C128D displaying random characters on startup screen (plastic case model)

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banman
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C128D displaying random characters on startup screen (plastic case model)

Post by banman »

Hello,


I have a C128D that displays a full screen of random characters.

I was wondering if anybody has any suggestions on this.


This occurs in both 128 and C64 modes.
IMG_20230112_135828.jpg
IMG_20230112_135915.jpg
IMG_20230112_135844.jpg
I have taken off the top cover only for a quick inspection. It appears that no body has removed any of the metal RF/heat sink shielding. Though I suspect that the case has been opened before.

I checked the 12v and 5v lines out on the power supply. They read 12.02v and 4.94v respectively.


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Re: C128D displaying random characters on startup screen (plastic case model)

Post by banman »

I can run a C128 deadtest cartridge and I get this image.

]
IMG_20230112_140312.jpg

I can hear the Sid chip playing during the test processes.

I am thinking that the CPU is okay.

I can run a C64 deadtest cartridge and I get this....

This is the part at the begining of the test where the screen is black

IMG_20230112_140614.jpg
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IMG_20230112_140448.jpg
banman
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Re: C128D displaying random characters on startup screen (plastic case model)

Post by banman »

Here's a few more pictures of different C64 teadtest cartridge startups....


IMG_20230112_140744.jpg
IMG_20230112_140701.jpg
IMG_20230112_140618.jpg
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Re: C128D displaying random characters on startup screen (plastic case model)

Post by banman »

I can run C64 cartridges like Avenger. It looks like this. Notice the sprites are recognizable in amongst the garbled characters.

The game appears to respond to keyboard and joystick actions. Sound effects can also be heard..
IMG_20230112_140957.jpg
IMG_20230112_140950.jpg
Visible solar system also runs. Notice too that the the orbiting planets display correctly in amongst the garbled characters. I imagine that this is sprites in action.

IMG_20230112_141013.jpg
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Re: C128D displaying random characters on startup screen (plastic case model)

Post by banman »

Before I go an open up further and start probing the mainboard, maybe someone can suggest me what is wrong. Any help would be much appreciated, thanks.



I am imagining that it may be memory related.

I don't think it is the character ROM, VIC II display chip or the CPU.
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Re: C128D displaying random characters on startup screen (plastic case model)

Post by banman »

What the....!


I pulled the top tin plate heat shield off and cleaned the dust around the main board.

Then this thing just came off in my hands!
16735756149617628700202096738197.jpg
I looked at it carefully and it appears that the video chip RF shield wasn't even soldered to the mainboard. That was or wasn't done at the factory....
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Re: C128D displaying random characters on startup screen (plastic case model)

Post by banman »

It looks as though that it doesn't even fit thru the PCB board holes.

IMG_20230113_131708.jpg

I imagine that the only thing holding it situ is a large Philips screw that passes thru the top tin plate heat shield.

Has anybody else experienced anything like this before?

I imaging that a whole PCB board run will suffer from this.

This is the serial number of this machine.
IMG_20230113_132331.jpg
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Gyro Gearloose
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Re: C128D displaying random characters on startup screen (plastic case model)

Post by Gyro Gearloose »

I don't have this particular model of 128 but we can go step by step to attempt the repair. I've done it before through the forum but it does take time.

So since it powers up and it affects both modes, must be something in common. Typically I'd check the power supply voltages first even if it seems to be working.

Do you have a meter and a logic probe?

In any case my first suspicions would be toward the DRAMs and nearby 74 logic chips, especially if they are MOS branded.

So some history would be nice, when's the last time this computer worked? How long was it in storage in what conditions? When did the malfunction occur?

What about a visual inspection? Loose socketed chips, corrosion from spilled Nescafe in 1986? etc etc etc
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Re: C128D displaying random characters on startup screen (plastic case model)

Post by Gyro Gearloose »

I think watching this video might be useful
The price one pays for pursuing any profession, or calling, is an intimate knowledge of its ugly side.
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