Mega Drive/Genesis picture noise

[Xaranar]

Hey all

So this is about the RGB output on every single Mega Drive I have ever owned, be it model 1 or 2. I'm not talking about the famous jailbars that are present on nearly every model 1 hardware revision of the console, rather, this is something I have noticed that is present in every MD console I have owned, at least, every PAL one. It looks like a sort of a moving concertina effect that moves back and forth, going vertically down the screen, if that makes any sort of sense. I imagine it will to anyone who has ever seen this before. Interesting to note, this weird interference is there on both models 1 and 2, even with an RGB bypass via a THS7314 amp, it is there on a PAL machine that I swapped the oscillator for an NTSC one, effectively making it an NTSC machine, and it is there when using the proper 420R attenuation on the sync line, as well as the fully shielded packapunch cables from retro gaming cables. Has anyone else seen this? Will post photos later on to show you what I mean.

Cheers

[ASDR]

Looks like you've done everything possible to get a good picture. Does this go through a Framemeister, OSSC? I've seen lots of artifacts appear/disappear based on the filtering of the particular TV/scaler. Only thing you did not mention is the power supply, not sure if the MD is picky there as I've only used the linear original one.

I know it's tricky, but maybe could you try taking a few pictures / direct captures? I have some rather noticeable jailbars on my unit and I need to adjust RGB gain & bias a bit on the OSSC to get perfect blacks and whites, but apart from that the image is really clean on my NTSC modded VA4 PAL MD1. I don't think I've seen what you describe.

Edit: Also just seen your other post regarding the whine issue. Puzzling, mine was clearly related to coupling from the video signals and changed with the screen content. Shielded cabling fixed it.

[Xaranar]

Okay, so I have a fresh MD2 board arriving today, I've been meaning to redo my mods as they are some of my earlier work and not the tidiest. I'm going to use shielded wire to connect the video signals to the RGB bypass board to see what sort of results that yields. I'll do the same with the Mega Amp board, to see if that makes a difference again.

[ASDR]

Please share your results I'm thinking of installing a bypass in mine. My soldering skills have gotten better and the installation doesn't look that scary anymore, basically just waiting for a new design to be released. I think dbElectronics and Voultar are working on one.

[Xaranar]

So, I've finished working on my replacement Mega Drive. I used shielded wire to wire up the Mega Amp mod, and that appears to eliminate any background noise, but most interestingly, the RGB bypass mod I found to be the culprit for the picture noise. The stock encoder provides a completely clean image, and colours are not any better using a THS7314. The only situation I can recommend using one is if you have a model 1 and suffer from the jailbars problem. I installed the mod just to be sure, and yes, it causes picture interference for some reason; removing the bypass and repairing the traces I cut completely eliminated the problem.

[Syntax]

I think maybe this issue is similar to the N64 noise on white screen thread.

Its probably dirty/noisy power supply to the THS chip.

You could fit a LDO to power the THS and see if it cleans the picture up.

[ASDR]

Not sure if I understand completely - do you still have noise with the shielded wires? I have jailbars on my MD1 and would like to do a bypass at some point. The image itself is fairly clean, so I also wouldn't consider a bypass if it wasn't for the jailbars. Do you use the ArcadeTV board or did you make your own 7314 circuit for the bypass?

[Xaranar]

Possibly, though I think I’m just going to keep the stock encoder, as the picture through it is already excellent, and the colour reproduction from the THS is no better. Furthermore, the picture from the THS is shifted slightly to the right, meaning it is out of alignment when used with the 32X.

[Xaranar]

To clarify - the image is only noisy with the bypass board installed, and it was one of ArcadeTV’s boards.

[ASDR]

Thanks for clarifying! Yeah, seems like you're better off without the bypass. Looking at your results, I think I'll also wait then till newer bypass boards (dbElectronics/Voultar) are available.

[Xaranar]

I think that's probably wise, I'm hoping the newer boards will include proper CSYNC 75 ohm termination, so as to negate the need for the 420 ohm resistor currently used in Mega Drive cables due to the CSYNC being driven at TTL levels. On the ArcadeTV board, the CSYNC just gets pulled up to 5 volts by a 2.2K ohm resistor, then through a cap and a 75 ohm resistor, which makes no sense at all, as the CSYNC line is already pulled up to 5 volts on the console's mainboard.

[zeruel85]

I think that's probably wise, I'm hoping the newer boards will include proper CSYNC 75 ohm termination, so as to negate the need for the 420 ohm resistor currently used in Mega Drive cables due to the CSYNC being driven at TTL levels. On the ArcadeTV board, the CSYNC just gets pulled up to 5 volts by a 2.2K ohm resistor, then through a cap and a 75 ohm resistor, which makes no sense at all, as the CSYNC line is already pulled up to 5 volts on the console's mainboard.

The original intention on that PCB was to lift the pins directly from the VDP (to avoid any interference), thus why the pull-up resistor.

I have one little PCB which takes care about the csync (proper 75 ohm attenuation), it works good, but keep in mind that if you need to use the 32X you must also have the TTL csync available, otherwise the 32X does not work at all.



I have a spare one if someone of you are interested in it. It provides a THS7374 with selectable LPF filter.

[ASDR]

Lifting the pins always seemed a bit too tricky for me, but from what I can tell soldering to the vias and them disconnecting either some SMD parts / disconnecting at the CXA or worst case cutting some traces should also do the job?

[zeruel85]

Lifting the pins always seemed a bit too tricky for me, but from what I can tell soldering to the vias and them disconnecting either some SMD parts / disconnecting at the CXA or worst case cutting some traces should also do the job?

That's correct.

[Xaranar]

Sent you a PM about your spare board.