Author Topic: CPC 6128: display cutting out, garbled characters - please help  (Read 300 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.







Online 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 »

Online 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!

Online 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?

Online 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.


Ray
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