29

While perusing the schematics for the Commodore 64C (titled COMMODORE-64 B/NE) I came across a suspicious section of the power supply: The unregulated "9V" is connected to the completely separate +5V with a 20 Ohm resistor, annotated with the following:

[7] FRANCE VERSION
R2: 20Ω RESISTOR & C63: 2200 µF CAPACITOR REQUIRED

What is this "France version" and why does it require special care?

I suppose a follow-up question is if this was ever implemented.

Here is the relevant section of the schematics:

Relevant section of the Commodore 64 schematic

This seems like a kludge for anyone into electronics:

  • It connects a "dirty" voltage, possibly with a lot of ripple, onto the clean regulated digital voltage through a low impedance.
  • As can be seen from the "5W" specification, this will waste a lot of power. "9V UNREG" is normally closer to 12 V, so this can dissipate up to 2.5 watts.
  • Pulling that much extra current from the unregulated line will increase the ripple, hence they also need to double the filter capacitor C63.
  • The regulated +5V is fed directly from the power supply using a linear regulator, and those are bad at reducing the voltage if it exceeds the target level.
6
  • 6
    Did the French version of C64 have a SECAM output? Is it related to that?
    – UncleBod
    Sep 16 at 6:39
  • @UncleBod that was my first thought as well but reading these schematics I'm completely baffled, it makes no sense and doesn't look like it even touches video output
    – OmarL
    Sep 16 at 8:09
  • 2
    @OmarL It does relate to SECAM output and it does touch the video output, just externally to the C64 motherboard - see my answer.
    – Justme
    Sep 16 at 8:59
  • 15
    What, have you never heard of the famous french resistance? Sep 16 at 19:58
  • 1
    @rackandboneman Godd*mn.. now I'm temped to write a new question title
    – pipe
    Sep 17 at 3:59
34

As noted in the question, the resistor is there to allow something to consume more current without voltage dropping too much.

But what consumes more current may not visible in the schematics.

France (and a few other countries) used the SECAM colour encoding for analog television. The SECAM encoding is very different from PAL and NTSC, so it was not possible to produce a VIC-II video chip that would directly output a SECAM colour signal, at least not without huge changes to the circuitry that is.

However, since France used the same 625-line 50 Hz TV system as most PAL countries did, a PAL version of the VIC-II could be used as usual, with an extra step afterwards for the color encoding to be converted to SECAM. This was accomplished via an additional circuit board inside the C64 manufactured by Procep, which takes in composite video or the separate luma and chroma signals near the AV connector, and decodes the color signals.

There have supposedly been two versions of these adapter boards, one version that decodes the PAL signal to RGB for connecting it to a TV via SCART (Peritel) connector, and the other one that decodes the PAL signal to YUV which can be encoded back to SECAM signal.

So most likely the extra resistor and bulk reservoir capacitor are there to allow powering the add-on board which allows connecting the C64 to a French TV. This arrangement allows the system to consume more 5V current, as some portion of the current drawn by 5V circuits is provided by the resistor from the unregulated 9V, so the same 5V regulator can still be used without it heating up too much.

Some people have also speculated that the C64 power supply sold in France was just too weak so they had to modify the C64 to work with it, but that is an unlikely explanation as the context is not seen in the schematics.

3
  • 1
    I wanted to improve this answer by linking a photo of the circuit board or schematic for encoding SECAM, but have not found any. Maybe someone here has a French C64c in their possession?
    – Brian H
    Sep 16 at 19:08
  • 3
    Added link to translated German C64-Wiki page that describes Procep board...
    – Brian H
    Sep 16 at 19:16
  • 2
    Comments should be related to the post; this discussion about the advantages of PAL over NTSC has been moved to chat.
    – wizzwizz4
    Sep 17 at 22:25
-1

Bigger C63 (2200uF instead of 1000uF) indicates the need to better filter ripple of 9V in France version - probably to fulfill more restrictive EMI/RFI requirements in France. In these days bigger capacitors were (and still are) more expensive and Commodore is known for "cost awareness". R2 is same in France and non France version, but anyway, I don't know why it is there. It's quite a strange design for me.

BTW: (9..12V - 5V)/20Ohm = 200..350mA -> 800..2450mW. Not good, but 2W lost in days when monitor consumes 60W it was not big deal.

3
  • There's no R2 on my non-french Commodore 64C. If it was included in every model why would there be a special note?
    – pipe
    Sep 16 at 6:19
  • Why R2 is required I am not sure. If 9V UNREG is a low power source of 9V, connecting it to regulated +5V through R2 can also improve ripple filtering - raising +9V will raise +5V and linear regulator will do its work, and so on. But it will only work when +9V sources low current (max few hundred mA?). Instead of two linear regulators (5V and 9V) you ends with one +5V and cheap R2. But it is a terrible design. Commodores thinkin: cheap + terrible? Lets implement it ;-)
    – ufok
    Sep 16 at 7:08
  • 1
    That kind of design relies on a guaranteed minimum current draw. Which NMOS and TTL circuitry has. In a way, a linear regulator isn't functionally more than a very intelligent, self adjusting resistor/rheostat... Sep 16 at 20:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.