Posted By
TLC
on 2021-07-07
14:45:59
|  Re: Finetuning trimmers Plus/4
Frequency tolerances for both PAL and NTSC are pretty tight (at least what the standards specify). In the Plus/4, every clock frequencies, including the color subcarrier, are derived from this single oscillator here. The trimmer there is for fine tuning this oscillator's frequency. With typical machines and TVs, by turning the trimmer, you'd notice the color to disappear and re-appear. The trimmer should be tuned to match standard frequency (---> the color subcarrier, as divided by the TED from this oscillator's frequency, should match that of defined PAL / NTSC subcarrier frequency within tolerances). In practice, as an amateur, one'd set that using a standard PAL/NTSC color display, by letting the machine warm up first, then finding points where the color appears / disappears by carefully turning the trimmer around - and finally setting the trimmer to the mid-position in the "color" area between the discovered points. (With a display seemingly immune to NTSC subcarrier variances, this unfortunately won't be possible.)
I don't know much about the RF modulator, unfortunately.
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