The CSG8701 (previously called MOS8701) is a custom timing generator chip for C64 and C128 computers. The DIP-16 chip generates two clock signals with a fixed phase relationship: One frequency is 4x the colour carrier and the second frequency is the pixel clock of the video chip. While the original frequency generation without this chip was PLL-based (very early C64 boards), Commodore has updated their circuit with this chip in order to save cost. However, the chip does NOT use a PLL, but instead it uses fixed delays to generate the two clock signals with their required phase relationship. This results in slightly different colour artefacts if you compare an old vs. a new C64 motherboard's video output.
Our replacement board does not only contain a replica of the original PLL circuit, but also the crystal. While the original chip took it's PAL/NTSC setting from the jumper wire on the C64 main board, there are two different versions of our replica board: A PAL and an NTSC version. The advantage of putting everything on one board is that you can use it for repair jobs without having to check the original crystal, and for PAL<->NTSC modding jobs without having to solder at all (after all, the exact crystal frequency is often hard to come by).
theory of operation
Our replacement board consists of three major parts: A crystal oscillator for the 4x colour carrier, a voltage-controlled oscillator (VCO) for the dot clock, and a phase comparator. The phase comparator cannot compare the two frequencies directly, which is why the dot clock is always divided by 4, and the colour carrier clock is divided by 7 (NTSC) or 9 (PAL). The resulting frequencies are roughly 2MHz, and these are fed into the phase comparator, which generates the control voltage for the VCO. This results in the VCO to swing at 8,18MHz for NTSC and 7,88MHz for PAL. Also see this page from the C128 service manual.
Just plug the replacement board into the socket that normally holds the CSG/MOS8701 chip. The original 8701 chip is not required any more, and there is no adjustment or soldering to do. Although the computer will run with the "wrong" VIC chip version, it does not make any sense to operate a PAL VIC at NTSC frequency and vice versa. We recommend to only operate the NTSC version with an NTSC VIC chip, and the PAL version with a PAL VIC chip.
The dual standard PAL/NTSC version has a jumper which should be configured like this:
- Instructions on how to fix the clock generator circuit on a ASSY250407 board are here.
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