I can probably find one but let's first finish the diagnose and see if your socket is really the reason
and if it can be repaired in the given case.
Switch on the Beomaster and choose Tape.
Unplug the cable and put the volume control a bit up.
Insert an unfolded paperclip into the pinholes in the tape socket, one pin at a time and listen for
a buzz from the speakers.
Two of the pins, that'll be left and right respectively, should produce a healthy buzzing noise when
you touch the paperclip.
If you have a buzz on two of the socket pins, insert the cable again. Touch the pins at the other
end of the cable with a finger (and/or the paperclip if that's easier) and listen again.
If you also get a buzzing sound from both speakers, the problem is in the tapedeck.
If you cannot get a buzz at all from one channel, no matter what, the problem is in the Beomaster
and it will need further diagnosing.
As mentioned, the socket pins can lose tension but that can usually be corrected by inserting
a tiny (watchmakers type) flat-bladed screwdriver into the socket, and gently push the bronze forks
in there closer together. You will be pushing from the outsides of the forks if you understand what I mean.
This will introduce a little more grip around the plugs pins when inserted.
The same can be done to the tapedeck.
Martin