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

[banman]

Hi Gyro Gearloose,


Thank you for your time and advice, much appreciated.


I too am thinking that it might be memory related.

I will go thru and document what I see on and around the board.


I purchased this of eBay recently from someone similar to an estate seller. I have been watching this person for some time. They seem to sell bunches/ lots of similar goods. I imagine that this seller acquired it from a another party. So I would think it has passed hands 3 times so so far.

It is actually in very good shape barring it's very dirty and dusty nature. Surprising since the case is all plastic.

I think that this is what they call the Euro model. From what I've read this didn't make North American FCC regulations and was sold outside of there. I

Looking at the board it looks very much like the slim wedge style version. I have been looking at a service manual that deals with the original wedge style and the schematic diagram in the manual seems to match what's in front of me.

This is what I've been using...

https://archive.org/details/troubleshoo ... 8/mode/1up



The handle is in good working order. The keyboard stows up underneath the case itself when in storage. The keyboard has lost all but one of it's rubber feet. I think you were looking for replacement feet in another thread.

Notice that the keyboard cord has a long term storage kink in it.
I imagine that this might have been from long term storage.

[banman]

It was dusty on the inside. I can't show you the real extent of the dirt because I have cleaned most of it up.

Around the fan intake to the power supply there was a black coating of dust.
Here are some off what the dust looked like....

Maybe it was used as a daily business machine and when it failed it was just shelved away.

[banman]

It is interesting to see that the main board is manufactured in Hong Kong and the case sticker says assembled in Germany.

The C64's I've seen have a Hong Kong manufactured board in with a UK sticker on the case. I imagine that Hong Kong was a colony of Britain back then.

I might be incorrect about this, please tell me if I am wrong. I think the case sticker denotes a 1984 date.

The chips on the mainboard all have a surprising tight date band around 1986.

Also note that the bottom PCB board RF shield isn't soldered but just twisted off.

[banman]

One will note the use of silicone thermal conductivity grease on the chips and the very very small contact point where the tin plate RF shield fingers touch.

At certain points around the PCB board there is small stripes of old style cream coloured paper masking tape. Maybe it stops other object from rubbing or making contact with the PCB board.

[banman]

Notice here how the plastic stem has cracking around it. It is not from overtightening however rather over a long period of time the plastic has in fact contracted around the brass threaded insert. Extensive investigation of plastic led me to this hypothesis.


Unfortunately all plastic over time shrinks. It it very noticeable in nylon gears where they crack around the drive spindles an can't provide thrust anymore.

This cable attached to another logic board which has to do with the 1571 disk drive...

The cassette tape port has seen some use too.

[banman]

Hi Gyro Gearloose,
I do have a multimeter, a logic probe and an aging oscilloscope.
One thing I did notice is that the ground plane on the C128 mainboard is referenced to mains earth.

I did have a bad experience one time with a earth referenced piece of equipment and an oscilloscope. I didn't think it was ground referenced. Plus I was totally aware of the dangers of this! Very stupid of me.

So I make it a habit of checking any thing that plugs into a wall these days whether it looks earthed or not.

[banman]

I did notice that the video display chips never really had a good heat sink connection ever.

[banman]

After reviewing some of the helpful information you had given me Gyro Gearloose, I started with removing the two CIA chips from the C128.

I removed the CIA chip responsible for serial port activities U4 CIA #2.
I noticed this when I started the C128 in native mode.

The cursor is there. It's a very transparent color it almost blends into the background.


And this in C64 modes

This defective CIA chip has an unusual identification number. I am not familiar with this item.

MOS 8521R0

I tested it in a C64's keyboard CIA socket and the keyboard does not respond.
I did replace this defective item with NOS MOS 8520 I had from another project.

[banman]

It is still not running quite right unfortunately.

One can see vertical lines at every character position. Not to be confused with RF interference.

When I type in a "Hello World" program in C128 mode it might run for a short period before crashing. One thing I noticed was when "listing" the program the line numbers are replaced be reversed characters.

I will run a diagnostic cartridge in both C128 and C64 modes.

I'll get back to you shortly.....

[Gyro Gearloose]

I did notice that the video display chips never really had a good heat sink connection ever.

The HMOS chips run much cooler.

I think you have some sort of jellybean logic chip failure somewhere, the DRAM is likely good as it's Samsung and the machine seems to want to live.

I am not a big fan of these test cartridges, they don't provide all that much useful information I think. These 8 bit computers don't have the power to really self-diagnose.

Unfortunately the 128 is a more complex machine than the 64, but the same step by step process should be used to narrow down the problem(s).

Just check the supply voltages and reset line first.