Retro Gen Adapter

[Chilly Willy]

Chilly, that is COMPLETELY wrong. The GOAC in the RetroGen Adapter is a surface-mount IC. Look at this picture:

Hmm - probably confused it with one of the others... I've got a bunch of different things with GOAC chips in them. Might be the Radica I'm thinking of.

The chip with a 1 on it is the TCT-6801 GOAC. It would still be a major pain in the ass to get another GOAC in there, especially with the presence of that mysterious glop-top labeled 6. Even if that glop-top wasn't there, you would need to manufacture a completely new PCB for the GOAC you want to use in the adapter.

Like I said - it would be a huge pain to replace, just not as bad as if it were the blob.

Also, before I forget, I uploaded the first part of RetroGen Adapter vs. Original Hardware on my YouTube channel. You can see the adapter in action against a 1991 VA3 Genesis Model 1 and a 1992 VA5 Japanese MegaDrive Model 1:

Neat video!

[KnightWarrior]

So it's possible to switch the GOAC on the RetroGen..But it might work

[Ace]

Actually, if someone can figure out the pinouts of the TCT-6801 and whatever GOAC they want to replace it with, one can wire up a different GOAC to the TCT-6801's solder spot and any missing signals from the cartridge slot could just be wired straight to the slot, though this would be a VERY difficult thing to do due to the very low distance between the TCT-6801's pins.

[Ace]

I feel kinda stupid bumping this old thread, but I have some things to say concerning the RetroGen Adapter.

First up, RetroGen Adapter vs. Original Hardware Part 2 is now up on YouTube:



Second, after I finished recording all the footage I had to record off the RetroGen Adapter, I decided to take a look at the audio circuitry to figure out what causes the distortion in the audio output. Here's the RetroGen Adapter's PCB again:



The audio circuit is at the bottom-left corner of the PCB. It's just a few resistors and ceramic caps; there's no amplification whatsoever. I will draw out a schematic later today, but I have figured out why low-frequency FM notes and PCM are distorted: resistors R1 and R2. Those are 10Kohm resistors, which while they bring the FM to an audible level without needing any amplification, cause distortion because the signal out of the GOAC is too strong. Those resistors need to be replaced with 75Kohm to 100Kohm resistors, which will make the FM very quiet, but completely eliminate the distortion. I will then need to replace two other resistors where PSG gets mixed in to compensate for the weaker FM outputs, and finally, I would need to find some way to amplify the audio signal, preferably with a single TL072 dual op-amp.

[chessage]

I'd say the composite signal is a much deeper concern.

[evildragon]

(I really wish people would stop using Image Shack, too much advertisements just to get a link to see the FULL picture - Photobucket only has ads when you upload then you can link to the full picture directly).

I find it interesting that once again, no care was taken to get the video OR the audio right.. The Genesis gets far too much neglect in that regard.

[Ace]

I'd say the composite signal is a much deeper concern.

The audio issues are what first caught my attention, so that's what I started with, but I will take a look at the video circuitry once I get some 1/8 watt resistors for the audio circuit and properly balance the PSG with the FM audio having 75Kohm resistors rather than the 10Kohm resistors it has out of the factory.

I find it interesting that once again, no care was taken to get the video OR the audio right.. The Genesis gets far too much neglect in that regard.

Well, the overall video quality is better than a real Genesis (not as much edge pixellation and less blurring than a console with the CXA1145), but the sync issues just kill it. Adding resistors will just make the video too dark, so there should be some other way to get around the sync issues.

As for the audio, it looks like nobody can design a proper amp for the YM3438 except for TmEE with his Audio Mod. With a few tweaks, it works perfectly with Geniclones. The thing with audio amps for Geniclones is that they seem to be designed to omit the use of an amp, which causes all sorts of audio distortion, and even when an amp is present, the signal going to the amp is far too strong. What would be needed would be 75Kohm to 100Kohm resistors at the GOAC's audio outputs and an amp built around a transistor like the FC3 Plus (this one actually reduces distortion as the audio before the amp is distorted just like the RetroGen Adapter on the FC3 Plus) or built around an op-amp like the Gen-X and GN Twin (but a TL072 or similar this time, not an LM358).

Oh, and one more thing: DO NOT use the PAL region settings (PE and PA). The TCT-6801 still outputs video in NTSC, but some games have MAJOR speed issues, garbled PCM samples and graphical errors. Here's an example: I have a PAL MegaDrive copy of Sonic the Hedgehog which I decided to use with the PE region setting as that's what would commonly be used for a European MegaDrive cartridge. The Sega chant was fine, but the top of the screen had graphical glitches, the drums were garbled, and in-game, EVERYTHING was REALLY slow, much slower than on a real PAL MegaDrive, but it would sometimes speed up between proper PAL and NTSC speeds. The PSG was also slowed down and the game had multiple instances of graphical garbage. That = fail. Is there a GOAC that has a proper PAL setting? Every time I come across a GOAC-based clone (with the exception of the RetroGen Adapter and some phony Russian GOAC-based MegaDrive 2s), there's always a region switch to switch between the US and Japan region settings, but never one to switch between PAL and NTSC.

[Ace]

Well guys, my RetroGen Adapter is dead and I just can't put up with this thing anymore. Mods on this thing are not worth it; the circuitry is VERY tightly packed in there, the audio goes completely crazy every time you remove components (weak audio still audible even with missing resistors and no shorts? WTF?), simply put: just take the RetroGen Adapter for what it is and don't mod it. That's my advice to you.

[Solkia]

Well, I'd rather just get a Genesis actually...

[Ace]

Who wouldn't? This was a curiosity, and it does work quite well, but as for fixing what RetroBit screwed up, don't even think about it.

[Solkia]

Haha, I can't even install the SD Card reader on my Dreamcast, I doubt I could do anything with that thing.

[Ace]

So it turns out part of the problem is the 14.31818MHz crystal on the back side of the PCB. I've replaced it, but the RetroGen Adapter still tends to shut itself off. It does eventually stop doing that after one or two resets of whatever console it's being used on. As of right now, the RetroGen Adapter will stay in my RetroDuo v2.0 as the audio output is far too low out of the adapter's A/V out and I've modified my RetroDuo to output the RetroGen Adapter's Composite video through the console itself as I broke the Composite pin from the RetroDuo's KA2198BD.

But my God, is thing poorly designed. I'm not even gonna DARE attempt a video sync fix for this thing; I'm not soldering anything to the RetroGen Adapter anymore.

[chessage]

I'm not even gonna DARE attempt a video sync fix for this thing...

A schematic of the stock video circuit would be nice...

[Ace]

That's feasible. I'm gonna open up my RetroGen Adapter tomorrow and post a schematic of the video amp.

[chessage]

^**Bump IV**