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Muffled/bad sound on 1 channel of Marantz PM4400 amplifier

The right-hand channel of my Marantz PM4400 amp produces very distorted and low volume sound compared with the left hand side which seems to be ok.

Obviously I am no expert but I have tried swapping speakers, speaker cable and using different phono cable from various inputs so I think I am now fairly safe in assuming the problem lies with the amp. I took off the cover but could see only one glass fuse, it looked absolutely fine although I have not tested it (I don't know how!).

Does anyone have any idea what this could be and if so how I could repair it with my limited (i.e. practically non-existent) knowledge?

Thanks.

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Go directly to the main amp, follow the leads from the speaker terminals. And you should find something on an heatsink. If there are two transistors see if they are running hotter than the other channel. If you see any damage to them (compare with good channel) replace both!
If all you see is a black box with pins coming out of it you need to replace that. They sometimes have the letters STK on them.
Replace both types with the letters and numbers of the same type. IE 2SC 1220 or STK 589.

Posted on Feb 16, 2009

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It sounds to me that all you are getting is the pre-amp stage of the amp and nothing from the power amp stage. As it is on both channels then it is something that is common to both. This could be something to do with the power supply of the power stage. Or whatever is on the heatsink. It could be that the amp uses a single audio amp IC for both channels. If it does my bet is that it's gone. If it uses two sets of semiconductors for the right & left channel it would be highly unlikely that both channels have gone, but not impossible.

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The distortion might be caused by a resistor. The loss of volume could be the capacitors, but you could try injecting signals into the transistors to see if any has gone. They can cause the distortion and loss of signal. If you apply a soldering iron to any part it will heat up quicker, or you could also use a hairdryer for the whole thing. You can also get a freeze spray that cools things that have heat sensitive faults (available from electronic parts shops).

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