Issue with Sound Output

[69tele]

Morning all, have a Korg Trinity the first original version, 20 years old now would you believe !

Having problems with the output, at first the slider was very crackly, cleaned that with contact spray however now I dont get any output from Left Mono at all.

Strangely enough if I connect it in stereo, it works BUT sound only comes out one channel.

Any thoughts ? I think I read in this or another forum years ago that the connectors might need re soldering ?

thanks !