I don't normally do Commodore 64 repairs these days, and even when I do, I don't bother writing them up as they are usually the same old faults, bad RAM or bad PLA etc.
Unless of course they are a bit unusual, like the 4064 / PET 64 I repaired last time.
This one was also a little unusual.
This is one of the Kalex boards, made in Hong Kong under license.
It is a 250425 board, I think the last of the large C64 boards with 8 RAM chips before the briefly moved to two RAM chips, on the way to the C64C.
Normally the later boards like this don't have the tin box around the VIC II and associated parts, like the early boards do.
This one does, but it is not the usual type of tin box with a removable lid, it is "n" shaped with no sides and is soldered in place.
Let's hope I don't need to access the bits inside there .... #foreshadowing
Like many of the later boards, all the chips are soldered in, other than the VIC II and the SID.
All seem to be early 1985.
What is unusual on this board is the KERNAL and character ROMs are not the standard PAL variants.
These are Swedish, C2G007 is the character ROM, C2D007 is the KERNAL. 901226-01 is the usual BASIC that is in every C64.
There is telltale flux residue on the back of the board, so it looks like those two ROMs have been replaced. Maybe this was a standard PAL board that had the ROMs removed and replaced with the Swedish versions to go with the modified keyboard (I don't have a picture, but I think they stuck stickers on some of the keys to get the extra accented characters in the character ROM)
So What's Wrong With It?
Well, there is no colour, but more than that, there is a weird pattern on the screen that changes with the colour.
Sort of a chequer board alternating with each pixel.
It seems to be working fine, other than that.
Different patterns on Dead Test, but still not right, and it confirms the colour RAM is testing OK.
I am going to desolder the tin lid, aren't I?
Bit of a job to desolder a large lump of metal from a ground plane, but I got there.
There is not much under there on these later boards.
All the video signal processing happens in the modulator.
All the timing is handled by a custom clock chip, the 8701 (the PAL version, NTSC is 8801).
I tried adjusting the trimmer, but it didn't make any difference.
Both of the chips are socketed, so I swapped them with a working board.
No difference.
The good chips gave no colour in the bad board, and the suspect chips worked fine in my test board.
Which is more than I can say for my camera, I can't get it to like the C64 colour scheme, it just goes all blue. But take my word for it, it was the normal READY screen.
This is why my development C64 has a custom KERNAL ROM with the SX-64 / VIC 20 colour scheme.
That is also the colours used on Dead Test, so I tried that.
The output again was nice and visible, and the camera liked it.
So both chips are fine.
Hmm, there's not much left.
Is it the modulator?
These is not much you can get to on the modulator, unless you desolder it.
I had the test board on the bench already as I had used that to test the chips. I wonder if I can "borrow" it's modulator without the use of a soldering iron?
Two Commodore 64 boards, is that how you make a Commodore 128?
Here I have the faulty board on the bottom powered up as normal.
I have tapped the power from the 9V DC unregulated supply, and the chroma and luma out of the VIC II.
Those power just the matching 9V unregulated rail on the test board, and the chroma and luma plug into the VIC II socket. The monitor now connects to the test board. The rest of the board is not powered.
No change.
Hmm, so it's not the VIC II. It's not the clock chip. It's not the modulator.
What's left?
I tried adjusting the trimmer again, I did get a brief flash of colour, but nothing stable, it could be flaky?
Before I turn on the desoldering station, let's just try something.
I have left the power as before, as that also feeds the regulator that generates the separate 5V supply for the VIC II and clock chip.
I have a clock chip on the test board and have tapped the two clock outputs. All the rest are supplies or fixed high or low options.
Those plug into the socket on the faulty board.
And it worked!
(I would insert a picture of a normal C64 READY screen if I was capable of taking one)
So that narrows it down.
Could it be a bad trimmer?
No, I replaced that and still couldn't get the colour back.
OK, so it's the crystal then?
I didn't have any new ones, so I borrowed one from a parts board.
And that fixed it!
(you know by now that I can't show the working READY screen, but it did work)
Testing the Fix
Everything seems to be working now, even the special characters.
Let's see if I can find something a bit more colourful.
That was the colour RAM test from DesTest.
Time to load some programs, out comes the SD2IEC Fastload Combo.
It's the easiest option when I just want to load things quickly and with minimal fuss.
I loaded the file browser and then scrolled down.
Ah, now don't panic.
That is a known problem with the early C64 KERNAL ROM, normally 901227-02. It was fixed in 901227-03.
I presume the Swedish variant must be based on the -02 with the colour scrolling bug.
It might be possible to patch the ROM with the fixes from the -03 KERNAL, but since it's soldered in, I will leave well alone.
I tried a few games, and all seemed to play fine.
Yes, we definitely have colour, thank you Toxic Frenzy.
Ghosts and Goblins was a lot more colourful than I saw recently on the 4064 (although it doesn't make me any better of a player).
That all seems good.
Wave goodbye to the inside of the tin.
That's getting entombed again (and about 2 days after soldering the lid back on, I found the bag of new 17.37MHz crystals)
I left it running the memory test I wrote for the 4064.
Ah, hang on, a couple of POKEs later and you can actually see the results...
I have run that through a couple of times as I have been writing this up (in blue on blue so I could see if the colour was stable).
That's all done and ready to go back.
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