exxos's Atari, Amiga & retro forum

Well, the 15pF +-5% capacitors should arrive tomorrow, and hopefully the VIC-II R3 chip too.

We'll see if changing those makes any difference.

stephen_usher wrote: 25 Apr 2022 19:05 Well, the 15pF +-5% capacitors should arrive tomorrow, and hopefully the VIC-II R3 chip too.

We'll see if changing those makes any difference.

Splendid

The capacitors (and CD4020BE 14 bit counters for my Apricot HD project) have arrived but no sign of the VIC-II unfortunately.

stephen_usher wrote: 26 Apr 2022 13:53 The capacitors (and CD4020BE 14 bit counters for my Apricot HD project) have arrived but no sign of the VIC-II unfortunately.

Good stuff, be interesting to see what the results are, shame no VIC-II though.

And the VIC-II has arrived!

stephen_usher wrote: 26 Apr 2022 14:33 And the VIC-II has arrived!

Woo Hoo!!

:girldance:

New VIC-II, no change.

I guess that there could be some sort of resonance in the circuitry down-stream of the VIC-II in the modulator (which also generates the composite video from the luma/chroma signals).

stephen_usher wrote: 26 Apr 2022 18:31 New VIC-II, no change.

:mad:

Having just watched the latest Adrian's Digital Basement II episode he mentioned that his ZIF-64 motherboard was a design which sometimes had jail bars and other times not so I decided to do an experiment and power cycle the machine a few times.

Although Adrian's machine switched between jail bars and no jail bars mine has a slightly different symptom, mine switches between high frequency colour tint cycling (one per character column approximately) and low frequency (once every two characters). If you power cycle quickly then the behaviour is consistent. If you leave a longer period it will randomly choose one or the other, which a preference for the lower frequency.

Now, what is interesting is that when I used the FFT on the crystal output I saw a lower frequency harmonic which was either one frequency or double that frequency, with the lower frequency preferred. I wonder if this is what I'm seeing and it's the crystal which is the problem.

Unfortunately the crystal frequency is an odd one. I can find it listed on Mouser though, but I'd want to make a decently sized order to Mouser so as to decrease the relative postage costs per item.

stephen_usher wrote: 26 Apr 2022 18:31 New VIC-II, no change.

I guess that there could be some sort of resonance in the circuitry down-stream of the VIC-II in the modulator (which also generates the composite video from the luma/chroma signals).

:sigh:

:headbang:


It could be the crystal I guess. Looking at the schematic, and I don't know if this matches yours, U20 seems to handle the dot and colour clocks, so what does the output look like at pins 6 and 8 of that?

Also, the modulator does seem to sit smack bang between the VIC-II and the video out, what do the luma/chroma signals look like either side of the modulator?

Is there a simple Composite signal you can tap into anywhere else where it won't be affected by the modulator, or does the modulator combine all the signals to make composite?

I don't know the C64 that well, but I do know the VIC-20 doesn't need a modulator for composite.

Pain in the bum analogue stuff!!