nikko alpha 230

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No bass in a power amplifier only? After first swapping channel input leads and checking upstream, I would start by looking at the faulty power amplifier's output stage by testing for failed semis - i.e. output and driver transistors. Next and more obvious problem area is the coupling and DC blocking capacitors capacitors - the smaller electrolytics.
 
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This is quite an unusual amplifier having a variable bias circuit. There is a DC/AC coupling switch located right on the inputs that switches a 2.2uF cap in circuit. Apart from that and its DC coupled all the way.

Definitely do the basic checks that Ian describes. Also check your speaker... a DC fault could have burned the woofer out leaving just mid and HF drivers intact. Unlikely perhaps but don't discount it.

Two basic checks in the power amp are measuring DC offset voltage (should be zero) and checking the bias current (idle current adjustment).
 
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