Posted By
MMS
on 2021-07-12
14:19:37
 |  Re: Commodore 16: Black and white screen on Composite, color on the RF
Hi Retroshine,
just few questions:
1) I saw the signal levels on the modulator, they look a little too high for the standars, but there are some further conversions till they reach video connector.
could you confirm, that all the output signals on the video connector at 1.0V P-P?
(actually the Chroma 1V is too high for the standard, some TV tolerates it, some others not)
2) the PAL(German) and PAL B/G (UK) standard difference influences the sound carrier, so it could not explain the situation
3) In case of NTSC and PAL difference there is no picture at all, just running garbage, because of the synch difference (25Hz and 29,97Hz). So it cannot be an explanation.
4) Thinking further the colorless output could be due to color carrier modification by intention. From outside could look like a PAL system, but hacked to SECAM.
CVBS can bring SECAM signal too, and the SVideo is just the separated chroma, luma and sync signals
PAL and SECAM is compatible with each other except color carrier information.
https://support.corel.com/hc/en-us/articles/219517967-What-is-the-difference-between-PAL-NTSC-and-SECAM
(though it does not explain OK RF signal)
If you have this Plus/4 from France or Eastern Europe, there is a chance it is modded to SECAM (just to fit to some old TVs). It was a big business back that time to make next to 64K RAM upgrade the SECAM conversion.
https://en.wikipedia.org/wiki/PAL#PAL-B/G/D/K/I
So worth to check some signs of home made soldering (but could be done by a professional too) or comparing good quality pictures to your PCB
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