[caveat: I do not own one, but I believe:] Give up on the bi-directional tape-to-tape dubbing aspect by unplugging an external tape deck and plugging in a DIN-to-4xRCA cable. Then connect the output pair of RCA plugs to a 2xRCA-to-3.5mm stereo cable, because 3.5mm plug (male) or 3.5mm jack (female) is what most inexpensive Bluetooth transmitters use. Connect one of the many, many, many available BT transmitter boxes. Use its app or manual pairing (if available) to connect your Bluetooth wireless speaker or headphones.
If you want the second deck back — for playback only — get a custom 2xRCA female -to- female DIN cable to take the output of the 2nd tape deck back to the input pair on the receiver. Or there may be some fancy bidirectional BT transmitter-*And*-receiver that could take BT input to the Beomaster, better than that second tape deck unless you’re trying to maintain historical accuracy.
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