TS115A...low output from one channel only

I have just bought a pair of (used) TS115a's and channel 1 (on one of them only) is putting out a very low signal...channel 2 is fine and the speaker works great through that channel. The low volume condition occurs with a mic direct into the socket and when hooked up to a mixer so I suspect a problem that needs repair? I'm in the UK (south east) so if you can recommend a service centre, that would be great
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