm


They are a tiny bit stiffer than the originals but I actually like it that way. I needed to replace only one that got lost but ended up replacing the springs for the Escape, Enter, and Return keys, just so they get a more satisfying punch.  :D


Maybe somebody will find that useful for repairs.

Offline Bryce

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The board just arrived. It will be on the healing bench tomorrow (after I've cleared enough space :D ).

Bryce.

Offline Bryce

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So... The good news which is also the bad news: I gave the board to my testing department (my 8yr old son who played games on it for most of the day). The display (it's connected to a 1084 Monitor) never lost the picture and it never showed garbled characters. This means that the CPC isn't the issue. As you've swapped the Powersupply without a change of results, I suspect the converter you're using or the LCD monitor is the issue.

Bryce.

Btw: Nice soldering. I usually clean up other peoples soldering, but the solder work on the CRTC was excellent. However, I suspect you may have used old or lead-free solder, so I replaced it with leaded solder anyway.
 

Offline Bryce

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Another day of testing, this time with temperature changes and PCB flexing, but still no garbled text or display flickering, the problem definitely lies elsewhere.

Bryce.

Offline Gryzor

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Yes but what does the QC dept say.

Offline Bryce

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The QC department decided that the problem lies either with the PSU or the Monitor. I monitored the CPC video output for hours* and it never once dropped out or went out of spec, so the board has been sent back.

Bryce.

* In case anyone is wondering how this is done, most digital oscilloscopes have a go-no-go option to mark the waveform and report an error if it ever wanders outside this region.

Offline Triggerhappy

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Hello Bryce and everybody else still interested!


First of all, sorry for the delay! I only found time this weekend to conduct further tests. To make a long story short, the problem seems to be the cable. I contacted the seller and he stated that this type "only works with CRTs", and that this was written in the description. This is a rather generous interpretation of a bracketed "CRT" (without the word "only" or any further elaboration) in the item description but I guess I should learn to read more carefully.


Anyway, does anybody know of or maybe sell a replacement cable that works with modern LCD TVs? Or is it possible to fix or rework the one I have? I am afraid using a CRT is really not an option, I simply do not have enough space to put one (also, I don't much like these old displays for a variety of reasons).

Offline VincentGR

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Yes you can rework the cable, there are some schematics on the wiki.
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Offline Bryce

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Sorry, but I have to disagree with the seller. The picture you posted shows bits missing, ie: a digital problem. The signal and cable at that point is purely analogue. That means the picture could be distorted or possibly blurred, but it definitely couldn't be missing bits due to an issue with the cable.

As well as that, there's no such thing as a cable just for CRT's, LCD need exactly the same signals at exactly the same levels. Tip for the future: Don't buy cables from someone who doesn't seem to have grasped the basics of what they are selling.


Bryce.

Offline Triggerhappy

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Sorry, I did buy other cables from this seller, wich all worked fine. Since this guy/company sells almost exclusively retro cables, I would assume they know what they are doing. Also, the "broken" characters did not appear the first time I powered up the machine. This might be an unrelated issue or maybe I did damage the cable somehow.


Anyway, I will consult the Wiki, as suggested. Although that might have to wait until next weekend. Thank you for your help!

Offline Triggerhappy

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Hello everybody! Well, it certainly took a while but the machine is finally running! I bought an "active" SCART cable, and that did the trick. Now, not everything is perfect. I cannot plug the scart cable directly into my TV but have to go through a converter/upscaler instead. And I have to restart the computer two or three times before I finally get a picture. But I do get one, that's the important thing! So, if anybody is looking for a working TV cable, I got mine from the seller ikonsgr74 on Ebay. (I hope this "endorsement" does not break any rules here.)






Some further notes:


- Somehow the power LED isn't working. Maybe I forgot to plug it in but I'll be darned if I take this thing apart again, because ...
- ... the Gotek mount was a b*tch to work with. I don't know if my print was bent or damaged or what the reason is but this thing barely fits inside the case, and only holds in place because I forced a screw into it that is way too large. I doubt I will be able to pull it back out without breaking the mount. I think I just might replace it with some DIY solution but that will have to wait.
- Oh, and of course you couldn't just "unplug the old drive and put the Gotek in its place", like the seller promised, because you need a special cable to connect the CPC's drive interface to the Gotek. Would have been nice of the seller to mention that.
- Sometimes, programs will just not load from the Gotek. I haven't figured out why. Just seems random. Restart, and it works fine. *shrug*
- The power adapter that I've built induces some nasty noise into the CPC's speaker circuit. Fortunately, it isn't audible going through the headphone jack.
- The case and keyboard still look pretty nasty, despite my best efforts. I covered up the discolored drive plate with a nicer looking sticker and repainted some of the most prominent scratches, but I might need to really rework the whole thing. If anybody knows exact matches (RAL, FS ...) for the original colors, I'd love to hear them.


I think that was one of the longest restorations I've undertaken so far, and it's still far from perfect. But the machine runs now, and I can relive some old memories! I thank you all for your support!





PS: Anybody able to identify this "frisky freak"? ;)
PPS: Just to be clear: The Gotek mount did *not* come from the same source as the cable but from a different Ebay seller that I do not want to mention by name.
« Last Edit: 19:10, 21 June 20 by Triggerhappy »