Does the bulb light if you select the tape input? - if not, you need to look at the socket. They often corrode to the point of not making contact any more.

If the bulb lights with other sources but not on FM, does it still play the radio in stereo (i.e do you hear any difference if you work the mono button)?

mika - I just tried another source and the stereo light came on. Relief. I then switched back to FM and as I went up and down the dial the stereo light would come on at different stations. Maybe the stereo light just needed a kick start from another source. All is better now. Thank you for the help. 

Good to hear it works 🙂

Those old things often just need to be used to stay in shape. It may have been a bad contact at the switch bank.

The reason for my latter question was that on FM, the stereo light has a separate sense/control circuit that may develop a fault even if the audio part of the stereo decoder is working. I have a 900 myself that needs to be left on a stereo station for a while before the light comes on, although it plays in stereo right away.