For the Beomaster 6000 (1980's non-quad) the remote receiver is an optional module plugged into the lower front of the Beomaster
(ie. not all Beomasters have it).
It can be taken out the same way or you can open the Beomaster
for inspection and service as described in the service manual.
Check for dust buildup at the sensor, especially at the inside of the plastic IR dome, clean if necessary.
Check any and all adjustments as per the service manual, check power supplies and check for bad solders, especially around the interconnecting PCB cables/plugs.
Finally, aging capacitors can cause the strangest problems. A complete kit for the Beomaster is available.
If everything else fails I may have a replacement remote receiver module
in my little stock of good used parts but check the sensor first.
Martin