Denon PMA 520 Left channel problems

[DR8907]

Hi I'm new to this type of electrical work but I have a basic knowledge of electrical repair. I have a Denon PMA 520, the left channel sounds like it isn't receiving amplification.. the right sounds perfect. Where should I start looking for problems at? Thanks.

[vtech]

Have you done any of the basics such as swapping speakers & trying different sources...any monitor switches etc?
Do you know about audio circuits or can you read/measure from a schematic?
Happened to be a schematic posted in the archive:
//www.eserviceinfo.com/downloadsm/40187/DENON_PMA520.html
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