the stereo light, should come on anyway unless you put it to mono, there are RCA jacks on the back you could use a mp3 player for tests, just put it  in tape , try replacing the bulb and see if that makes stereo light work or put a volt meter on the contacts for the bulb and ,see if it is getting a voltage ,

you could swap the wires from the preamp board to the main board this would narrow down where the fault is.

It has never come on even with ipod conneced through tape or phono.

The stereo light only comes on when tuned to an FM station that broadcasts in stereo - and that's only with a signal strength of an adequate
level to open the stereo decoder.

Check if the Beomaster is actually playing stereo (tune to a strong and clear FM station and listen for difference in the ambience of the
sound when switching to Mono and back).

Got stereo sound but no light? - Check the stereo indicator lamp, socket and leads.

Got no stereo sound? - Check your antenna and its connection to the Beomaster.
Antenna etc. all good? - Check the IF stages and the stereo decoder in the Beomaster.
A problem in the tuner frontend could also cause this, but it's rare and will usually cause a much more pronounced tuner problem.

Martin