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Commodore 64 ‘breadbin’ repair log #7 – bad characters

Dec 182016

This c64 gave me the following screen on power-up. Flickering characters with artifacts and a vertical line through the middle of each character. After going through the symptoms on Ray Carlsen’s site, I thought this may be bad colour ram. Swapping out the 2114 didn’t make any difference and colours associated with the characters looked good anyway, so I don’t know what I was thinking there.

After thinking about this for awhile & looking at the pattern closely, I had an idea that this may be a bad character generator ROM at U5. To prove this theory, I could type in a short basic program to copy the entire character set from the ROM into RAM, then tell the VIC 2 chip to use the character set in ram and reprogram one of the characters to one of my choice. The Commodore 64 has great features such as programmable characters which is very useful for games.

Luckily I don’t have to think too much as there’s already an example at the link below. So I begin typing some basic commands. The problem is my typing has to be super accurate because I can’t see what I’m doing and any typo will screw the troubleshooting up. It took me 3 or 4 goes before I finally got it right.

https://www.devili.iki.fi/Computers/Commodore/C64/Programmers_Reference/Chapter_3/page_109.html

After typing in “POKE 53272,(PEEK(53272)AND240)+1” as shown above, the flickering stopped although the artifacts obscuring the characters were still there, which was to be expected because we’ve just copied corrupt data. The computer was now getting the character set from RAM instead of from ROM. Now to change the letter T to a smilie face. I enter the following program and run it.

10 FOR I=12448 TO 12455: READ A:POKE I,A:NEXT
20 DATA 60, 66, 165, 129, 165, 153, 66, 60

Pressing a T shows a smilie face which looks complete without artifacts or a line through the middle of it. So I know that the VIC 2 chip is OK.

I remove the old mask rom and install a socket. My EPROM programmer is currently out of action so I don’t have the ability to verify the character rom or program another EPROM in its place.

But for now I can extract a known working EPROM from my own C64 into this one for troubleshooting purposes . After powering up, the character set shows up perfectly. Now all I have to do is program another EPROM when I get around to it and this c64 can go back to its owner.

NebulasRay repair log #2

PCB Repair Logs Comments Off

Dec 152016

Got this genuine NebulasRay for a repair:

Game played fine but suffered from a color issue, screen was all yellowish, self-test on boot reported a problem on palette RAMs:

A yellowish screen means that problem is in the BLUE color generation so I started to study the hardware and figured out the RGB circuit:

As you can see from the above picture, there are three 8K x 8 bit static RAMs, each one for a each color.These SRAMs are addressed by the custom ‘156’, their data go to the custom ‘116’ which processes them and generates the different color shades.These digital signals are converted to analog and formed into a single color by three 1K Ohm resistor arrays.Lastly each is color is amplified by a PNP transistor and routed on JAMMA edge pins.

With this knowledge for first I checked the 6264 SRAM @5X (which does the BLUE color), it showed normal activity on data/address bus until I probed its pin 27 (/WE) , it was not toggling like in other two RAMs but it was stuck HIGH so RAM was never written:

/WE signal of this RAM (like the other two ones) are generated by the custom ‘116’:

But with my multimeter I found no continuity, at a closer look the pad of pin 27 of this RAM lost contact with the trace.I promptly restored it and this gave me a fully working board again.End of job.

Taito F3 (PCB version) PAL update

PAL Updates Comments Off

Dec 152016

In the past days I got two working Taito F3 boards (PCB version) : Grid Seeker and Super Cup Finals).So I could test the dumps we had from Arabian Magic (thanks to ‘luiskiko’).Some dumps were from PALCE devices so I could successfully test them on GAL without any conversion, the remaining where from PAL devices so I first converted them in GAL format.PAL location and marking are the same between Arabian Magic and Grid Seeker since boards have same layout while Super Cup Finals has a slighty different one but it shares all the PALs (at different location) except for one which is unique and one with different label but same contents.

Commodore 64 Game Killer cartridge

Technical Info Comments Off

Dec 152016

Found this oddity on eBay not too long ago.

Its a crude device that attempts to disable collision detection for games that used hardware sprites.
It device itself wasn’t too successful but it was cheap and love my C64 so here it is.
I spend half an hour drawing up a schematic for the device and also dumped the ROM which is in the downloads section.

Cant imagine anyone ever needing to use the schematic for anything. Its very simple and any fault finding could be done in moments and who in the right mind would want to create more of these things but it was a small side project.

Crystal Castles (two layer PCB revision) PAL dumps added

PAL Updates Comments Off

Dec 142016

In the past days I dumped the PALs from an unusual two layers Crystal Castles PCB (with Atari EPROM strickers)

PALs were two PAL16L8 and three PAL10L8, all were unsecured.Then I converted them to GAL16V8 fusemap and Andrew Welburn successfully tested them on his board, thanks to him for the feedback.

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