Hi Christian!
You should re-post on the Workbench or Vintage forums methinks. Many more people (including me!) are watching these!
See you there!
In the meantime, there are several things that would need attention.
On Tim Jarman's and Martin (aka Dillen) I dismantled the thing to clean everything, especially the motor pulley which will have black goo around it, hence provoking wow, and I fitted a new pinch roller. The tape path will have to be very clean too. It would be nice if you published pictures of you mech!
Tim says:
Check the brakes, there are three felt pads that act on the spool
carriers (best accessed with the cassette tray removed), they can get
gummed up with dirt and cause this sort of thing. After repair work they
can also get re-fitted wrongly, check this too. I assume that your
mechanical overhaul has included removing the spool carriers from their
shafts, cleaning away the horrible old grease and lightly re-lubricating
them? This is very important in all Beocords as stiffness here can cause
needless head wear.
Check the head azimuth and of course the Dolby level (if you are using
the NR function), if it still sounds dull there is a procedure for
checking the frequency response in the manual, it isn't easy but may be
worth doing.
Remember that Nakamichis on the whole have an incorrect frequency
response with rising treble (one of the many reasons that I don't use
them) as it sounds "good" in the showroom but makes them unsuitable for
use as a "reference" machine. The 9000 has a slighltly falling treble
characteristic too which highlights the effect hat you have observed.
The Pioneer CT-A9 and the Aiwa AD-F770 / AD-F990 both have perfectly
flat IEC frequency response characteristics when in good condition and
correct adjustment, these are far more suitable to use as reference
machines than any Nakamichi.
Also think of lubrication. The spools will have sticky grease, and the motor will certainly need the odd drop of Singer oil too!
Well that's I did on mine, and it worked nicely. The only person who can supply THE belts is Martin. All else is c**p. He is all dedicated to keeping B&O stuff alive and he's very nice and has the science and most importantly the parts you need! The pinch roller is rare, watch the Bay for an original one. I have a good spare one if you like.
Cheers,