Probably the tape transport needs cleaning.

I can only reliably get about 4 or 5 passes of
the tape before I have to reclean the critters.

What I do is keep about 3 or 4 spare TK50 tape
transports on the shelf, precleaned and ready
to go, then if I run into trouble, I can remove
and replace the drive in about one minute or so.
Then, I can clean it at my liesure and put it
back in the ready to use pile.  I use the long
signal cables and a add-on power dongle to allow
the drive to be pulled out the front and changed
without having to pull all the panel covers off
the BA123 cabinet.

If the heads are dirty or the led control lamps
and receivers dirty, then the thing loses track
of reality and become totally confused.

Supposedly, if the front panel light blinks
rapidly, you can clear it with four on/off
presses, fairly rapidly, on the light switch.
Sometimes that does not work.  If it doesn't
you may have problems.

To clean, I pull the tape transport, remove the
tape takeup reel aluminum cover, then wad up
some soft tissue de toilette and dampen with
rubbing alcohol (isopropyl alcohol), then very
gently manually pull up on the head to get it
up far enough to swab it clean.  After than,
I poke around with a small artist's brush down
into the led/receiver lenses and clean those.
There is probably a better/more official way
to clean it, but I find the above to work well
in my hands.

I am still at a loss as to what to properly do
to rewind a tape when the transport loses all
track of where it is and won't time out or reset
so that it can be rewound.  I thought I saw a
TK50 manual online in one of the archives.
Maybe that has into on how to clear a stuck
tape and get it to rewind.

Good Luck.....

Bob