AV Famicom NESRGB Mod... Solid Color Screen. Help! [SOLVED]

[flex]

Hi Guys,

Having a bit of a drama with a NES RGB mod I am doing on a friends AV Famicom and hoping someone can help / think of anything I haven't yet. Long story short I completed the RGB mod but upon powering on I am was just getting a solid grey / color screen like when there is no game inserted. In fact game in or not the result is the same.

I was really happy with the removal of the PPU as this came out very clean without messing up any pads etc. In my troubleshooting so far I have removed the RGB board out of the equation entirely and plugged the PPU directly into the new socket and now I get a solid blue color screen (Fortunately I hadn't cut the original composite line yet as I had removed C5 / Q1 so I soldered these back in. Effectively returning the system to its stock configuration with a socketed PPU.

With the aid of the Famicom schematic I have tested continuity of every PPU pin with a multi meter a number of times to all components e.g. CPU / SRAM / Cartridge slot etc. and have verified that there are no broken traces and that the socket and the chip itself is making good connections. I have also verified that the PPU is getting 5v.

I have also swapped the PPU chip into my own RGB modded AV Famicom and can verify the PPU is still in good working order.

As I haven't touched any other part of the board except the PPU socket and removal of C5 and Q1 (2SA937) which I have both reinserted, have verified the new socket and PPU is connecting everywhere it is supposed to be I am struggling to figure out what is wrong!

If anyone has any ideas it would be much appreciated as I am getting close to giving up and buying him a replacement

Here's a few pics of the board:

https://imgur.com/a/4JEWhYO

[Link83]

Have you got any close up pictures of the AV port pins? Also its blurry and hard to to be sure, but is there some corrosion or a broken solder joint on the power supply input?

Also, just a thought but have you tested with multiple different games? and have you tried pressing the reset button down a number of times when the system is on?

Some posts on a similar issue here:-
https://forums.nesdev.com/viewtopic.php?f=9&t=9832

[flex]

Have you got any close up pictures of the AV port pins? Also its blurry and hard to to be sure, but is there some corrosion or a broken solder joint on the power supply input?

Also, just a thought but have you tested with multiple different games? and have you tried pressing the reset button down a number of times when the system is on?

Some posts on a similar issue here:-
https://forums.nesdev.com/viewtopic.php?f=9&t=9832

Here ya go...

As I have been playing about I have put the NESRGB board back on (hence the wires) as I get a solid grey screen with it on and a solid blue one with it not. They probably look a little bit ragged in the first photo as I had just wipped em off quickly. The corrosion / sticky stuff around the power supply input was already there on this board i just assumed it was probably some flux / gunk left over from the factory or something.



I see that thread earlier and the reset button appears to have no effect when pushed... and I have tried it a lot!

After watching a few videos and reading other peoples experiences I have a gut feeling it might be CPU related for some reason... why it would decide to just give up at the same time as doing the mod though I have no idea.

I am going to measure the resistance and capacitance of some of the components with my meter and see what comes back see if anything is out of the ordinary, although I would appreciate some pointers on points to check. I don't have any spare parts e.g. CPU laying about here though to test and I would have to unsolder it and put another socket on for this anyway.

Don't suppose if anyone knows if you can purchase an NTSC / Famicom CPU from anywhere? Had a quick look on eBay and couldn't find much.

[flex]

Would a CPU from an original Famicom HVC-001 work in the AV Famicom ok? or should i be looking for a later revision?

[flex]

SUCCESS... I Found the issue!

I had managed to damage a trace which ran past pin 5 (which connects Pin 36 of the PPU to Pin 14 of the U2 chip) but it hadn't actually broken and had just moved into the pin causing a short. It was however still connecting to its intended components but shorting out on Pin 5 on the way through. When I looked at this trace by eye it looked like it was intended to go to this pin and didnt look out of place at all ! I finally found it by really going over the traces of the board in fine detail and under a different light using a head torch!

It must have been caused by a bit too excessive heat during the desoldering process that weakened it. Anyway I couldnt see a way to fix the trace and pull it away from the pin without breaking it so I just cut it and soldered in a bodge wire.

All is working good now. Not bad for a first attempt at this mod. Think I might invest in a proper Hako desoldering tool rather than my cheap chinese knock off one i have currently.

Couple of pics below of the dodgy trace and the fix.

Hopefully this may help anyone else who has a similar issue and what to look out for.

[Link83]

Great to hear you fixed it and well done on spotting that!

Thanks also for following up on the thread to let us know what to look out for (Its always a shame when you find old threads that end with no solution)