Tip & How-To about Marantz PM7200 2-Channel Amplifier
The amplifier would sometimes not turn on, when it did there was a loud click from one channel of the amplifier.
Inside the amplifier there is a protection board that plugs into the amplifier, the design uses 3 x 25V 47uF capacitors charged up by 22K and 220K resistors.
As the amplifier ages with time, a parasitic resistance across each capacitor gets less and less, this means that the capacitor can not charge fully up and reach thresholds, in the **** design of a delay circuit. The parasitic capacitance is affected by the room temperature making the fault intermittent.
The pop sound is caused by DC offset, it was 25mV, this was nulled using a trimmer on the amp pcb to stop the click-pop sound, this offset could of damaged a loudspeaker.
Posted by gary on
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