Accessing the vast knowledge source here.... Realistic Am/Fm Auto-Reverse Tape Deck The tuner and output/line level amps are ok on both channels. The cassette deck left channel is very weak-right channel OK. I adjusted the head for best freq. response but still no left channel. Auto reverse decks use 4 channel head and switch the channel pairs for rev. So, it can't be 1 dead channel in the playback head, as the left channel is dead both directions. Ditto for the phase switch. Preamp? Or am I missing something? JR Dweller in the cellar
Well ... one possibility is that the actual switch which changes head windings may be damaged. Something like a wiper contact bent, broken off, or held clear of contact by some insulating debris.
Another possibility -- especially given that it is weak, not silent, is that there is a partial short in the shielded cable from the switch to the preamp.
Of course, the preamp could have a failed transistor in it. You have obviously eliminated the final amp stage, by it having normal behavior with the tuner.
How old it it? It could be that an electrolytic capacitor has dried up and that could be causing the problem.
Or the switching from the preamp to the amp may be defective in some way.
And one final suggestion is that the heads (or the tape guides) may be dirty or worn, lifting the tape clear of the head pole pieces on the left channel (which I *think* is the topmost one, but I'm not sure any more.)
Can you try swapping head windings and seeing whether the left channel remains quiet? If there is a plug-in connector for the head, you can probably just turn it upside down to interchange the left and right heads. Obviously, the audio will be playing backwards (which is an interesting experience, *once*. :-) But it will let you test whether it is in the tape transport mechanism and head stack, or the preamp. Hmm ... maybe not, if the head is badly worn. The tape will be running like this in the worn notch:
so the center two tracks (right tracks) will be in better contact than the outer two tracks. So reversing the connector will still leave you with the same bad channels.
It may be time for a new head for the drive. How many hours of play do you have on it?