If you aren't getting sound from any channel on any input (including the tuner), check the tape monitor switch(es). Even if you don't use the TM switch sometimes the contacts will get dirty and cut the loop. If you do have something in the loop (tape deck, decoder, EQ, etc.) check to make sure the signal is getting through that unit.
Do you get sound from headphone jack (using headphones)? If so, this indicates a problem in the main speaker output stage.
If this receiver has 'pre-amp out/power (or main) amp in' jacks and you have something in that loop check that too. If it does have those inputs/outputs and you have 'jumpers' (metal u-shaped rods) connecting them, you can pull those out and connect the 'pre-amp out' to another amp to see if you are getting signal through. If not, that indicates a problem with the pre-amp (or at least something before the power amp). If you are getting a pre-amp signal, the next thing to check is the power amp by plugging an external device into the 'power (main) amp' in jack, BUT IT MUST BE SOMETHING WITH A VOLUME CONTROL because if the power amp is working it will be running wide open (same as turning your volume control all the way up). Whatever device you use to test the power amp, start will the volume on that device all the way down and bring it up slowly after you plug it into the Marantz. Be sure to turn the receiver off when changing connections and conducting these test. Do not pull jumpers or plugs out or put them in with the receiver on.
Good luck. Hope it is nothing $erious.
