Multi-mix 8 USB: Very old but still sort of working...

I've just pressed my old, and not much used of late, 8 into service as a USB input for recording on a mac.

3 of the 4 xlr channels produce a loud earth hum on the the left side only (headphones in mixer). This is with only one mic plugged into the good channel.

Mac (El C) finds the CODEC no problem, and apart from a quiet digital sizzle (which does not record) it seems fine, but when you go to record the track hears random digital junk at irregular intervals. I sometimes get a longe enough gap to record a track, but other times all sorts of clicks glurks shash and werble.

Is there anything easy I can do?

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