SMS II RGB-mod screwed up my console... (FIXED, but now it's noisy audio)

[metrolf]

Where do I begin? I modded my Pal Sega Master System II for RGB. I connected the wiring to a port which fits the Mega Drive 2 RGB-cable, consulting both mmmonkeys guide (different cable port, but otherwise identical) http://members.optushome.com.au/eviltim/gamescart/gamescart.htm and Tims cable-pinout page, http://members.optushome.com.au/eviltim/gamescart/gamescart.htm. The first question I had was whether to connect the mono audio to the mono pin or split it across the stereo-leads. I removed the RF-modulator as well. The RGB-part of the mod worked perfectly, super crisp image, but no sound. I soldered everything directly to the chip-legs.

For some reason I tried removing C31, a cap on the audio line and that actually gave me sound, but also a horrible background-noise. On Tims page for SMS FM Audio he does the same thing, passing the audio trough the audio board if I'm not completely mistaken. I then tried another game other than the built-in Alex Kidd, and Alex Kidd still booted up... The other game has been cleaned and tested, and the connectors have been tightened and cleaned as well so that is not the issue, it all worked great with RF. The game actually started up once, but blacked out quickly. Once I got the text "Software Error" on screen.  Other than that, the audio is gradually getting weaker as I keep trying to fix it, the noise is getting louder and Alex Kidd keeps glitching out more and more.

I've only finished one year of my electronics-bachelor so as of yet, I have no clue how to diagnose this. Does anyone have any ideas?

[ApolloBoy]

Try replacing C31 and move the audio wire from that to pin 8 of the CXA1145.

[metrolf]

Ok, I can try that, but the label on the cap is rubbed off. Is it a 10uF 50V cap? So after the signal is passed trough the cap, I will run that to pin 8 on that chip?

[metrolf]

Consulted a service manual online, seems like its 1 uF. Better start looking for one to harvest somewhere.

[ApolloBoy]

Consulted a service manual online, seems like its 1 uF. Better start looking for one to harvest somewhere.

It's much better to buy a new one, it's a very common value actually.

[metrolf]

Consulted a service manual online, seems like its 1 uF. Better start looking for one to harvest somewhere.

It's much better to buy a new one, it's a very common value actually.

It would be, but I live in Norway where there is no retailers carrying electrical component, so the only option is to order from digikey. 25$ for shipping 1 cap! No thanks^^

[metrolf]

Finally got the cap replaced and wired it to pin 8 and to some extent; success! The console actually works now so yaay, thanks for the help so far But the audio was ridiculously noisy, so I figured something was still wrong. I ended up desoldering the audio-wires in the scart-plug itself and leaving only the mono. This made it A LOT better, it's actually acceptable now, but the noise is still there. I even tried installing separate gold plated RCA-jacks to see if the cable was the perpetrator, but the audio was, you guessed it, still noisy :/ So I guess it's something on the board... Could it be related to the quality of the cap I replaced (C31) or the cap used to amplify the audio I installed alongside the rgb-mod?

[People?]

Did you install a capacitor in series with the audio signal? It needs to be 10 µF minimum, with negative leg going "out" to the TV.

[metrolf]

Yep, it's all like it's supposed to be... Can't figure this one out...