Strange loss of highs on Technics cassette deck

[benyig]

Hello,

This is very strange... When I try to play any old tape on my Technics RS-BX747 deck, it sounds fine for a few seconds then it turns dull losing almost everything above ca. 1kHz. If I stop then start the tape again it all happens exactly the same: all is fine at first then in a few minutes completely dull. It happens regardless if it's at the beginning, the middle or the end of the tape. On other decks, the tapes sound fine.

Do you have any idea what may be causing this problem? My only theory is that when the tape gets tightened after a couple of rotations it somehow moves away from the head a little, but it does not really make sense.

Thanks,
greg

 

[mech986]

Could be a lot of problems. I would first clean the heads and the tape path of any tape residue and be sure the tape path is clean, clear, and in alignment. Then I would get a decent new unrecorded tape, and record from a CD or other good source, and then play it back and see if it sounds good or just as poor.

If it sounds good, then something in the tape path is skewing the old tapes which may upset Dolby sensing or gets alignment way off with the previously recorded tracks. If it sounds just as bad, but the recording sounds good with another deck, then you may have an electronics issue.

 

[dr*audio]

The tape is skewing (riding up or down) between the pinch roller and capstan. This is usually caused by the pinch roller being dried out and needing to be replaced. If it's a dual capstan deck, the pinch roller condition is even more critical to proper tape handling. The rubber on the pinch roller should be dull black with no brown bands on it, and not shiny. If the pinch roller has a brass hub, Terry Witt can rebuild it. If it's plastic, you have to find a replacement of the same size that fits the same diameter shaft. You can try cleaning it with ethanol and treating it with Caig RBR.

 

[Rey1]

Heads may also be out of alignment. The same thing happened to me and also on a technics deck. Look at the head alignment screws below the heads. After factory alignment they dab some paint over the screw and the chassis. If the paint on the screw does not align with the paint on the chassis, then someone has messed with the alignment. If that's the case you can realign using headphones and a few good prerecorded cassettes. However, using this method re-alignment may not work as well for recording.

 

[joehughes]

I'm experiencing the exact same issue with a Technics M205 but only on a handful of tapes. Just wanted to add my experience here since this is the only description of this issue I could find. If I fix it I'll report back.