• Please review our updated Terms and Rules here

Repairing a C128D

Nicolas 2000

Experienced Member
Joined
Apr 23, 2022
Messages
132
Location
Belgium
I've just tested my C128D. It appears to boot OK in C128 mode, also in C64 mode. I've successfully loaded games in C64 mode. Problems:

-no power LED. Might just be the LED itself. Ignore for the moment.
-bigger problem: part of the keyboard doesn't work. In particular:
...-everything between 8 and DELETE on the top row above the letters
...-the P and the three keys to the right of it
...-the 8 and 5 on the numpad.

Possibly more; I'm not sure if the numpad + and - are supposed to do anything in BASIC, they're not.

So would this problem likely be located inside the keyboard or inside the C128D? I'm not familiar with the technology, but as the nonfunctional keys appear to be grouped, I'm thinking either cut traces in the keyboard or something with the scanning matrix in the C128D?

Things I've tried: reseat the keyboard connector. Test cable for continuity: without further disassembly I could check all but 1 wire and all of them found their way to the connector, so I think the cable is OK.
 
Here is the C128 keyboard matrix:


I can't really find logic between the matrix and the faults I'm having; there isn't any one pin that would capture those groups of keys. So I assume that means the problem is more likely to be found in the keyboard than in the C128D itself? I'll take a look inside the keyboard.
 
Last edited:
Was pouring acid into a keyboard a thing in the eighties? I've opened it up. The metal plate has rust on many places. The PCB has some corroded traces, just like those you'd see when you have battery leakage. On first sight, these traces do seem to coincide with the nonfunctional keys and they fail continuity.

So the good news: it looks like I've found the problem, and it's not inside the C128D. Also, I've learned about C128 keyboard functionality.
The bad news: it seems like I'm in for a job of replacing quite a lot of quite closely spaced traces, and I have no experience with that. Any pointers to a good tutorial or tips? I know the general idea of scraping off the mask from good parts of the trace and soldering a wire in between. I'm not sure there is enough space here though; I shouldn't interfere with the nearby keys! So, are there methods that avoid this issue? I don't know, perhaps when I take off the mask from the affected spaces, I could solder over them to reconnect?

And how -apart form the obvious keeping it dry- do I prevent further corrosion?
 
Last edited:
Hallo Nicolas,

In the worst case I can help you with another keyboard, or a friend of mine who lives near Lokeren. About the corrosion: just make sure things stay dry. About the wiring: use fine transformer wire.

Good luck!
 
I have recently repaired a C128D keyboard

The traces on the circuit board had corroded.

Just a matter of scraping off the green insulation to expose some healthy copper then using a conductive PCB repair pen to redraw the traces back in.
 
Last edited:
IIRC, internally the 128DCR keyboard is the same as the one in the all-in-one C128, so if your plastics and cable are all still good, that would be a cheaper option than trying to find something specifically for the 128DCR. I don't know if that still hold true for the plastic-case European 128D though.
 
The 128CDR keyboards I have are different switches and keycaps than my standard units. The locking keys sit above the board, and don't solder to posts.
 
I would not try to solder, the tracks are quite fine and easily damaged further.

The silver conductive pens are probably best but don't spend a fortune as the cheaper ones have low enough resistance to make a keyboard work. Use a meter to locate the defects then as I said above, lightly scratch the green covering off to expose bare copper then simply draw a new track. Let it dry then test the conductivity between the far end pad and the cable connection. Repeat until all connections are good.

If the pads on the keys are poor, there used to be some substance that would revive the conductive properties, but that seems not to be on sale any more. However the conductive pen can be used to draw on the rubber pad and this seems to work as well.
 
Ah shoot, I could have sworn... Sorry about that! Maybe I am thinking of the A500 and the A2000 or something!
 
Thanks for the input all!

I've also read that C128 and C128D keyboard PCB's are the same except for the cable which is soldered directly to the PCB on the D version and uses a plug on the all-in-one. Mine is a metal case D by the way, but I don't see a DCR designation. My locking keys had soldered posts. I would not be surprised to find there are many variants, as per usual.

Anyway, I've repaired two broken traces in the numpad section by soldering thin gauge network cable from a healthy part before to after the corrosion. I've scraped away as much corrosion as possible and conformal coated using 2K glue as that was easier to apply on small sections than my coating spray. I've also used the glue to keep the wires in place. The numpad works now. I can't get the corrosion out of the pad parts nor coat it there, so let's just hope the corrosion doesn't advance there now that it will be kept dry. For the moment, the keys work perfectly.

I'll try the same approach on the other corroded part and see where it ends. Worst case, I'll contact Ruud about a spare keyboard. But I have good hopes that I can get this one up and running as long as the corrosion doesn't advance in the coming years. And even then, the conductive pen approach might work for further repairs and pad repairs. As I don't have one now and the pads are working, I'll stick with wire repairs now. I have a decent digitally adjustable Weller and quite OK skills; I'm not too afraid of damaging the traces with this approach as I keep the trace quite cold.
 
Last edited:
I've finished bypassing the broken traces and coating it all. The keyboard works like new now! But it still looked awful so I've removed all keycaps, gave them a soak, and now they're drying. The black metal plate below the keycaps has quite some surface rust. I've scraped all loose parts off, and next task will be to paint it black. With a tiny brush, between all the key guides. Joy... But after that, it should be working and looking mighty fine.

So I guess this means I'll have to check my two C64's and prepare them for sale, as this C128D is all that and more! (keep ignoring the nonfunctioning power LED...)
 
Thanks! Let's hope the evil rust monster is put to rest now; I wouldn't like this to be a repeating feature of my C128D ownership...
 
The black metal plate below the keycaps has quite some surface rust. I've scraped all loose parts off, and next task will be to paint it black. With a tiny brush, between all the key guides. Joy...

I've had some luck using that spray-on phosphoric-acid-based rust converter for that. You brush the loose rust off as well as you can, then spray that stuff on and it converts the rust into a nonreactive matte-black coating. If you can't get to the surfaces to prep them very well without removing all the keyswitches, you could apply that with a brush and it might not flake off as bad as painting over a surface that you can't get to to prep well, I mean.
 
I had that thought too, as it indeed becomes hard and blackish. I didn't have it here though (last used 15 years ago on a classic car...), so I just went ahead with black enamel paint from Revell. If it flakes off in a way that bothers me a few years down the road (you hardly see the backplate when all the keycaps are on) I can try the rust converter approach.

For the moment, the keyboard is complete again. Looking fine, working fine. Happy! Now I'll look into some nice upgrades (thinking of an SVHS to SCART cable and a tape interface SD card reader) to fully enjoy the C128D. But the SVHS cables are sold out, as is a lot of stuff...
 
Back
Top