Repair : Commodore 128 blackscreen repair - part 1

Quite a while ago I bought what I thought would be an easily fixable Commodore 128.  On the pictures it booted into 128 mode and c64 mode just had the blue screen with no text.

When the computer eventually arrived the green screen started with no text, that's odd and the C64 screen was not working at all.  Then after a few restarts nothing worked. Ugh oh!.

Let's open her up!

 

I've bought and fixed two 128's so far and previously I've used the rocking method on a soft surface with mixed results.  This time I tried a spudger and it work miraculously.  

 

Insert the flat end just above the joystick port and slowly move it to the high point.

As you get close to the highest point lift the spudger up, this pulls the bottom of the case out and pushes the top in.  Intern this releases the catch.

 

Once complete do the same on the other side.  

Maintenance on the Character and Kernel Roms

On opening the case, the first blatantly obvious issue was that the character ROM had fallen out.  It was a custom ROM in an adapter so that a larger 8K ROM would fit in the 4K ROM socket.  Reseating the ROM did not fix the issue either.

The second thing I noticed was that the Character and Kernel ROMs didn't have any tape over the windows.  So I covered them immediately and popped them out to verify the ROM contents.  Surprisingly both ROMs where 100% fine.  So I put them back in.

What is going on with the PLA?

I left the machine for a few days and when I started it again 128 mode was working again, but this time it only lasted for one reboot then disappeared.  This really confused me.  I removed the SID and still nothing.  I was about to pull both of the CIA's when I noticed the PLA had one of those round pin sockets.  I am really dubious about them so I tried booting again with my finger pushing down on Chip and it booted back into the familiar 128 boot screen.

I tried this a number of times and it was really really inconsistent. In the end the best method I found was to try to twist the whole socket.  Even bending out the pins to make better contact with the socket failed to help. At this point I knew the socket had to come out so I ordered a proper socket and waited for it to arrive.

Previous repair damage, what *%# happened to this board?

While waiting I decided to remove the old socket.  I used the desoldering gun and some solder wick clean up most of the pins. The socket itself proved to be difficult to remove, so I ended up carefully cutting it up the removing it piece by piece.  The disaster struck, part of a via and the connecting trace slightly lifted. However the the damage was minimal.  On inspecting the board I also noticed that one of the other vias was damaged as well, this one I definitely was not responsible for, it just was not there. 

Then I had a really good look at the board and noticed the damage on the Ram sockets where someone had applied way too much heat and burnt the board and then soldered in the sockets really badly.

This machine was a mess and it was pretty obvious a lot of repair work had been done on the board prior to me receiving it.

The damaged vias I decided where probably the primary source of the issue of the socket.  So I decided that was the first item on my list to fix,  I would deal with the rest later.

Replacing the vias

I've never replaced vias or riveted a PCB before so I took a fairly pragmatic approach.  I worked out the current diameter of the vias by inserting micro drill bits into the holes and then double checking the diameter with digital calipers.  The diameter worked out to be 0.8mm.  I then sourced some vias from EBay, specifically this item here.

As the internal diameter was correct I needed to drill out the holes very carefully using a hand drill to 1.2mm.

 After inserting the rivet I placed tape over it so it wouldn't fall out and them placed a standoff under the via to stop it pushing out.

Ensuring the rivet is sitting on the standoff by pushing it down a little, hopefully you'll meet the resistance that the standoff offers.

Using a circular motion slowly flatten the rivet down. then use the other end to ensure it is flat to the edge.

Once you are happy and the rivet is tight, you will need to solder the track back to the rivet and then more importantly check the continuity of the new via and track!

A working C128, well just the 128 part!

Soldering in the socket and inserting the PLA back in gave me rock solid C128 mode.

The final stretch, fix C64 mode

The final part of this repair wasn't straight forward either. I was using the C64 dead test cartridge and using the old put the new RAM chip over the top of the old RAM chip technique. I just couldn't get it work 100%.  On closer inspection every single one of the already replaced and socked RAM chips had terrible soldering. There were huge blobs and non-existant solder on every single socket. The first job was to resolder every single leg.  After that I was very quickly able to complete identifying and replacing two RAM chips.

So really there wasn't very much wrong?

In the end there really wasn't much to do.  However it does pay to inspect your unknown dead boards for bad repairs before you start the repair work.  I'm pretty happy how this turned out.

The final step for this will be to recap the electrolytic capacitors and I might look at a video memory upgrade using modern parts, but that is a whole other project.

But hang on wait . . . I've only check the C64 mode . . . does the C128 mode actually work properly?

See part 2

Indulging myself with a C128D

A while ago I bought a pretty cheap C128D keyboard on Ebay.  I have been using it a lot with my MiSTer using an Arduino as a USB HID interface.  The recently I got an Amiga 2000 keyboard to build a new interface for my MiSTer ** so the C128D keyboard was about to become surplus to requirements.

