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 Post subject: PAL NES With Video Fault
PostPosted: Thu Apr 06, 2017 9:38 am 
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Joined: Thu Apr 06, 2017 9:26 am
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Hi guys, my first time here and I was wondering if anyone has any ideas to narrow down my search for a fault with a PAL NES that has a problem outputting video.

The system loads the cartridge and the music plays normally, I actually get a flash of the game picture for half a second or so when it's powered on and then I get a black screen thereafter.

I've swapped out the PPU, CPU and the Power/AV box which are all working on another system with no problems. I cut pin 4 on the CIC to make sure it wasn't playing a part. On the scope I believe I'm getting a genuine black screen produced (not a problem picking up a dodgy signal). Because of the cartridge tray setup it's a challenge to take any measurements with a game running so I thought I'd see if anyone has come across anything similar and can point me in the right direction and if I should start looking at the RAM, Logic chips or individual components (which all visually appear fine).


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PostPosted: Thu Apr 06, 2017 11:32 am 
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Joined: Sun Apr 13, 2008 11:12 am
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Location: Seattle
First, to summarize to make sure I understand:
* You have two PAL NESes, and you have independently verified that both CPUs, PPUs, and RF modulators work on one board but not the other ?
* Music continues to play even after video has stopped ?

If both are true, the first place I'd look is the buffer transistor between the PPU video output and the RF modulator video input. (On this NTSC mainboard it's labelled "Q1" and is a 2SA937)


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PostPosted: Fri Apr 07, 2017 9:54 am 
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Joined: Thu Apr 06, 2017 9:26 am
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Thanks for the reply lidnariq.

I do have two PAL NES's that I'm using and you're absolutely correct that the sound continued to play normally after the picture went black. The system appeared to be outputting a black screen and not no signal, the monitor I'm using reverts to a blue screen and cuts the audio if the video signal isn't present - I was getting a black picture with the game sound and I could see a signal with the scope.

I did suspect the Q1 transistor earlier and did an in-circuit check and it tested the same as my other board. I swapped the 2SA937's to be sure and it didn't make a change. Probing the video signal at the PPU with the scope it appeared consistent with the black screen given when no cartridge is present (except not flashing in this case).

I think the 74LS373 is the next likely cause and will investigate.


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PostPosted: Fri Apr 07, 2017 11:03 am 
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Joined: Sun Apr 13, 2008 11:12 am
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Location: Seattle
I, um, ok. That's a lot weirder than I was anticipating.

It sounds like you have access to an oscilloscope? And are somewhat familiar with CVBS video?

Is the output video still well-formed? (i.e. relatively sharp hsync tip & depth? colorburst?)
Does it look different if you 'scope PPU pin 21 (where video comes out) ? Or on the RF modulator input ?

I'm kinda confused, because to the best of my memory, the portion of the video path from the PPU to the RF modulator is really pretty simple: just a PNP voltage buffer and a lowpass filter.

I guess "on both sides of the PNP transistor" is another obvious place to verify signal integrity.

If the '373 failed, it shouldn't cause an all-black screen. It should cause a weird banding effect, where the screen appears to be divided into 4 zones on-screen...


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