Dreamcast A/V Out Sync Issue

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Odyssey113
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Posts: 6

Dreamcast A/V Out Sync Issue

Post#1 » Mon Feb 20, 2023 12:11 pm

I've been working on a second-hand DC i picked up on the cheap a few weeks ago, one sold as-is with disc loading issues from ebay, I was hoping to make an easy GDEMU mod out of it and make it my second DC in my house. Turns out it's got a bit more of a lingering issue going on that's effecting it's ability to sync on first boot with VGA devices (using a Kaico adapter out of the DC) and sync at all with my RGBs consumer set (at least with my RGB modded 27S42 that is syncing on S-video port).

One thing to note before I get into this- My original DC that is also a VA01 motherboard works perfectly fine with both of these setups I will be referring to so we can go ahead and rule out cables right off the bat, unless this mobo revision I just picked up ends up requiring a sync on Luma cable or something where as the slightly newer mobo revision I've owned for a while does not.

Also please note, I've done swip-swaps of parts like the known working power supplies and the GDEMU's themselves into the working consoles and ruled out any of the parts themselves, outside of the motherboard.

Port looks fine, pins look fine. I've retouched up almost every solder point and hit them with flux. Retouched all the AV pins doing a very nice drag soldering job. Inspected all pins for bridges. They all look fine. Retouched almost all components on any sync line I could find within the schematics especially focusing in on the Y (Luma) line. I think I've ruled out cold solder joints by now. The new board also got a full electrolytic recap that I did myself, which didn't make it better or worse. The issue persists. When it syncs, it works great.

Specs:
Known working DC, board revision 837-13778/04, only mod done to this is GDEMU - Console works fine on both setups. It's been my main DC for a while.
Problem child DC, board revision 837-13778, just added GDEMU and I did do a full re-cap in hopes it would fix this. The re-cap did not change anything for the better or worse.

Problem Description:

On VGA (connecting to VGA monitor via Kaico Adapter) the console will trigger the monitor to power up, it will white/static screen on first boot and usually no audio, then if you unplug the AV and replug it back in, the console works fine and operates as normal as long as you have it on.

On my RGB modded 27S42 Trinitron consumer set, however it is visibly out of sync, with what appears to be at least Vertical sync if not if not horizontal as well. The same behavior that happens with VGA where I unplug/replug in the AV doesn't seem to make this setup work. I will post a video of that below. I was able to get it to sync one time by dumb luck a few days ago but haven't been able to since. It was a dumb luck situation like I get with the VGA, only VGA that trick tends to work everytime.

One very interesting thing to note about this issue, is that on my consumer set, if I sync on my composite line going into my TV instead of the S-Video line, is that it syncs on composite just fine. Also regular composite video output also works fine as well, so I do get that it seems to either a problem with syncing on Luma itself and this console, a problem with the selector pin (pin 6?) maybe not grounding and not "selecting", or just some thing else i'm overlooking maybe to do with the port itself.

Video showing sync issue on consumer set: https://imgur.com/RdPHKph

I appreciate any input you might be able to offer! I feel like it's right under my nose somewhere here, I just don't know how to sniff it out!

Also I'd probably be okay if there is a way for me to just hard-wire this into accepting VGA off the bat without needing to detect, as that is the goal for this console any-hoo!

EDIT / Update: This is issue semi-sorted itself. I can't really explain it great especially since my first console never gave me issues with either setup, but I believe the console I'm referring to might have a weak C-Sync signal or something. I got my upgraded scart switcher in the mail that utilizes a power supply. Seems to maybe carry the sync signal stronger or something. Again, it really doesn't make a lot of sense because the first console worked fine with both, but hey it works now so FUCK IT!... Guess it always did lol!

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