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CX2600 no sound (just static) cheap patch.


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I got two Atari 2600 from a forum friend here in Sweden, one light sixer and one Vader, both NTSC and both were missing sound over RF...


I had the same problem with my own Vader but as I was going to s-video mod that anyway it was never a big problem, same goes for this Vader but the light sixer was in very good condition and picture is knife sharp so it was a shame there was no sound.


Took it apart, this tank of a videogame with a casted metal box over the whole PCB... Adjusting just the audio coil didn't help at all, checked this and that, desoldered the two 820pF Polystyrene capacitors for the sound modulation and measured, a bit off but it was uncertain with my measurement instrument. Tried some SMD caps close to that value but nothing helped. Adjusting audio coil nor fine adjust in the RF-box there at the RF connector.


So after finding sound on the Vader at a different frequency than the sound I understood sound circuit was OK but off by a a little... The available adjustment was not enough to get the sound in the ball park so to speak.

As this Vader is going to be s-video modded anyway I decided to give the light sixer a go again, with E.T. started on the Harmony I soon discovered I had sound from the TIA, checking with the oscilloscope I concluded that I also had modulated sound where the two 820 pF caps met... somewhere around 4MHz or so - very uncertain as I had a hard time getting a stable signal on the old borrowed CRT 'scope.


I decided to short the coil that's inline with the sound adjustment coil, took a crocodile clamp wire I had built the day before (swapped wire from the crappy original one). Behold - after some audio coil adjusting I had sound!


I wrote to my friend that the problem was solved and replaced the croc' wire with a soldered one, straight over the coil - but then I got nothing, no sound... So I snipped it and tried another croc clamp wire a little shorter - sound again but a little weaker and on a different adjustment. So the initial length of about 90cm wire (35") was pretty good, the 50cm (19") not as good. So I grabbed 90cm of multi-strand hookup wire, soldered to the coil and sound was back in all its glory!


I soon realized I can't have a loop of wire hanging out of the console so I wounded up into a coil over two fingers and taped it with some paper tape then put double sticky tape to hold it in place, then I needed to adjust the audio coil again but it was still within range and I ended up with a fully working and only slightly modified console still in its original shape.




Adjusting the audio coil with a cartridge inserted is a bit troublesome, I have previously purchased professional adjustment tools made of plastic that was bendy enough to make it doable, you either need a very short tool, shorter cartridge or a flexible one to get to it.


I'm pleased, even though I had already ordered new caps and transistor, but they may come in handy somewhere else.

I don't think this is a PAL/Multi-system TV problem as I have other NTSC units that plays sound perfectly fine, I assume there's some aging components that has let the sound carrier drift a little bit too far.

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