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.