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How does this circuit oscillate?


I've got a old Atari 2600 I'm trying to repair without any audio. The audio from the TIA chip goes through some kind of oscillator circuit (From what I've read online) and combined with the video which is then fed into the RF modulator circuit.

Video is working fine but there is no audio. From the schematic the audio pin goes to this circuit which forms some kind of oscillator?

Can someone explain how the oscillator is works? I've checked with my scope and don't see any of the pins around the transistor oscillating? Is there enough info in the schematic to explain what its supposed to do?


Active Member
If the top half is an RF oscillator (or modulator to mix into video signal), your scope may not see anything in the higher ranges. Seems like the audio is only fed via C208 and the R207. Since the video goes thru, then the oscillator part is maybe working?. Check the audio C and the R. It is possible the C208 is faulty, depending what kind it is.
Then again, not sure about this circuit. Is it feeding to the video "mux" to the right? If this feeds to another video mixer, then audio may only be modulating the oscillator, which may be "dead". In that case, check the transistor.
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In the US, NTSC video on an TV channel is a combination of things. The audio is used to modulate a 4.5 MHz FM subcarrier. The modulated FM signal is then added (summed) with the video signal (which is limited to 4.2 MHz bandwidth to make room for the audio subcarrier and its sidebands. This combined signal is then amplitude modulated on to an RF carrier for the TV channel number.

The circuit looks like a variation of a Colpitts oscillator. The transistor's capacitance changes with the voltage across it, and that changes the frequency of the L-C oscillator.


Nigel Goodwin

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Common base oscillator, feedback from collector to emitter - the sound system is FM, and called 'inter-carrier sound' - in the UK we used to use a 6MHz carrier, and the rest of Europe used a 5.5MHz carrier.

As it's only low RF, any reasonable scope should show the carrier.


Thanks for the feedback. I worked out what the problem was from your suggestion. Faulty cap C208. Replaced and I can now see the waveform on my scope.

The other bit of the circuit that wasn't shows was the video portion which then combines with the audio section and is passed to the RF modulator.

Atari is up and running and audio working. Takes me back many a year :)


Well-Known Member
Most Helpful Member
retro computing.... there's actually an archive of old computer games, and an in-browser emulator
[Atari 2600 Library]
although now that you have your 2600 running again, that's probably more fun than emulating it online...

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