Re: C64 to C64C mobo conversion

On 6/29/22 22:28, tokafondo_at_tokafondo.name wrote:
> 24 de junio de 2022 5:46, "Gerrit Heitsch" <gerrit_at_laosinh.s.bawue.de> escribió:
> 
>> On 6/24/22 03:26, tokafondo_at_tokafondo.name wrote:
>>
>>> I have a pre-8701 C64 motherboard and a C64C motherboard.
>>> If I wanted to test 8580 and 8563 chips in the C64 motherboard, would it > be just enough with
>>> replace the 7812 with a 7809?
>>
>> No, the VIC from a C64C expects 5V on Pin 13. You can get that from Pin 40, but it's a good idea to
>> run it through a ferrite bead and put a 100nF capacitor between Pin 13 and GND.
>>
>> Also, you might have to rework the modulator since the 856x VIC expects a 1kOhm pullup while the
>> 656x VIC wants a 500 Ohm pullup on the luma signal. This pullup is found inside the modulator. Just
>> replacing the VIC might result in a dark video signal.
> 
> FYI: I don't know if this is the right thing to do, but by only getting the pin 13 fed with +5V and putting a 100nF capacitor as you said, it's working with no ferrite bead, and no 1kOhm pullup replacement. The video signal is clear and bright.

On some boards I have seen flickering in the background with no ferrite 
bead. Looks like you got lucky.

As for the pullup. The output driver of your VIC will now have to work 
harder, not sure if that's good in the long run.

  Gerrit