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 Post subject: Famicom AV mod problems
PostPosted: Thu May 24, 2018 2:53 pm 
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hi guys I was hoping you could help me with the following:

I av modded my famicom (board HVC-CPU-07). It does produce video clearly and the audio *did* work. The problems are:

- suddenly the audio stopped working while testing (i took it from pin 46 and use a 220uf cap, you do hear the music softly together with a buzz however when you put the volume on the tv at max, and the audio does work when I take the signal from one of the single channel caps)
- although the video works, the colors are off. They're washed out/strange (blue is more purple, brown is greenish, it differs a little bit between games however) as you can see in this picture:
Image


I do not know what to try further, I tried both these mods:
https://assemblergames.com/threads/guide-av-mod-your-famicom.49263/
http://jpx72web.blogspot.nl/2016/11/famicom-av-mod-new.html?m=1
and still got the same results... Thanks in advance!!


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PostPosted: Thu May 24, 2018 3:04 pm 
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Just quickly double-checking: does the TV still produce correct colors when given other sources?


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PostPosted: Thu May 24, 2018 3:10 pm 
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yes it does (it is a sony pvm-14m2e by the way)

I realize now I forgot to mention that the colors are displayed correctly on my regular CRT as well as my LCD tv...


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PostPosted: Thu May 24, 2018 3:18 pm 
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So you only get bad colors with the modified famicom on this specific TV?


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PostPosted: Thu May 24, 2018 3:22 pm 
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Yes.


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PostPosted: Thu May 24, 2018 3:29 pm 
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.... Is there any way you can find out if the TV is decoding the video as NTSC-M vs PAL-M ? Showing just purple and green is the axis from +U+V to -U-V in YUV; maybe PAL chroma subsampling is canceling out the +U-V to -U+V axis??? Maybe the TV automatically detects standard based on whether the colorburst is 3.579MHz vs 3.576MHz ?????

Feels like a huge stretch, tho.


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PostPosted: Thu May 24, 2018 11:28 pm 
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well... I had look at the front of the monitor and it appeared the phase control wheel was accidentally turned to the right... this caused the colors to go weird! I usually use PAL consoles so I didn't notice... I am sorry I did not see this before!

Now there's only the sound problem left, do you have an idea about that? The strangest is that it did work.... but testing later it did not anymore... I tried with another cap and without as well but with no luck. You can however hear it though when you but the tv volume at max (but it's really low volume and there is a buzz)


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PostPosted: Fri May 25, 2018 12:16 am 
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Sounds like the signal isn't getting the entire way through the audio path anymore, and what you're hearing is loose coupling by some means or another (e.g. power rail or capacitive)...

The best advice I have is to carefully follow the audio path out of the 2A03 until it disappears. There's a schematic of the original Famicom here: check both sides of R4, R5, C2, R7. Pin 45 is the cartridge side of R7; pin 45 is connected to pin 46 by the cart. Following advice will depend on when it disappears.

Note that audio will be much quieter on both sides of C2 compared to next to the 2A03.


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PostPosted: Fri May 25, 2018 12:33 pm 
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thanks a lot for the advice, i'm looking into it this weekend :)


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PostPosted: Fri May 25, 2018 2:43 pm 
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i already got a moment ;) thanks a lot, with your clear explanation it was quite easy! the audio is gone after resistor R7. If I measure everything (system turned off) I get about 71 kOhm (should be 100 kOhm). The values of R4 and R5 are not correct anymore either: both 7.4 kOhm while these should be 20 kOhm and 12 kOhm. Color codes are correct with values printed on the board. Do these need to be replaced?


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PostPosted: Fri May 25, 2018 4:24 pm 
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bastiaan86 wrote:
the audio is gone after resistor R7.
    There's a bunch of possibilities then:
  • The inverter-as-amplifier (U7 pins 14 & 13) is broken
  • The resistor R7 (100kΩ) is broken
    Quote:
    If I measure everything (system turned off) I get about 71 kOhm (should be 100 kOhm).
    oh, nevermind. 71k seems out of range, but won't cause things to completely not work. (Only very large or very small resistances would). And 71k is plausibly correct for an in-circuit resistance anyway.
  • The cart is shorting pins 45 and 46 to something else altogether
  • Something's wrong with the RF modulator (after pin 46) and it's shorting audio out.
  • The expansion port is shorting out audio
Quote:
The values of R4 and R5 are not correct anymore either: both 7.4 kOhm while these should be 20 kOhm and 12 kOhm. Color codes are correct with values printed on the board. Do these need to be replaced?
That won't be causing the symptoms you're seeing. Because of how the audio mixing circuit works, you're going to see a resistance of 12k//20.2k or 12.2k//20k in-circuit anyway, or ≈7.6kΩ

If you have a cart with expansion audio, that'd help to isolate things.

If you don't, I'd be tempted... although it's not an amazing plan because it poses a small but nonzero risk to the various electronics ... to pull a cart in the middle of operation (you need to keep the audio generators running, so you can't reset or power cycle things), isolating cart pin 45 side stuff from cart pin 46 side stuff.


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PostPosted: Sat May 26, 2018 12:37 am 
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thanks a lot for your extensive answer :) i will hopefully be continuing this evening!


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PostPosted: Sat May 26, 2018 11:25 am 
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Maybe you could put electrical tape (or anything thin, easily removed, and nonconductive) over cart pins 45/15 to isolate the audio path instead. Then there's no risk at all. (A game that relies on cart IRQs won't work, so pick something that doesn't use MMC3)


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PostPosted: Wed May 30, 2018 1:27 pm 
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thanks to your explanation I got the audio to work again :D the U7 chip was defective. I have a broken famicom here of which i used the U7 chip and I soldered it on this board! Thank you so much :D


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