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  • Joined: 13 Sep 2016
  • Posts: 4
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SG-1000 Composite video mod help
Post Posted: Tue Sep 13, 2016 2:25 pm
I've been trying to hunt down all the necessary information to complete a composite mod for my SG-1000, (Original model).

So far I've found out that the TMS9918 chip emits Composite on pin36. Does anyone know if this needs to be amplified to function properly with a RCA jack? And if so anyone have instructions on how to make a simple amp?

Next I'm needing to know where to get audio out of. I believe the sound chip is SN76489, do I just pull it from that? Here's a wiki on it https://en.wikipedia...ruments_SN76489. Or is there another area to get sound from?
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  • Joined: 30 Jan 2007
  • Posts: 270
  • Location: San Jose, CA
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Post Posted: Sat Sep 17, 2016 12:59 am
It doesn't need an amp, all you need to do is wire up a 470 ohm pulldown resistor to pin 36 of the TMS9918 (meaning you wire the resistor from pin 36 to ground) and then wire your video jack to pin 36.

You can get the audio from the SN76489 as well, but it's fairly noisy so you'll want to add a 0.001 uF cap across pin 9 of the chip and ground.
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  • Joined: 13 Sep 2016
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Post Posted: Mon Sep 19, 2016 1:40 pm
ApolloBoy wrote
It doesn't need an amp, all you need to do is wire up a 470 ohm pulldown resistor to pin 36 of the TMS9918 (meaning you wire the resistor from pin 36 to ground) and then wire your video jack to pin 36.

You can get the audio from the SN76489 as well, but it's fairly noisy so you'll want to add a 0.001 uF cap across pin 9 of the chip and ground.


Thank you for your help! I got a couple more quick questions.

Is this the right type of capacitor? (sorry cant post a link but I can post the brand but found them on Amazon)

E-Projects B 0002 B13 Ceramic Disc Capacitor

and does the polarity mater for the capacitor?

I found these resistors, are they okay?(Also on Amazon)

E-Projects 25EP514470R 470 Resistors

I'm assuming the polarity of a resistor doesnt matter correct?

Also when you say "across pin 9" do you mean to solider one leg of the cap to the chip and one to ground?

Lastly is a super noob question, can I ground using the RF Modulator? Or do I need to ground some where else?
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  • Joined: 05 Jun 2010
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  • Location: Pennsylvania, USA
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Post Posted: Tue Sep 20, 2016 4:21 pm
Ceramic capacitors are OK. Polarity doesn't matter for them. Same with the resistors.

With Pin 9 just solder one leg of the capacitor to pin 9 and the other leg to ground. Solder your center wire of the audio to Pin 9 as well (or the leg of the capacitor running to pin 9).

You can ground to the RF modulator.
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  • Joined: 13 Sep 2016
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Post Posted: Tue Sep 20, 2016 5:53 pm
Maraakate wrote
Ceramic capacitors are OK. Polarity doesn't matter for them. Same with the resistors.

With Pin 9 just solder one leg of the capacitor to pin 9 and the other leg to ground. Solder your center wire of the audio to Pin 9 as well (or the leg of the capacitor running to pin 9).

You can ground to the RF modulator.


Thank you! :) You guys rock!
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  • Joined: 04 May 2020
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  • Location: England
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Post Posted: Mon May 04, 2020 2:35 pm
Maraakate wrote
Ceramic capacitors are OK. Polarity doesn't matter for them. Same with the resistors.

With Pin 9 just solder one leg of the capacitor to pin 9 and the other leg to ground. Solder your center wire of the audio to Pin 9 as well (or the leg of the capacitor running to pin 9).

You can ground to the RF modulator.
hi i know this post is old but ive been trying to do the same mod and not having much success. Basically i done this mod just how you guys said in the post and im not getting any sound only video So i tried changing the capacitor to a different type of one same value rated capacitor tho and that didn't work either. Only time i get sound is by turning my TV's volume up to 100 and only then i can faintly hear the sound with alot of static.

Only thing im doing different is im not using the rf for ground im using various different grounds around the board the square solder pads round edge which works for the video.
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