Eliminating jailbars on HVC-CPU-GPM-02

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grievre
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Joined: Tue Sep 06, 2022 12:31 pm

Eliminating jailbars on HVC-CPU-GPM-02

Post by grievre »

There's some debate over how to get rid of Jailbars on Famicom and I think I found a way that works very well on the later revisions.

The changes I made are:
  • lift the Vo pin on the PPU (pin 21), you want to get it completely out of the hole and flipped up
  • desolder the transistor Q1
  • On the bottom of the board, add a 10uF tantalum capacitor as pictured, between the inboard leg of R6 (which is 5V) and the ground trace near it.
  • Solder a wire across pin 20, the hole that pin 21 used to be in, and the negative of the tantalum cap you just placed.
  • short the pads on the Q1 footprint for collector and base (these should both already be grounded, so this may not be necessary, but it was done on the unit I tested.
  • take the transistor you desoldered and solder the base directly to the lifted PPU pin, the emitter to the outboard leg of R6, and the collector via a short wire to the center pin of its original footprint (which is now grounded).
Steps 3-5 may be unnecessary but I have not tested.

To me the fact that this works so well indicates that the jailbar noise is being coupled in at the PPU output, which is presumably high enough impedance that it's vulnerable to this noise.

I don't know what the analogous mod to the HVC-CPU-07 board would be, since the transistor amplifier circuit is significantly different there (and the RF module maybe also expects a different output impedance/voltage level). I also can't presume that any such mod would actually improve that revision as it also has different routing and no third ground layer on top.

Attached pictures are the mod, and a comparison of the SMB3 1-1 with just the capacitor and ground jumper, vs the capacitor, ground jumper, plus the transistor relocation. Please excuse the other artifacts in the image, the setup was a bit hasty.
Attachments
PXL_20220917_063023015.jpg
PXL_20220917_063038759.jpg
Super Mario Bros 3 - transistor lift test_2022-09-16_22-26-36.png
Super Mario Bros 3 - transistor lift test_2022-09-16_22-24-32.png
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TmEE
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Re: Eliminating jailbars on HVC-CPU-GPM-02

Post by TmEE »

These are really good results ~
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aquasnake
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Joined: Fri Sep 13, 2019 11:22 pm

Re: Eliminating jailbars on HVC-CPU-GPM-02

Post by aquasnake »

make sure that keep the transistor close PPU pin21 as more as possible

The principle is that the long route of small signal will be interfered by radiation. In order to increase the anti-interference capability, the source should be amplified first before long routing
JanosX
Posts: 1
Joined: Mon Jan 09, 2023 9:55 am

Re: Eliminating jailbars on HVC-CPU-GPM-02

Post by JanosX »

Hello, I installed this mod : https://backofficeshow.com/shop/famicom ... ment-board
it worked very fine but I had the vertical lines,
so I did this litle mod from Grievre and there's zero video signal anymore :? Halp ! i haz no idea why :(

going to revert to normal with vertical lines for now.
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