Help, please!

My much-loved 30 year old A60 has recently developed a fault, but having
no electronics experience, I have no idea how to diagnose it. If anyone
can at least give me a clue, and an idea of how easy it might be to
repair, I would be very grateful.

The symptoms are that the left channel, after the amp has warmed up
distorts very badly. This is only when using the loudspeakers - the
headphones output sounds fine.
The symptoms are the same from any source, using either the switched or
non-switched terminals - I've also swapped speakers to confirm that the
left speaker has not developed the fault.

Thanks in advance for any advice - I really don't want to dump this amp...

Ade.