Hong Kong Famicom - no trim pot for clock oscillator?

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Hong Kong Famicom - no trim pot for clock oscillator?

Post by Anomunus » Fri Oct 04, 2019 10:59 pm

Note - this is somewhat continued from another thread. I branched it out here to allow my situation and findings to-date to fit better into the larger goal of better documenting the inner-workings of, modding, and troubleshooting the Hong Kong Famicom, for which I have found the info is not well compiled on the Internet.

Context: I have a Hong Kong Famicom with board revision HVC-CPU-NPC-26-01 (the newer one). The HK Famicom, for those of you who don't have the background, uses NTSC CPU and PPU but replaces the original RF/power board with one that contains an official Nintendo circuit (complete with a Nintendo-stamped chip) that converts the NTSC signal into a PAL signal for RF output, and has a switch that toggles between 50Hz and 60Hz output, but lacks the switch for selecting the output channel. User mark_k on this forum discovered back in 2009 that this Famicom's clock frequency is 21.3125MHz (compared to 21.47727MHz for the normal Japanese Famicom). Link to that thread here.

After years of dreaming, I obtained a suitable AC adapter for the unit and thus started to try making it work. At first, I ran it through an RF switch from my American SNES (this is known to be acceptable for connecting Famicoms to a TV, even for the HK version) to my Sharp LC26SB24U flatscreen, which tuned it on Channel 85 (assuming the TV interpreted the signal to be UHF), with a black and white image and no sound (switch toggled to 60Hz mode). At this stage, I thought that the monochrome image was caused by the TV being NTSC only trying to display a PAL signal.
Next, I ran it through an external TV tuner called MTVBox that can tune PAL signals (and outputs to VGA, which my TV does accept). It tuned on the channel that has a frequency of 590.05MHz. The sound was present and clear after setting the sound mode to I, but the image still came out black and white and somewhat noisy despite having the correct PAL color mode (closest possible to PAL-I, which is what UK and thus HK was using). The MTVBox allows for noise filtering which did help the image quality a bit. The output was pretty dark though, sometimes hard to see the things on the screen.

Since the signal was still in monochrome, I decided to try doing an AV mod to it to see if that would provide the colour of the image signal (since AV modding taps directly from PPU). I followed the procedure on this post from this thread here.
(Shout-out to all the people who contributed to that thread especially Ben Boldt, a lot of progress has been made on improving the AV mod circuit design this past year)
After successfully building the AV mod circuit, my testing indicated that, although the image through RCA plugs is more clear than from RF, it ended up still being monochrome.

Board in its current state after AV mod, for reference (Top and Bottom):
Image Image

Test images after AV modding here:
Image Image

User lidnariq suggested that the monochrome issue is because my board is missing the trim pot at this spot, in the picture below. Without this component, combined with the fact that the clock frequency on the HK Famicom is slightly different than the regular Famicom, it may have the wrong ppm for my TV to interpret the colour signal.

I just dug around some more and found out that for the Hong Kong Famicom, the later revision of this board was not equipped with the trim pot, whereas the earlier revision was (although it's on the bottom of the board). Notably, lidnariq actually previously commented on a thread where the differences in HK Famicom hardware revisions were highlighted, linked here. On the old revision, the trimpot is the green "thing" that was mentioned in the linked thread.
See this person's Dropbox folder to compare those hardware revisions: Link
(Also just realized that the special RF board on this thing also has some screw-components of some sort that can be adjusted, maybe that could have fixed my colour issue earlier on, darn... I can upload pics of my board if needed, so that y'all can attempt to figure out more of what happens on that magic board)

Conclusion: So I guess now I gotta ask, what kind of trim pot should I get to rectify this? I will most likely solder it to the bottom of the board to follow the old revision of the HK Famicom, to allow for easier adjustment, and also because I am terrible at soldering. :P Thus, this part's height must be less than the vertical clearance from the bottom of board to the Famicom case bottom. (estimate ~5-6mm??)

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Re: Hong Kong Famicom - no trim pot for clock oscillator?

Post by lidnariq » Fri Oct 04, 2019 11:12 pm

Assuming you want baseband system M NTSC out, and don't care about the RF output...

If the crystal is still 21.3125MHz, you probably don't "need" the trimcap: you'll just need to replace the crystal with a 21.477MHz crystal instead. This also explains why they didn't bother to populate the trimcap: in the HK famicom, it doesn't matter what frequency comes out (well, within 1%), only that the later conversion IC has access to the same clock.

( The leaked official famicom schematic ( http://nesdev.com/Ntd_8bit.jpg ) uses a 33pF trimcap there ... but once you have the correct crystal, you may be able to get away with just trying a few random small capacitors instead of buying a trimcap )

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Re: Hong Kong Famicom - no trim pot for clock oscillator?

Post by Tyree_Cooper » Wed Oct 23, 2019 6:27 pm

that's why i keep that dropbox folder online :mrgreen:

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