Re: Dating a C-64 board

From: Richard Atkinson (
Date: 1999-04-18 16:56:57

On Sun, 18 Apr 1999, Ethan Dicks wrote:

> > The schematic of the original board suggests that there is a PAL/NTSC
> > jumper, it should be a jumper wire like in the version A board that can
> > be put either on location E2 or E3. The board layout shows an empty
> > space near U29 in the video cage, maybe this is where the jumpers are.
> Indeed there is.  I missed the legend because part of it is obscured by
> a large cap, C33.

I was getting worried for a minute there. Fortunately, this means I *do*
still have an original board in my very oldest C64. It has the NTSC jumper
cut on the PCB and a link wire soldered in for PAL.

> > Maybe they made a last minute change in the C64 for unknown reasons, and
> > the video chip and the ROM were left configured for 1MHz.
> The only crystal in this box is the 14.31818 MHz one that you would expect.

The C64 was originally going to run at exactly 1MHz like the Max, but I
think Commodore changed the design because a VIC-II with an 8MHz dot clock
asynchronous with the 14.3MHz colour clock looks *horrible* (I have tried
it). I don't think it was done for cost-reduction reasons because the PLL
circuit required to lock the dot clock at 8x the cpu clock must have cost
far more than an 8MHz oscillator module.

> > And I think the VIC-20 runs at significantly higher clock speed,
> > perhaps 1.2 MHz. I forgot ;-)
> 1.0Mhz, IIRC.

The NTSC VIC-20 runs at the same speed at the NTSC C64, but the PAL one
runs at 1/4 PAL colour subcarrier frequency (or 1/8 of a crystal twice
that frequency), hence running about 10% out of spec for a 1MHz part
rather than merely 2% :)

> > It's a pity that you don't have your original C64 anymore. There's
> > always the search for early chips to explore funny bugs. 
> We didn't have to search for bugs... I got bit every time the machine
> got warm.  I used to save every five minutes.

I need to search out these early VIC-IIs and rather quickly, for my VIC-II
reverse-engineering project, so if *anyone* has any early NTSC VIC-IIs
that they're prepared to donate to the cause or swap for PAL ones, please
get in touch. I'm particularly looking for a 6567R5 and this plain '6567'
if indeed it hasn't just had the revision number scratched off. I'm also
looking for the later 6567R9 and if anyone knows of any other NTSC VIC-IIs
beside 6567R56A or 6567R8 (or indeed PAL VIC-IIs other than 6569R1,
6569R3, 6569R4 and 6569R5) then I need them too.

> > I think Andreas Boose wanted to know once if the reset bit, that is
> > disabled in all known VIC-II chips is perhaps active in very early
> > versions. Perhaps you could try it on your machine?
> Sure... give me some more details.  I can write a quick BASIC program if
> I know what addresses you want wacked.  How does it behave in later C-64's?

I think VIC-II just ignores it - I need to know this for the project. I
wonder if I can reclaim it as a mode bit as I am doing with bit 7 and bit
6 of that register.


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