Posted By
TLC
on 2021-07-13
13:07:29
|  Re: Commodore 16: Black and white screen on Composite, color on the RF
RF is basically COMP + SND, modulated to a high-frequency carrier. COMP is basically LUMA + CHROMA added together (with some band limiting). For RF to have color, there already needs to be a perfect COMP. Now, in this case - there is color with RF, but not COMP. Pretty odd.
I could find the schematics of the NTSC modulator. 251844 and (alternatively) 251311. I'd risk that the PAL one produces at least the COLOR, LUM, and COMP outputs very similarly. As you can see, the composite signal which is produced for modulation to the RF carrier, is very similar to COMP (which eventually leaves the computer on the Video socket). Both of them are produced from the same source signals / just using different voltage dividers and series capacitors (R9, R10, C5 for COMP, and R7, R8, C4 for the internal comp). From the fact that there is color on RF, we can rule out any problems before the voltage divider part. Which basically leaves us with three options.
1.) There's a problem somewhere in the signal path from (the PAL equivalent of) R9, R10, C5 and on.
2.) There's a problem with the connection.
3.) There's a problem in the display.
Myself, I'd probably leaning to test options 2.) and 3.) first (because they're easier to rule out, and because they look more likely). For 2.), test if/whether COMP is really taken from PIN 4 of the DIN socket (...and additionally, whether some other Commodore machine produces colours using this cable). For 3.), checking whether a known to well working C16/C116/Plus/4 produces colours on this displays using the composite cable, should do it. For 1.) by itself (without any other knowledge i.e. no verified env at hand), one could really only make sure whether COMP has colour, using an oscilloscope.
Ps. the diodes you mention have been regularly added as overvoltage (ESD) protectors to the serial + tape port inputs.
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