From what you are describing, that the sound starts OK and then gets muffled after a couple seconds just on some tapes, I would first start thinking about pinch rollers or something else related to how the tape gets moved through the transport, not certainly about the PB azimuth, otherwise it should be muffled also during those first couple seconds.
The sound changing over time lets me think more about some kind of tape skewing issue or a not good/constant tape tension between the two rollers/capstans and/or a variable tape/head contact.
On which brand/model of tapes you are experiencing this problem?
In the past, i had a similar problem on a totally different deck and it turned out to be the pinch roller. And it happened mostly on those cassettes where the tape moves more freely, with less friction, and especially while playing the beginning zone of the cassette side... while it didn't happen, say, on TDK which had some more friction within the shell.
Try to make this quick test... when the sound gets muffled, put a finger on the left/supply reel hub, to apply a light/gentle pressure on it like trying to reduce a tiny bit the speed of the supply reel, and see if treble do return as they were before.
Doing this way, you are adding some more back tension and this might mean you have a pinch roller problem or some other kind of transport issue (back tension, a belt, maybe head penetration, slightly too low pinch roller pressure maybe due to hardened grease or a weakened spring, etc).
Unfortunately, this kind of test isn't too definitive in telling you exactly where is the root of your problem but it worths a try.
But i think PB azimuth alignment is less likely to be your problem there.
Maybe, someone who knows such Nakamichi decks much better than myself, might chime in with some better ideas.
The sound changing over time lets me think more about some kind of tape skewing issue or a not good/constant tape tension between the two rollers/capstans and/or a variable tape/head contact.
On which brand/model of tapes you are experiencing this problem?
In the past, i had a similar problem on a totally different deck and it turned out to be the pinch roller. And it happened mostly on those cassettes where the tape moves more freely, with less friction, and especially while playing the beginning zone of the cassette side... while it didn't happen, say, on TDK which had some more friction within the shell.
Try to make this quick test... when the sound gets muffled, put a finger on the left/supply reel hub, to apply a light/gentle pressure on it like trying to reduce a tiny bit the speed of the supply reel, and see if treble do return as they were before.
Doing this way, you are adding some more back tension and this might mean you have a pinch roller problem or some other kind of transport issue (back tension, a belt, maybe head penetration, slightly too low pinch roller pressure maybe due to hardened grease or a weakened spring, etc).
Unfortunately, this kind of test isn't too definitive in telling you exactly where is the root of your problem but it worths a try.
But i think PB azimuth alignment is less likely to be your problem there.
Maybe, someone who knows such Nakamichi decks much better than myself, might chime in with some better ideas.
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