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Originally Posted by frenchy
Has anybody ever done a 'stereo' mod to one of these old tvs that have two speakers? I.e. add a stereo modulator to the circuitry so you get stereo from the speakers? Seems like it would be cool on one of these sets with the speakers separated so far apart in these big cabinets and would get great separation effect.
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TV stereo sound is very similar to FM stereo sound. The subcarrier frequency is lower on TV (same as the horizontal deflection frequency) vs 19KHz for FM stereo. Another small difference is that the L-R channel (which is double sideband supressed carrier, like FM) has some noise reduction applied to it, but you can safely ignore that. You could take an FM stereo detector box (the sort sold as add on for FM tuners), modify the tuning of the LC circuits to reduce the resonant frequencies down to match the TV frequencies (add caps across the LC circuits to shift them down within range of the adjustment slugs). You'll need to tap the TV sound detector before the FM deemphasis circuit, usually an RC circuit, to feed to the stereo demod box.
Another approach is to use an FM stereo demod chip. You could use an old walkman set, or an old ghetto blaster circuit board pretty much intact. Disable the FM tuner and FM IF circuits. There should be a small trimpot and series resistor associated with the stereo decode chip. Also a cap going to the same pin. Add another cap in parallel to that cap, the additional cap about 20% the capacitence of that cap. This should make it possible to adjust the trimpot to lock onto the TV stereo signal.