Twin Famicon No video, audio, power led

[jonathanb9595]

Hello all,

Drug my twin famicom out of storage to install Tim Worthington's nes rgb mod. To preface, I (stupidly) did not check if it was operating before I started the mod.

Anyway got the ppu out, socketed, put in the mod, got the debug led lights, but no video. Fiddled around some more, couldn't get a picture. Figured I'd go back to square 1 and try to run it stock. So I remove the fpga board from the mother board and try with just the ppu installed. Now I get no led for power, no picture, audio etc.

I can feel　the cpu (?) Getting warm (not super hot) and ppu is cold to the touch. I tried measuring the 5v on the ppu, but I don't get anything.

I've got 2 or 3 other working famicoms I can take ppu's, cpus etc from if need be, but right now i'm trying to figure out my power issue.

Thank you!

Jon

[Skawo]

No power LED would indicate a ground short somewhere to me. Check the voltage at the voltage regulator.

[jonathanb9595]

Hi Skawo,

With the power board disconnected from the main board, i get 7.7 in and 7.7 out. When I connect up the main board my output drops to .3 volts. Is that a sign of a short? I also removed the ppu from the circuit and still see a .3v out.

[Skawo]

Yeah, the voltage dropping dramatically is a pretty clear sign of a short. Reading 0.3V probably means your power supply's protection's kicking in.

The regulator reading 7.7V out is also weird, though - that's meant to be 5V, I believe.

[jonathanb9595]

The twin famicom needs 7.5v to operate iirc.

Not having too much luck finding a short. Is this the point in the game where I just start pulling chips?

[Stephen]

Been there, done that. It ended up being C13 IIRC. If you show me a photo of your power board it may jog my memory.

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[jonathanb9595]

Thanks for the reply, I'll rip out C13 and see what happens. In the meantime here is my board:






Looks like its a different revision to the one you posted.

EDIT:

Did more poking, apparently there was some gunk on the PPU 5v line. I read the CPU and the FDS CPU chips voltage, they both have ~4.7 volts (looks like this is on the low end). Scrapped some junk off the PPU 5v line and see the same ~4.7 volts.  I tried to probe the power LED, but I get 0V on it (I tried scraping the leads just to make sure it was making a good connection).

Looks like the voltage regulator was a red herring? Wonder if my PSU is going, I modified an original PSOne power supply to work with this system when I got it.

This still doesn't explain on the voltage regulator why I'm reading such a small voltage, but with the 5V line being so low, is my next step to try a new PSU?

[ericj]

The traces on the bottom-left of this pic look weird, but maybe it's the lighting.

[Stephen]

Looks like its a different revision to the one you posted.

Looks like the same PCB revision as my board, maybe I'm missing a detail?

[jonathanb9595]

Looks like its a different revision to the one you posted.

Looks like the same PCB revision as my board, maybe I'm missing a detail?

Gah, my mistake. Yup thats the same revision, i was looking at the components and they looked different.

Got busy yesterday and didn't take out c13 yet. Hoping to get to that tonight.

As for the traces @ericj, its just a weird reflection. I saw that as well and checked, but they look normal.

[Stephen]

Got busy yesterday and didn't take out c13 yet. Hoping to get to that tonight.

I've had a chance to look more closely at the power board. I can't recall if it was C204, C210, C206, C207, or C208 -- but for me it was one of these smaller capacitors. I would recommend taking them out and testing their ECR / capacitance.

[jonathanb9595]

Got busy yesterday and didn't take out c13 yet. Hoping to get to that tonight.

I've had a chance to look more closely at the power board. I can't recall if it was C204, C210, C206, C207, or C208 -- but for me it was one of these smaller capacitors. I would recommend taking them out and testing their ECR / capacitance.

Thanks for the ideas, I will test these tonight. I've completely removed the famicom board from the power board, and found if I "enable" the 5v rail (shorting the pwr switch pins), I get ~4.8V on the 5V rail, but checking back on the 7.5v voltage regulator, I'm only getting 2v out. So the problem is (hopefully) isolated to the power board.

[jonathanb9595]

Next day update, I believe c206 is bad, but I just went ahead and ordered all the caps that are on the psu board. Should be here in a couple days and I'll report back.

[jonathanb9595]

Next update.

Swapped out all the caps on the board, no change on the voltage regulator (just power board, nothing connect but simulating the power on gives 2.4v out on the 7.5v voltage regulator).

Not really sure were to go next. Could somebody confirm that their operating twin famicom is giving a full 7.5v off the voltage regulator when its in operation?

[Skawo]

Where exactly are you measuring so we can be definitely on the same page?