Posted By
TLC
on 2021-07-14
04:37:36
|  Re: Commodore 16: Black and white screen on Composite, color on the RF
@Retroshire: O.K., I see. All that I can say now that this looks even more odd, given that the machine also didn't produce colour over s-video  . (This rules out anything around the luma + chroma addition part in the modulator. Also, since the RF part takes chroma off the same bandpass + buffer amp where COLOR OUT originates, chroma i.e its presence and analog properties must be correct as-is. On a related note - displays are usually very forgiving about chroma amplitude problems, they're known to catch even very subtle remnants of chroma).
Possible, further options.
- Some problem around the common GND of the Video out socket. (Whatever the problem is, it affects both composite and separate luma-chroma mode, i.e. it can't be because of some continuity problem on one of COMP or COLOR). Can be verified using a continuity tester between pin 2 of the AV socket and the machine's GND traces, even the modulator box).
- The presence of chroma (and correct amplitude) could be verified by a simple test. Switch on the machine, set border and background colours to some saturated one, say, red*. Measure the voltage on pin 6 of the AV socket vs. GND in AC setting using a multimeter. Repeat the test using a known-to-work machine. Compare the results. (--> A very crude method, given the "AC" characteristics of regular multimeters  , yet it should be good for a comparison and indication test).
- The main osc may be a bit off spec. (This is only possible if the display has different colour subcarrier tolerances for RF and baseband video modes). Was the main osc circuit of this machine modified recently (or, anytime before)?
*Ps. I was stupid. Every colours of the TED palette, by-design, have the same saturation level. You can use any colours (except black and white).
|
