Twin Famicom, no red

[nihonmasa]

Hi guys,

I got a Twin Famicom (the standard black one without Turbo). It was not working (garbled screen).
I swapped the PPU with one I bought on ebay and it now works, except that I don't have any red color on the screen.
I desoldered it using my Hakko, and installed an IC socket.

Any idea what could cause the loss of red? PPU itself? A trace I broke whole desoldering/resoldering?
Anyone has any idea or schematics so that I can try to work my way out?

Many thanks in advance!

[Skawo]

Could be the PPU, could be the timing crystal getting too much out of spec.

Were you seeing any red amongst the garble with the old broken PPU?

[nihonmasa]

I think so? Not 100% sure

Do you know where I could find a PPU outside of ebay? Issue with ebay is that the sellers are all from the US and charge me the same price as the PPU in shipping, ending up at 50$ a pop....

[Skawo]

Could try getting a clone PPU on Aliexpress - an UA6528. Problem there is that it has to be an actual UA6528 - sometimes you end up getting a UA6528P instead, which wouldn't work, and they're not always tested so you could get a dud :/

Alternately, if you're in the EU, I have a few spare working non-clone PPUs I'd be willing to sell, too, if you'd be interested.

I'll say, though, the PPU being bad in this specific manner seems a bit unlikely to me. Since you've put the PPU into a socket, could you try the old PPU in again to see if the red comes through?

[nihonmasa]

Hi Skawo,

I am in Switzerland.
I actually did "even better"; unsoldered the PPU from my working twin, and I have the exact same result.
All the traces I could follow and test have continuity.
Changed the capacitors, same issue

Took a pic, actually unsure if it's the red that is missing or something else but the picture ain't right. (No, my CRT is not magnetized )

https://ibb.co/RjT7M9M

[Skawo]

So, definitely not the PPU, then.


Regular famicoms had a little pot you can twiddle if the timing crystal gets out of spec slightly, but it seems it has been removed in some Twin Famicoms. See if yours has it - it's next to the crystal at position C106.

If it has been removed, then try swapping the timing crystal (you need one that's 21.47727MHz).


I guess it could also be the transistor that's part of the video amplifier circuit, but I'm not sure which one that is on the twin, sorry :/

[nihonmasa]

So, definitely not the PPU, then.


Regular famicoms had a little pot you can twiddle if the timing crystal gets out of spec slightly, but it seems it has been removed in some Twin Famicoms. See

I guess it could also be the transistor that's part of the video amplifier circuit, but I'm not sure which one that is on the twin, sorry :/

So, definitely not the PPU, then.


Regular famicoms had a little pot you can twiddle if the timing crystal gets out of spec slightly, but it seems it has been removed in some Twin Famicoms. See if yours has it - it's next to the crystal at position C106.

If it has been removed, then try swapping the timing crystal (you need one that's 21.47727MHz).


I guess it could also be the transistor that's part of the video amplifier circuit, but I'm not sure which one that is on the twin, sorry :/

I don't think a crystal issue would give me a clear image like I have. But a bad resistor or transistor, maybe!

I'll keep looking

[Skawo]

It can. I had a Famicom that went completely black and white because the crystal went out of spec, but otherwise the image looked fine.