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2022 Games have wrong colours, etc

 

 

 

Version 4.0 (NESRGB40) boards sold between August and September 2022, marked with 22.31 date code on the silkscreen, most likely contain buggy firmware.

 

There are four bugs identified.

 

Colour tint firmware bug

 

Description

 

The RGB attenuator which mimics the colour tint (a.k.a. colour de-emphasis) feature of the PPU is broken. Instead of attenuating the signal it is boosting it.

 

Symptoms

 

In Rampart, the scrolling messages appears brighter than the background, instead of darker. The screen distorts under the message because the signal is being boosted above standard video levels.

 

 

In Noah's Ark, the top of the water appears yellow instead of Blue. The red and green channels are being boosted instead of attenuated.

 

 

Games affected

 

There is a list of all games that make use the colour tint feature in Nesdev Wiki.

 

Solution

 

Remove the three resistors (very carefully) that are responsible for RGB attenuation from the NESRGB video DAC circuit. This will disable the colour tint feature. This does not restore correct operation, but looks better than the unwanted boosting effect.

 

 

 

Palette RAM metastability firmware bug

 

Description

 

Under the right conditions (including temperature), reading/writing to the palette ram will fail. Seems to happen more often on NTSC NES front loader and PAL NES top loader consoles.

 

Symptoms

 

Incorrect colours in all games, symptom may come and go as console warms up.

For example, Castlevania looks like this

 

 

When it should look more like this

 

 

Sometimes the colour changes are accompanied by colour fringes on the edge of objects, which looks like PPU de-sync, described in the clock hardware bug below.

 

Games/hardware affected

 

 This bug seems to only manifest on a small number of NESRGB installations. I suspect something like 2% of customers are affected. The best method I have found for making this bug manifest itself is to put the console in the fridge. Take it out once it's cold, play Super Mario Bros 3 and leave it on the map screen and watch the screen as it warms up. The map screen updates the palette every second frame so you will notice any palette corruption in the video output immediately.

 

Solution

 

A slight change in the timing of the PPU's CE# (pin 13) signal should be enough to make it go away. To accomplish this, solder a 100pf ceramic capacitor between PPU pin 13 and pin 20 (ground) on the underside of the NES motherboard. The exact value of capacitor is not critical, try 47-220pf if that's what you have at hand.

 

 

Interrupt detection firmware bug

 

Description

 

The NESRGB uses the INT# signal from the PPU to detect that the PPU has stopped rending and is ready to accept palette writes. The INT# signal detection logic was poorly implemented and this can cause the blocking of palette writes to occur in some games.

 

Symptoms

 

Incorrect colours in some specific games. More likely to happen in NTSC consoles than PAL.

 

Bomberman 2 road is black instead of grey.

 

 

Final Fantasy town screen looks like this normally.

 

 

Go into a shop, come out, it looks like this. Green grass/trees turn white.

 

 

Games affected

 

Only Bomberman 2 and Final Fantasy have been reported so far, but I'm sure there are more.

 

Solution

 

There is a modification board available to fix this problem, called INTFIX10.

 

Installation instructions.

 

 

Clock hardware bug

 

Description

 

Version 4 hardware is more susceptible to noise on the clock signal than version 1/2/3 hardware. Previous versions had a clock input with schmitt trigger hysteresis of 400mV. Version 4's clock input also has a schmitt trigger on the clock input, but the hysteresis is only 200mV.

 

Symptom

 

Noise on the clock line can cause the NESRGB board to become unsynchronised with the PPU. This looks like colour fringes on the edge of objects and the picture moving left/right by one pixel.

 

Hardware affected

 

Normal NESRGB installations should be unaffected. The Bakutendo mod for installing NESRGB boards into original Famicom consoles is very likely to be affected.

 

Solution

 

There is a modification board available to fix this problem, called CLOCKFIX10. It is installed onto the Nintendo motherboard.

 

Installation instructions.

 


Last updated 15/2/2023.

 

 

 

Copyright © 2024 Tim Worthington