Hi Livewire,
Thanks very much indeed for your reply, very helpful indeed
Thanks for the ID on the thermistor, it was mounted to the heatsink between the two output transistors so that makes complete sense.
A quick test shows it's reading zero ohms at room temperature, so I need to get it colder and warmer and see what it's up to.
I don't suppose you have any service information relating to this radio do you?
I'm loathe to replace the output transistors (I have replacement transistors - AD161/162 - on order) to find there is another "upstream" component I also need to check/replace.
Thanks very much indeed again, it really is appreciated.
Martin