Hello. Some time ago I restored my C64, my FD-148 floppy drive and buy a second cassette player, a second 1541-II drive and backupped my floppy disks, including the VIC20's one . Now I found my old faulty VIC20, that shows characters alteration (seems a realtime bit-changing) as can be seen on the following video:
https://youtu.be/1lelO3Bn5vQ
I would restore it and preserve with a full maintenance...
I think that the character ROM can be faulty, but I need some clue since I don't know the VIC20 circuit very well...
The logic seems to be working correctly, I can load files, change colors and the BASIC seems to be working fine, but any char is represented as the same identical mess...
I'm studing the schematics, in the meantime, someone can help me focus the fault?
Thanks a lot!!!
Fulcro
VIC20 and bad characters
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gordonf
- Vic 20 Newbie
- Posts: 12
- Joined: Fri Dec 16, 2022 6:22 pm
- Website: https://www.youtube.com/@greatwhiteretro
- Location: Winnipeg
- Occupation: IT Systems Engineer
Re: VIC20 and bad characters
That does look like a bad or even missing character ROM. The apparently changing characters even suggests a lot of those data lines are floating (hovering between low and high).
I'm starting a Rev 1 VIC board repair so I might try removing the character ROM and seeing if it does something like this.
It could also be any number of things attached to the character ROM, so as Adrian Black warns us, try to probe around there and inspect things before trying to desolder anything.
Would a purely graphics cartridge still run? The Penultimate Plus cartridge or later uses custom characters to produce a bitmap menu screen, so if that works but plain text mode does not, you can focus on the character generation almost exclusively.
I'm starting a Rev 1 VIC board repair so I might try removing the character ROM and seeing if it does something like this.
It could also be any number of things attached to the character ROM, so as Adrian Black warns us, try to probe around there and inspect things before trying to desolder anything.
Would a purely graphics cartridge still run? The Penultimate Plus cartridge or later uses custom characters to produce a bitmap menu screen, so if that works but plain text mode does not, you can focus on the character generation almost exclusively.
Great White Retro - Vintage computing
Re: VIC20 and bad characters
I tried to hijack the char memory from 32768 to other memory zones, filling them with fixed bytes and showing the screen poking proper values on 36869 address: that showed correctly the relative memory with no floating bits. I loaded some games from disk and in some of them the char were reprogrammed form the game, so it showed the correct new set of styled chars.
I'm thinking that the only way to test the solution is to replace the 901460-3 chip...
I would try the availabe mods in which a 27C256 is used in place of the original chip, but my EEPROM programmer is a little old to work with modern PC (is a Willem Prog connected by a parallel port, unavailable in my PC...)
Any other clue is welcome!
Thanks!
Fulcro
I'm thinking that the only way to test the solution is to replace the 901460-3 chip...
I would try the availabe mods in which a 27C256 is used in place of the original chip, but my EEPROM programmer is a little old to work with modern PC (is a Willem Prog connected by a parallel port, unavailable in my PC...)
Any other clue is welcome!
Thanks!
Fulcro
Re: VIC20 and bad characters
If you can get you eprom burner to work (maybe with a LPT to USB adapter?!), then you can also use a 2732 eprom instead of the original character rom. 
All you have to do is switch pins 18 und 21 (A11 and OE/CS2), but otherwise is is pin compatible, so no adapter is needed like you would need with the 27256 eprom. (you would also need a small adapter then, because there is not much space next to the VIC case....)
All you have to do is switch pins 18 und 21 (A11 and OE/CS2), but otherwise is is pin compatible, so no adapter is needed like you would need with the 27256 eprom. (you would also need a small adapter then, because there is not much space next to the VIC case....)
Overdoc