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I have a service manual for my Zenith 29JC20. I do not know if this will work in your set.
Couple of things from the 29JC20 service manual.
1. The beat between the two IF carriers (sound and video) is detected by crystal diode X2 resulting in 4.5 megacycle signal being amplified through 1/2 the 6AU8 (I do not know what tube your set uses) tube providing adequate sound level for sound take-off. The video signal is also taken from this amplifier and coupled to the sync clipper for sync separation.
2. Proper alignment of the 4.5MC intercarrier sound channel can be made using transmitted Tv station signal. However, the signal to the receiver antenna terminals must be maintained at a reduced level BELOW the limiting point of the 6BN6 Sound discriminator. This lever can easily be identified by the hiss which often accompanies the sound. various methods may be used to reduce Tv signal level; however, a step attenuator is recommended for most satisfactory results.
1. Connect the step attenuator between the antenna and the
receiver antenna terminals.
2. Tune in a Tv signal. Adjust the step attenuator until a signal
is reduced to a low level where a HISS is heard in the sound.
3. Alternately adjust T5, T7, L23 and R52 (BUZZ ADJUSTMENT) for
the best quality sound for minimum buzz.
NOTE: Any one of these adjustments my cause the hiss to disappear, necessitating a further reduction of the Tv signal. The HISS must be present for accurate sound alignment.
I would check the BUZZ Adjustment if there is one. Also check the Limiter or sound discriminator stages.
3. It says in another test to adjust the volume to near maximum. Pull the sound output tube and reinserting it into its socket while listining for a slight "click". If a click is heard, it can be assumed that the speaker and audio putput are operating normally.
Do the same for the Sound Discriminator and the sound limiter tubes. If a click is not heard then that is the bad stage.
polaraman
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