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PostPosted: Tue Jun 16, 2015 12:54 pm 
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Posts: 759
I'm working on repairing a front-loading NES for a friend (not the toploader I was fixing earlier), which initially just output a solid brownish-green screen, but now after some work I have it displaying in black and white (no chroma?). Any ideas how to troubleshoot this? So far I have:

Cleaned/replaced the cart connector
Removed and tested the CPU/PPU (both work fine on another board)
Replaced the 7805
Replaced all aluminum capacitors
Adjusted the trimpot (~10 full turns in each direction), and also swapped it out with one from a working board

Thoughts for a next step?


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PostPosted: Tue Jun 16, 2015 1:43 pm 
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Displaying in black and white, without color ... do you still see the dot crawl? Do you have access to a 'scope? Is the output smeary? (Can you measure the frequency of the crystal? to what precision?)

Chroma decoding is disabled if the TV doesn't see a valid colorburst on the scanline. I'm not certain how picky the frequency rejection is... it can't be that sharp, given that it only has 9-ish cycles during the color burst to measure.

As far as "do, not measure": Maybe try swapping the crystal and/or the two BJTs next to it.


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PostPosted: Tue Jun 16, 2015 2:42 pm 
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Oh, FFS. I accidentally bumped the blue-only switch on my RGB monitor. It works fine.


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PostPosted: Thu Sep 24, 2015 8:23 am 
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You must have to check the circuit part of your screen which is controlling these actions. I mean the circuit which is controlling the colouring of the screen. Normally the main parts are TFTs but also for their control a trigger and control circuit is there.

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