[SMS2] Sega Master System II mod - no audio!

[djlooka]

Hi everyone,
I've been tinkering with an SMS II (integrated Alex Kidd) for a while:
I've read tons of howtos, guides, etc but I can't still make this work.

My aim is to have a SCART socket on the back of the console for A/V output.
This tutorial http://kaikoshumi.webs.com/mastersystemiimod.htm
seemed to fit my needs, I also had the same chips on the board, but was intended for a captive SCART cable, not for a socket.

This guy http://www.electronicspoint.com/adding-rgb-scart-sega-master-system-t241450.html
wanted to perform the same mod I'm trying to do, but had a different motherboard revision (with different chips).

This is my setup so far:


I can see a nice video (composite, since the C-video signal goes to pin 19), but have no sound.

Notice that the RGB switching and Audio pins are not wired:
- If I wired the RGB switching signal to the correspondent SCART pin
(I've read different stories on this one: pin 16; pin 16 and 8; directly to pin 8, then 180 Ohm to pin 16)
moving the C-video signal to pin 20, I should get RGB picture instead of Composite: tried them all, all lead to massively distorted picture.

- I can't get the audio signal from any known contact on the board!
(all these contacts have been tested with either a 10uF or without it)

• SEGA 315-5246 - pin 15
• Sony CXA 1145P - pin 9 (also tried pin 8 in a moment of extreme discouragement)
• Negative leg of cap marked as C31)
• probably more

This mod should have been extremely easy, but...

Now, I don't care if all I can get is Composite video; all I want is a decent audio!
My idea is: since there is audio output if I plug the ol' crappy RF, where can I get it?

[albino_vulpix]

Have you tried wiring the audio and video outputs to an RCA and plugging that in? The simplest explanation I have is the input/outputs on the SCART socket have been wired up wrong.

[djlooka]

> Have you tried wiring the audio and video outputs to an RCA and plugging that in?
Nope, since I have no spare RCA connectors
> The simplest explanation I have is the input/outputs on the SCART socket have been wired up wrong.
I've tried both 2 - 6 and 1 - 3 (which mean audio IN and OUT), without success.

The best workaround would be to get the audio signal directly from inside the Mitsumi RF converter, but I can't find any schematic...


P.S.: I just found out the only inputs of such RF converter are 3 pins. Tried every one, without success.
My guess now is that there's a sort of amplification which happens inside that converter and if I could get the correct pin inside it, I'd finally have some output...

[albino_vulpix]

Have you checked the continuity in the scart cable itself? My SMSII has composite video and audio wired as in your diagram and works well.

[djlooka]

After another session of "solder - connect - curse - desolder", I made an interesting discovery:
- tried different cables (thanks @albino_vulpix), same result -

• if I connect the audio output (negative leg of C31) to just ONE audio channel, I get a sharp mono sound (left and right are separately both fine)
  (+ C31 -)------------(+ 10uF -)--------------(scart pin 2 OR 6)

• if I connect the same audio output to both the scart pins, I get no sound
  (+ C31 -)------------(+ 10uF -)--------------(scart pin 2 AND 6)

• if I connect the audio output from 2 different sources to both the scart pins, I get barely audible crackling sound
  (+ C31 -)------------(+ 10uF -)--------------(scart pin 2)
  (SEGA 315-5246 - pin 15)------------(+ 10uF -)--------------(scart pin 6)

Anyway, even if I wanted an RGB stereo mod, I guess I'm fine with composite and mono.

[viletim]

djlooka,

Take a look here:
http://members.iinet.net.au/~stinkyfist/reviletim/smsfm/smsfm.html

While the page is intended to be brief instructions for installing the FM sound board, it may as well be subtitled "Where to find the audio signal on every model of Sega Master System". There is only one sound channel however. Perhaps you are thinking of the Sega Game Gear, which does feature stereo sound.