There is something about the C128D that I really liked so I picked up a working one all the way from Germany.  I'm really happy to say that it arrived safely and it works beautifully.  I am going to set this up with my 1084 monitor as my center piece for my Commodore retro collection.

It's so pretty!

** If you haven't seen the MiSTer project, you are missing out.  It is an Open Source project with a FPGA the runs a huge amount of retro computers, console and arcades.  I've been using it since it was first released, having been a Mist user previously.
MiSTer on Atari-Forum
MiSTer on GITHUB

Enhancement : Upgrading my C128's VRAM from 16K to 64K

I've know for a while that it was possible to upgrade my C128's VRAM from 16K to 64K of RAM.  I'd recently pulled it apart to make sure the caps were all still okay so I took the opportunity to give the upgrade a go.  Also I'd acquired quite a few ram chips for C64C repairs.

Ray Carlson has an article here on it :
http://personalpages.tds.net/%7Ercarlsen/cbm/c128/RGB%20V-RAM/64kvram.txt

I had seen the easy upgrade option using a daughter board.  I wasn't really happy about not being able to put the cover back on the graphics chips properly with that method.  With my new found de-soldering skill I just decided to remove the old chips and put in some sockets.

. . . and well the result . . . I wasn't really expecting any difference for the most part.  My C128 started, I tested it using Ray's test code.  I loaded and ran a demo that used it which for the most part worked however my Commodore Monitor did have some issues with some of the tested resolutions.  I even ran GEOS with the 3.3 Mega patch.  That's pretty much it.  Well if I need my chips back at least I can swap back in the old 8K chips in to the nice new sockets.

Enhancement : Gonbes VGA upscaling

Gonbes 8220 and C128 RGBI

I have quite a number of C64's, C128's and Amigas, but I only have one proper monitor.
New TV's really don't really work very well so off to the inter webs I went.

I've been looking at heaps of projects for RGBI conversion for the C128.  None really took my fancy until I saw this one from Mattis Lind, it was small, it was compact and it was surface mount.

• https://github.com/MattisLind/CGA-RGBI-to-Analog-RGB

The only thing I didn't like about it was the connections.  I'd rather have them on board than solder wires on to the board.  Luckily it's opensource and the Tool Mattis used was probably the easiest PCB design tool I've ever tried ( I've given up on every other one ).

So I added in some bits including mounting holes. I did make one mistake and that was to break the ground.  I used a jumper to fix that with the original holes for the wires that I left in place.

 As you can see the over all result was really neat with all of the colours displaying nicely.

Gonbes 8220 and Amiga RGB

I also wanted to get my Amiga's working nicely on a monitor too and not the TV I was using.  The colours were really washed out.  The pin out on my Amiga monitor cable was not compatible with the RGBI board.  However I was able to use it a as template to build a nice connector to the Gonbes unit on a bare RBGI card PCB ( note it's just point to point wires and a resistor, nothing else ).

Details for the hook up came from this link

• https://ianstedman.wordpress.com/2015/01/25/gbs-82xx-experiments-part-2/

Which I found under this video

• https://www.youtube.com/watch?v=xqlrOcGZx38

 As you can see the results are pretty good.  

(The monitor I used to test was really crappy)

 

Comparing the Amiga Board to the C128 Board.

Repair : C128 PSU working ( but was DOA ).

I've done a lot of reading about how crap the Epoxy filled c64 power supplies are ( do not use them, put them in the bin ).  Everyone however seems to agree that the c128 power supplies are great and worth using.  I never got a power supply with either of my c128 so I thought I'd get one.

Off to Ebay I go and get a "working" unit . . . yeah right.  When it arrives it's totally dead.  I have a look inside and the Caps have leaked and one of the fuses is in parts.  On further inspection BOTH fuses have not survived the bumpy ride to Northern Australia and to add to that the 1.4A fuse was only 1A . . . Hmmm . . .

That's cool, I can fix this and I was planning on re-capping it anyhow.

Here is the unit after I gave it a first initial clean and flushed the entire board.

You can still see some gunk around the 4th diode and where the resistor and the two wires join.  I used a desoldering gun to remove both of the caps and then used an interdental brush to make sure it was all as clean as possible before flushing the board again.

Desoldering guns are awesome, so after 5 seconds the Old caps are out and the New ones are in.
They are not exactly the same voltage spec, but the uF is exact.  You can go higher with the voltage without an issue ( just don't go lower ).  They are also good quality Panasonic ones.  Don't buy cheap caps.

 

Back it all goes into the case, I'm not turning this on until everything is in it's correct place.

During transport this tab broke off.  A little Tamiya liquid cement and you can't even tell it was broken.

At this point I've taken it outside and plugged it in with extra an extra RCD just to make sure the magic smoke didn't escape.  After a good run I tested the voltages, which ended up being 10V AC and 4.9V DC.  I'm pretty happy with that so on to the final phase.

 

 Plugging it into my secondary C128 and hey presto.

There's only one thing to do now and that's to replace the plug with so I don't need a travel adapter.