Hello all! Thought maybe you fine folks can help me diagnose this problem I'm having with my AV mod. As soon as I switch on the power, I get a perfect picture but then the picture slowly becomes worse and worse and gets darker and darker until it becomes almost pitch black. If I switch it off and immediately switch it on again, the picture stays black. But if I leave it alone for about 5 minutes and switch it on again, the picture is perfect again, but then repeats the process of becoming worse. Here's a video to show what I mean: (Please ignore the jailbars, at the time I had already I took off the 1uF caps underneath the board to diagnose this problem) And here's a pic of my mod: Though the pic may not really show it, I'm pretty sure I don't have any shorts or anything. Any thoughts on what it could be? If anybody more knowledgeable could perhaps offer me some advice, I would truly appreciate it! Thanks!
Looks like something is pulling too much current from the transistor and overheating it. Does it get extremely hot? I would hope it's not pulling too much current from the PPU instead, otherwise it will cook it. I did a simple 2N3904 mod on mine straight from the RF modulator input pins. I still have slight jailbars because I never isolated the pin, but not enough to bother with. I have never seen this particular mod...
I've been using this as my guide: http://jpx72.detailne.sk/modd_files/fc/avmod.htm The transistor doesn't seem to be getting hot though I can't really tell for sure as it's underneath all those wires and resistors (see pic above). Do you think it's a problem with the transistor I'm using? I've reused the one from the Famicom (2SA937) but I do have a 2N3906 as a spare.
Look at the schematic again and match it to your picture. I don't think that's how you have it wired... Do you have the pin next to video out (/RST) wired as the 5V supply? That's probably not a good thing- run this to a proper 5V supply. There should be a 7805 on the board somewhere, or at least use pin 40 on the PPU.
Hmm. I have that resistor connected to pin 22, which is one of the 5V on the PPU. Am I not supposed to do that? EDIT: ah just saw your edit. Ok, I'll try wiring it up to pin 40 instead. 7805 seems a bit far away, but I'll try that if pin 40 doesn't work.
According to this pinout (https://wiki.nesdev.com/w/index.php/PPU_pin_out_and_signal_description) pin 22 is the /RST line- you probably shouldn't be pulling current from it.
Hmm, so I've rewired it to pin 40. Still the same problem. I suspect even if I were to rewire it to the 7805 it would yield the same results but I'll give it a go anyway.
Ah, oh well, it was worth a shot. It's possible it's a damaged transistor, but could also be damaged PPU. That would be a bummer... I gather you have tried multiple displays also? You could also reverse the mod and see if RF still works.
Try a different circuit then. Don't instantly blame the PPU or transistor, or something like a failing capacitor somewhere else on the board. If you have access to an oscilloscope, hook it up to it and see what the waveform does. I've had success with this circuit in the past, and also adding bulk capacitance to the power inputs of the PPU to remove jailbars. .
