Genesis model 1 stereo mod, video distortion

Hey folks

I'm going to be modding my genesis for stereo audio soon but I have a concern. I have a model 1 genesis like the one shown in this link

http://imgur.com/DIxRDRs

Notice it does not have the "High Definition Graphics" writing around the cart slot. It also has the TMSS upon booting fyi.

Here's my issue: I've seen various mods online for stereo audio on early genesis models and some utilize the volume control on the system while others do not.

When I use the headphone jack, I get significant video distortion, horizontal wavy lines that grow more intense with the volume slider. I figured since that was the case, I should probably skip the amplifier altogether and do the mod where I tap the audio chips pins directly. But then I've read that doing that can result in sound levels that are far too low.

Any ideas or recommendations for this problem? Essentially I'm looking for a good stereo mod (red/white jacks out the back) that will bypass the defective amplifier on my system. All help is appreciated!

 

Notice it does not have the "High Definition Graphics" writing around the cart slot. It also has the TMSS upon booting fyi.
High Definition Graphics is only written om the MEGADRIVE( i think)

 

High Definition Graphics were written on the earlier American Model 1 Genesis consoles. At present, I have audio in my model 1 Genesis tapped off the Sony A1034 chip inside similar to how this page describes it - http://retrorgb.com/genesisaudiomod.html. The mod was performed by the previous owner of this Genesis, and technically isn't amped the way it should be. The stereo output is too low, however it is certainly functional. Better to run it to a small amp circuit prior to output, but you could certainly do the mod as described on retrorgb and get stereo audio out.

That being said, my intention is to eventually put a proper amp in the Model 1 and get closer to line level stereo audio output on my model 1 Genesis.

I am curious about the distortion issue you mention. There are some Model 1s in the middle of the internal revision run that have terrible caps inside. What power adapter are you using, and have you cracked the system open yet?

 

Some more information about my specific model. The connections on the back look like this
http://imgur.com/Kvv4F15

Which according to this thread
http://www.sega-16.com/forum/showth...-good-Genesis-1s-and-Genesis-2s-from-bad-ones

indicates that my model is VA6.8. I have opened it (did the s-video mod) and it looks like the one in this picture below from the retrorgb page you linked
http://retrorgb.com/images/GenesisAudioModPage03.jpg

Except my sound chip is a Sony CXA1034P. So I'm pretty sure I have a Genesis 1 model with the good quality sound.

Again to my problem: What are my options if I want to have amplified sound? Is it possible that disconnecting the wires that lead to the headphone jack could solve the problem? Or is it more likely a problem with the circuitry on the board? Should I replace the amplifier somehow and, if so, with what? Any links?

Thanks for the help!

 

I looked into this recently as I had done the mod as described at http://retrorgb.com/genesisaudiomod.html and while I could amplify that loud enough with my stereo I wanted a proper amped signal from my genesis. I found a post by Fudoh from years ago, http://shmups.system11.org/viewtopic.php?p=738046#p738046, mentioning another way to get the stereo line level by adding another LM358 like already in the genesis to make a stereo line level output. Here is the post Fudoh linked to with the schematic on how to do it, http://shmups.system11.org/viewtopic.php?p=638564#p638564.

But again there was still some things I didn't quite like about this solution. It requires making irreversible changes to separate the stage that mixes the left and right channel that eventually is amplified and fed out the mono out. And since I was already buying a LM358 which does two channels anyways I decided to just recreate the amp section and not mess with cutting traces. From a schematic of the snes http://i.imgur.com/jcTCW.png it looked like the spots where you tap for sound on the CXA-1034 is the same line just before resistors R42 and R43 in Konsolkongen's post. So basically all together you need 1x LM358, 2x 10k resistors to replace R42 and R43, 2x 33k resistors for the feedback resistor on the LM358(R74 on the schematic), and 2x 10uF capacitors after the output of the LM358(C30 on the schematic). Here is a pinout of the LM358: http://3.bp.blogspot.com/-6pj_6jvzC.../s1600/Simple+Preamp+Mic+using+IC+LM358+2.GIF

Anyways I gave it a shot and it seems to be working great but use at your own risk.

 

http://www.sega-16.com/forum/showth...ive-1-stereo-mod-no-headphones-volume-control

I have a board on there you can buy for dirt cheap to make properly amped stereo on any model 1.

 

Update: I did the stereo mod, first trying to tap into the line out solder points so that the volume control slider controlled the volume going out to the TV. Turns out this worked with no video distortion! So I didn't have to work around the amp after all. Stereo mod complete and working beautifully.

Thanks for the input all. : )

 

Very cool Helder! What difference does the TL072 make compared to the LM358?

 

The stock amp is considered crappy in comparison but I'm sure people will complain about the TL072. That circuit was tested against the stock genesis headphone audio at max and matched it without the distortion from faulty slider. It's a good cheap way to properly get stereo audio directly from the audio encoder chip.