Swamp C64 repair attempt

[VintageVic]

I recently acquired something to fix.

First set, a 'swamp' C64 and 1541. Extremely dirty and corroded. Most likely spent years
outside exposed to humidity, wind, dust and what not.



Inside looked really corroded.



repair attempt started with dishwash liquid, brushing and generous amount of water rinsing.
After drying it over night, it is not perfect, but something you might put your hands on to.



As one might guess, it was a black screen. But I quickly found out that the power switch is not functional.
it's probably the reason why the unit was abandoned to suffer the weather in the first place, because I noticed
that the switch contacts were soldered at some point in the history of the unit.

I then replaced the pwr sw and got some life into the unit:



Not perfect, but I can see the cursor blinking and that is encouraging.
Second boot looked like this:


I still see the cursor blinking.

Most chips are not socketed, but the VIC2 is. Already checked the VIC chip on a working computer and it's fine.
I suspect the IC socket of the vic and that's what I'll probably replace next.

 

[VintageVic]

Well, replacing new IC-socket for VIC2 did not do the trick.

Something funny going on at kernal rom - some excess solder there?
But perhaps character rom is more likely suspect now. I'm having a break to
absorb the result before doing more to the pcb tonight.

 

[rittwage]

You are doing God's work here.

It looks like it could be character ROM or maybe the PLA. Hopefully you don't have some bad traces, but I wouldn't doubt it.

 

[VintageVic]

Okay,
carefully desoldered/pulled char rom out. Gladly no damage to the chip or the pcb.
ROM itself was ok so no luck finding out the problem just yet.

Very good point to check the traces. I checked traces from the VIC2 to the nearest chips,
but no fault there either. That is slow and tiring work so I'm having a break again. Perhaps
it is worth pulling / desoldering the PLA next.

 

[bladamson]

That'll buff out. :3

 

[VintageVic]

Back to the board after the rest break.

Clean pull for the PLA.

And the PLA was OK! Part of me is of course happy. If I fail to repair this
pcb, at least there are valuable spares readily desoldered from it.

Just to make sure, I put ic-socket there and tried out the board with another PLA
and no chance in behaviour.

I guess I need to keep on checking traces from here on.

 

[rittwage]

Looks like the VIC-II is not seeing RAM correctly. The computer is running into BASIC, so RAM is OK. Path from VIC to RAM or other chips is being interrupted in some way I assume. Do you have any cartridges to run? Get a dead test going, at least... You can run one as a kernal ROM if you don't have a cart.

I think at least one of the 6526 is involved in this also, and probably some of the TTL logic.

 

[VintageVic]

Right,

after failing a few days ago with the swamp c64, I needed a change and went on repairing my summer car seat 770 (Fiat 600).
Spent several hours trying to get it ready for MOT test, replacing the water pump, most of the steering components only to find
out oil pressure is gone and the carburator is flooding.

Now, I'm ready to fail with the commodores again :D.

So, yesterday I was getting into a bit similar conclusion with Rittwage, the computer boots to basic, but is not showing
the screen right. Something wrong in getting the data to the VIC-II.

I revisited Ray Carlsen's site and the commodore pictorial guide. From there I picked up two choises, U25 and U26
can cause graphical errors, but computer can boot to basic (and basic can run right under the garbage / distorted screen).

I've repaired a few C64's by replacing U25, so I thought statistics may help here and pulled that out. This time I struggled
a bit of course, ended up damaging one leg of the chip and broke one trace. Fixed the trace, inserted IC-socket and tried with
a known good 74ls257, but no luck - no change. A good moment to go to sleep after another failure .

But, it was not all bad. I let the board on for a few moments with the new chip at U25 in place. Quickly noticed, that
the U26 was a MOS 8715 (Miserable Or Shitty -chip) AND although it did not run extremely hot, it was obviously running
hotter than the U25 next to it.

MOS chip that seemed to run a bit warmer than it normally should - I should have noticed and checked this in the first place.
So today, I pulled it out. Struggled with it again, it took me long to take it out and broke yet again one trace. But after
installing IC-socket and trying out with a working chip, I seemed to get SUCCESS!

Finally. About time.



Broke the trace here, leg 19. Cannot see it here, but it's broken. A good practice to very
carefully inspect the pcb after a pull.



A little bit more peppy C64:



I'm not out of the woods just yet.

I do have a dead test cart, but the cart slot is so corroded on the board, that I might be better off by replacing the
connector from a spare board. Another task I have yet to try.

Keyboard connector is very oxidized, both on the keyboard and the motherboard.
And most likely I have to dismantle and clean the keyboard completely anyway if I hope it to work at all.
Joystick ports are oxidized... and there must be something else what I'm missing now. But
the right step has been taken forward!