I have some Panasonic RQ-S models that have strange behaviour that is hearable better when tape was recorded with Dolby B (as I don't record with it those tapes are mostly bought prerecorded). It seems like the sound is fluctuating, it's volume changes to lower and higher pretty much at the same pace (like sinusoide). That's os either not a problem with no Dolby recorded tapes or it is so slight that is not annoying. I thought that it could mean two that prerecorded tapes were cheaper so their mechanics sucks, and pinch roller have to be close to perfect state to push it the way there is no problem. That could be a nice test of rollers.
As I've changed rollers for new, I will test this. Fast forward, only one tape. It seems it's gone! Will check other tape that has this problem more pronouced.
