Twin Famicom graphical issue

Started by noex90, May 29, 2023, 03:19:56 pm

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Hello guys,

I just got a twin famicom from Japan, I totally disassembled it to clean it up because apparently it has spent some time in a basement or something and was full of dirt.
It's a lot better now and looks brand new (almost), so I tried to power it up and see if it was working correctly in disk mode, but, it has a few graphical issues:

Don't mind the black and white picture, my screen isn't really composite friendly..

I also tried in cart mode, the game runs well and is playable, the sound also works normally, there is just something wrong with the graphics. Note that everytime I restart the console, pattern is exactly the same.

Do you guys have already faced something similar? Might it be caused by a bad VRAM, or broken traces?


It could always be broken traces, but I'd guess that either the PPU or the PPU RAM is bad.


Thanks for your feedback, at first I thought it was caused by bad VRAM chip but after a few researches, it appears that the sprites should appear normally if it was a bad VRAM right? In my case everything look a bit scrambled..

Which chip should I check regarding PPU / PPU Ram?

Thanks a lot!


May 30, 2023, 04:48:19 am #3 Last Edit: May 30, 2023, 05:04:52 am by Skawo
Could also be one of the latches, I guess, but not sure which ones are for the PPU

You can get replacement famiclone PPUs that should do the job well enough, though they're not the cheapest things. The SRAM should be pretty easy to get.


Thanks a lot for the picture and the explanations, I'll begin by checking traces around these chips to see if there is nothing wrong.


Replacing chips isn't exactly the easiest to repair so I would rule out all easier fixes first (caps etc).

How are you hooking it up to the TV? I had a similar picture when trying to use RF on a PAL TV which supposedly doesn't support NTSC via RF (via composite is fine though).


So you think I should check all the caps as well? Bad caps could also cause similar graphical glitches?

The twin is hooked up to the TV via composite so I should be fine on that part. What makes me think that it's caused by a bad chip / cut trace is the pattern "D3D5 DBBDDF.." which is always the same everytime I power up the console.

I also tried to swap IC105 / IC108 and the result is the same, so I assume RAM chips are OK?!


Yeah, they're either both OK or both bad :v
Whichever is more likely is up to your discretion.

I would wager it's the PPU, then, though... I've never seen one fail THAT badly. Worst I've seen was like this:

Does the PPU get hot at all when it's been powered on?


By switching RAM chips, I thought the console won't boot at all if one or both are bad since there is a "Work RAM" and a "Video RAM" chip.

Indeed, nothing like compared to your picture, mine is on a level above yours  :-[

Yes I just checked, CPU & PPU are getting hot while the console in ON.


...Like, noticeably hot, or just warm?
They, uh, shouldn't get "hot" per se.


It's not really "hot" but warm yes! Hard to tell the temperature but I can keep my finger on it without being burned :)


OK guys, I've finally managed to find the issue! Let me explain if it can help somebody with the same issue.

Seeing your picture showing what are the symptoms of a bad PPU which were totally different than mine, I've decided to really check for bad traces, assuming that all my chips (CPU, PPU, RAM) were OK.

In my knowledge there is no schematics available for the Twin Famicom so I took the famicom's one:

My issue is related to graphics so it was time to check every connections from PPU to other chips.. and as we can see on the schematic, PPU AD4 (pin34) is supposed to be connected to S-RAM D4 (pin14), in my case there was no continuity between the 2 points.

Unlike the schematic suggests, it's not a direct connection between these 2 points, as you can see on my picture :

PPU AD4 is connected to pin 60 of cart connector -> pin 16 of HM6264P-20 chip -> D4 (pin14) of S-RAM.

In my case there were oxydation on the last pads under the cartridge connector and the trace was cut just above pin 60 (red line on my pic).

Repaired it with a bit of soldering and here is the result:

Thanks a lot for the help, now it's time to repair the disk reader :)


Awesome! I assume the b/w picture is due to a PAL TV, or something?


Yes I'm using an upscaler to output via HDMI and it's not really compatible with composite so it displays in B&W :)