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

Repair : C64 No signal, black screen fix - part 3 - the final chapter, success!

Well . . . I was totally wrong about the ROM's I replaced both to no effect.

Back to the drawing board.

Looking at diagnostic pictures and my remaining socketed chips I was getting down to a last man standing situation.

I only had 5 chips left, CPU, CIA2, PLA, Color Ram and two identical logic chips.

PLA - ease don't

I guessed the PLA ( I had two spare ) and got to work de-soldering it very, very carefully
WARNING : apparently it is very easy to kill it by removing it.

I eventually got it out very cleanly.  I put in a socket and then inserted a new PLA and got this.

Ugh Oh,  that's not right. I put the original PLA back in the socked and got the same picture. I pulled out the multi-meter and began checking all of the connections, nope everything was fine.  I then checked for shorts.  I already knew that two of the pins short when a cart is attached so I removed it.  I quickly found the short between two pins connected to the Colour RAM.  The solder had joined under the socket.  With a piece of copper solder wick I jammed it against the side of the socket and it cleared the short and I went back to the black screen.

The Final Culprit - CIA2

Time to pull CIA2.  So I got to work and removed it and powered it up without CIA2.

Now we are getting somewhere.  I quickly socketed it and jammed in my two spare good CIA's.

Now it's all working . . . so what was the damage?

I replaced 3 x logic chips ( status unknown, one was probably dead ).

1 x ram chip ( dead ), 2 x CIA chips ( dead ), 1 x SID ( dead ), 1 x 8701 ( dead )

Was it worth the repair.  Cost wise I'd have to say no.  If you wanted to have C64 that works just buy a working one.

Education wise, yes it was a great exercise. On the plus side I now have a test board with almost all of the IC's socketed.  I buy this dead stuff to learn skills and my through hole soldering is so much better now.

I'm also pretty pleased I got the PLA out in one piece.  I've read it's quiet hard to do without killing it.

Repair : C64 No signal, black screen fix - part 2

Who's the suspect?

So at this point I was stuck.  I scoured and read every bit of Ray Carlsons C64C chip notes.
http://personalpages.tds.net/~rcarlsen/cbm.html

I'd looked at every single picture of the C64 pictorial fault guide.
https://derbian.webs.com/c64diag/

The only cart I had that sort of worked was my dead test cart.

But then I had a flash of inspiration.

Ultimate II 1541

Now I mentioned Gideon's Ultimate 64 in part 1 of this article.  However I also had an Ultimate II 1541.  Initially I wanted to use it to make sure the disk test I had done with the 1571 was okay.

When I plugged the cart in and turned it on . . . nothing.  The disk light flashed but the screen was black, but then something great happened.  When I brought the menu up with the middle button on the cart I got this!

Looking at it I could make out where the menu text was meant to be.  There was a lot of junk on the screen but I could definitely see the menu.

 

The Kernel is a bit iffy

Looking back at the Ray's site and the Fault guide, everything started pointing to a Kernel Fault.  This also made sense as the Kernel and Basic ROM are on the same chip.  So when I returned back to Basic I'd get a black screen, but then when I went back to the Cart menu, it didn't use basic.

So it's definitely the Kernel Basic Rom and probably the Character Rom too.

I didn't have anything that could replace the Kernel Rom, OR DID I!!!  One of the great things you can do with the Ultimate II is to use a replacement Kernel Rom and VOILA!!  When I return back to basic I get nothing, but that's to be expected as the Cart can not replace the Basic or the Character Roms.

Slow boat from China . . .

At this point I'm waiting for some EPROMs and a Burner to come from AliExpress and for some replacement parts from Ebay . . . so stay tuned.

Repair : C64 No signal, black screen fix - part 1

 So as I’ve mentioned previously, I’ve bought a whole stack of not working C64’s from the net.  Mostly they’ve worked or needed minor work to get them going. This one however was dead as a door nail.

I actually gutted it to house my Ultimate 64 from Gideon, if you haven’t seen or bought his stuff you’re missing out. https://ultimate64.com/

This was actually one of the first c64s I bought for my C64C revival so I just put it away as I had no clue how to fix it.

No now some two years later I thought I’d give it a crack. So here is my adventure.

Oh SID where art thou ( status : dead )

I’d already discovered that the SID was bad when putting it into my Ultimate 64. It ran red hot and just didn’t work.  So I just left it off of the board as according to Ray Carlson’s website it can cause black screen if dead.

No signal, no picture

When I turned the computer on there was no signal at all,  the power light came on and the fuse was fine.

The 8701 timer ( status : dead )

My first step was to switch out the 8701,  I have a working c64 and luckily the chip was socketed on the both of them. When I turned it on again I got a Pal signal detected on the TV, but it was now a proper black screen, progress.  The next thing I did was order a replacement, but more modern TOLB from Ebay.

Old single sided sockets are crap

Unfortunately when I put the chip back in the working C64 it stopped working.  It turned out the socket was bad. So the first thing I did was replace those sockets.

Pulling out a CIA ( status : unknown )

The next item on my agenda was to pull the only socketed CIA on the board (near the keyboard connector).  It made no difference, but I left if off because I could not test it and I could not swap it.

Swapping the VIC II ( status : good )

Again I was lucky that both VIC II's were socketed so I swapped it into my good board and it was all good.

Swapping the RAM chips ( status : unknown, probably good ).

The two RAM chips where socketed. I had bought some replacements but didn't know if the worked.  Swapping them made no difference.

Ugh Oh, no more socketed chips!

At this point I had no more chips I could easily swap so I decided to remove and socket the replaceable readily available logic chips that I had on hand and this is where I can unstuck.  Although I had had no trouble removing the sockets, removing the IC's using my desoldering gun proved to be a total pain.

I removed the 74LS14 first,  solder came away nicely but it wouldn't budge.  In the end I cut the legs off and pushed them through with the soldering iron.

I removed the 74LS08 second, it was much easier this time, practice you know. and I only cut a couple of legs off and the chip fell out.

At this point the C64 started to show some life, when I flicked the switch I'd occasionally get a flask of colour or with the dead test Cart in I even saw it working in the total mess of characters. Then it'd die and reset.  But I got nothing with no Cart in.

Lastly I replaced the SN7406 Hex Inverter, I finally discovered how to desolder properly using my gun.  Put the nozzle over the pin for 2-3 seconds, then swirl it around so you feel the pin loose. Then pull the trigger.  Once I removed all the solder I grabbed the chip with pliers and instead of pulling it I gave it a slight twist to show where I'd missed solder.  I went back to those pins and it fell out intact.

I found after this I got an occasional black and with flashing screen which would stop with the dead test Cart in and every now and then the border and background only would appear in the colours of the dead test cart.

 

What now? I've go no more chips!

At this point I was stuck, I'd order some chips but I had nothing more on hand . . . or did I.  I decided I'd recap the electrolytic capacitors.  The result being that I started getting the flashing more often when I turned it on, with far less power cycles.  From my IC extraction adventures I was an old hand now and they all fell out first go, which was really nice.

Winning!!

 

Does my 1571 reset on power cycle and can I load a disk typing blind.

I plugged my 1571 into the board and the missing CIA chip and a keyboard as I wanted to test that the SN7406 was okay and also to see if could get the drive to do a directory listing.  Resetting the computer also reset the 1571, but sadly no bananas with loading a disk.