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Pioneer A400 amplifier servicing in Surrey, UK

Hello, can anyone recommend an engineer to service a Pioneer A400 amplifier in Surrey UK, and advise on likely cost? The amp drops the left channel at low volumes like when I listen to TV at night, the volume dial crackles. Also, one or two of the input channels don't work as well as the others with intermittent crackling/static noticeable and drop in volume level. Many thanks. James