Annied, just a thought, but are you sure all your ancient cassette tapes were recorded from stereo originals? If any of the original inputs were mono that could account for the one channel only output.
Thanks for the suggestion, but no, that's not it. It seems to have something to do with temperature as I've now discovered, quite by chance, that if I leave the combi unit switched on for a couple of hours before I start using it, I seem to get sound from both speakers. It also occurred to me that if I hooked up my old Technics cassette deck, I'd get much better quality recordings than by using the cassette deck in the combi unit. So now I have a cassette deck plugged into the combi unit, which in turn is linked to my good speakers and I'm getting much better results, even if I do have a binful of rejected CDs. If I could just get the record player to play at the right speed, I'd be firing on all cylinders.
This past few days I've been having a whale of a time resurrecting old tapes. My taste in music is quite varied, so I've been changing from one genre to another. About the only thing the recordings have in common is that I can't keep still to any of them. "Showboat" had me dancing round the room, I was chopping onions in time to opera, ironing in time to Rod McKuen and a German singer/songwriter and tap dancing to pop music. Luckily neither my house here nor the flat in Germany are overlooked!