I have a mystery. Two famicoms with black screens. CPU and

[numbski]

(...and my title got truncated...)

CPU and PPU tested good in my spare front-loader NES and I have a solid 5v on both systems.

***

I basically spelled it out in the title. Black screen, and high-pitched whine for sound. On one of them, whatever CPU is installed starts to get hot if I leave it on, but apparently no worse for wear otherwise. The other one I had installed an NESRGB into, and was getting checkerboard-video-corruption. I figured it was just a dirty cart slot, but within a few minutes...black screen.

I don't seem to own a multimeter with a capacitance test function. The one with the NESRGB on it has the new power board on it though, and both test a strong 5V. I am thinking of desoldering and testing the work ram on both next. Both of these had composite video straight from the PPU working just fine when I received them - apparently everything l touch turns to ash.

[numbski]

Quick update - work ram tested good. Can't swap my front-loader vram because it is physically the wrong size. I am beside myself, because if not vram, then it is almost certainly a capacitor, and I though I had replaced all of the electrolytic ones. There's a ton of tiny ceramic caps. Could take me forever to run this down.


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[SamIAm]

Are you sure all of your new electrolytics are in the right way?

If you put a polarized electrolytic in backwards, it will quickly break down internally and cause a short circuit. Eventually, it will rupture, too.

[DirkSwizzler]

Quick update - work ram tested good. Can't swap my front-loader vram because it is physically the wrong size. I am beside myself, because if not vram, then it is almost certainly a capacitor, and I though I had replaced all of the electrolytic ones. There's a ton of tiny ceramic caps. Could take me forever to run this down.


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I've never done a cap replacement. So I'm not an expert. But I believe ceramic capacitors have a much longer lifespan and usually aren't worth replacing.

[numbski]

You're right about ceramics lasting a long time, but I have seen two chalking away that I replaced.

@SamIAm - good call, worth double-checking polarity. If it shorts, continuity should be there across the cap.


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[Makinx]

I've never done a cap replacement. So I'm not an expert. But I believe ceramic capacitors have a much longer lifespan and usually aren't worth replacing.

This is true for the most part. I recently had to replace a ceramic capacitor in a crt that burned out, so they CAN fail. They don't gradually deteriorate in the same way electrolytic caps do though.