Super Nintendo alternating between B&W and color picture?

Working with a flaky Super Nintendo (SHVC-CPU-01 PCB) that keeps alternating between black & white and color picture during Burn-In testing using a Nintendo Test Cartridge. It's passing all of the hardware tests, and the sound module tests okay, so I've ruled out a bunch of potential causes right off the bat.

A/V Cable is genuine and works fine on my Super Famicom and a GameCube, and a visual inspection of the board does not reveal any obvious corrosion or broken traces. The cart connector looks good and all of the solder joints are clean. The caps also look okay, but I'm not sure how to properly test (if someone tells me, happy to do it, I have a multimeter). Power adapter is good and tests fine, no issues.

I've seen and read various things via web searches, and two things stood out as potential causes:

• Bad traces. This didn't pan out.
• Bad crystal oscillator. Not sure how reliable/likely of a fix this is, but for a dollar, I ordered 10 anyway.

If anyone has any other ideas or recommendations, happy to give them a go. I don't have scopes and I can't do surface mount work, so anything along those lines is out of the question. Basic to intermediate soldering and a multimeter is about as far as I go. ;p

Thanks!

 

Does it do this with a RGB (SCART/JP-21) cable? If not, then there is something wrong with the circuitry that creates Composite Video and S-Video from RGBS (Red, Green, Blue, C-Sync). If it does, there is something wrong with the PPU.

 

I've had it running via JP-21 RGB to a Framemeister for about 15 minutes now and it seems to be consistently displaying color. No sign of B&W picture--I haven't been staring at it non-stop, but long enough, and I don't think it's gone B&W at all--so it seems that your idea that something is going wrong when the console produces composite video is potentially correct.

As an aside, it's hilarious watching the Framemeister struggle with the Burn-In tests, hehe.

Any ideas where to go from here? Do you think it's an issue with the BA6952F? I can snap some photos of the chip, the caps and the surrounding areas if it's helpful. I'm pretty sure nothing in the area is corroded, rusty or otherwise damaged, so not sure what's going on. I'll have to sell this for parts if it can't easily be repaired.

 

Almost certainly the xtal that generates the color reference is slightly off frequency.

 

Since RGB works fine, then my theory is probably correct. If the xtal that TriMesh mentioned can't be fixed or replaced, you could always build a new Composite video/S-Video encoder that uses the RGB signals on the Multi-Out. You would then need to cut the composite video/s-video traces leading to the Multi-Out and solder the outputs from the new encoder to the correct Multi-Out pins.

Shouldn't be difficult if you can gather the parts. This site has a list of the parts needed and a tutorial on how to build a RGB to NTSC composite video/S-Video converter. It's being used for JAMMA arcade machines but it should work on the SNES. You could even make it external and use a SCART socket wired for JP-21.

 

The crystal can definitely be replaced, it seems to be this one:
http://www.nesrepairshop.com/Catalog/index.php?main_page=product_info&products_id=1084
http://www.ebay.com/itm/New-21-4772...rystal-Oscillator-HC-49S-10PCS-/221760288872=
I already went ahead and ordered a set, just in case.

Some people seemed to indicate messing with a pot located next to the crystal can also resolve the issue, but I opted not to do that for the time being as I have no idea how to properly adjust it, nor do I know if the issue is that adjustment, or the crystal itself.

 

Ah ok. Hopefully it fixes the issue.

If you decide to mess with the pot in the future, make sure to measure it with a multimeter and don't turn it too much or you could damage something. I learned that the hard way with my original Xbox's laser.... try replacing the crystal first before the pot.

 

That's a trimmer cap - it's used to fine adjust the frequency of the xtal, and will probably need adjusting after you replace it. Normally you turn it from one end to another and find the places where the color drops out, then set the cap to the middle of the range where the color is correct. It's much easier if you use a non-metallic tool, since if you use a metal one the setting will change when you remove it.

 

@TriMesh, thanks for the info. Do you think I should just try adjusting the frequency as-is, or replace the part and then adjust it?

Also, it the adjustment doesn't seem to be a flat or philips screwhead, it's sort of an odd, round-ish shape, so not sure what to use (thin pliers?) to adjust it.