I purchased two of these mono blocs and one has a problem with distorted audio.
First you need the service manual. What kind of test equipment do you have?
Marantz MA700 Manual - Monophonic Power Amplifier - HiFi Engine
Just multimeter mate
Ok, read over the manual and get familiar with the circuit.
The first thing is (with amp unplugged) to check all the fuses.
If a fuse is bad, try another of the same rating, and if it also blows determine why it fails.
There may be bad electrolytic power supply capacitors, or shorted output transistors.
If the fuses are good, then measure the emitter resistors (six per channel, 0.27 Ohms each).
Also measure the base resistors (six per channel, 4.7 Ohms each).
If any resistors are bad, replace and check the associated transistors with the meter's diode function.
If the resistors are all good, plug in the amp, and check for output offset voltage, less than ~10mVDC.
If the output offset DC is ok, measure the power supply voltages, +/-44VDC wrt ground.
If the rail voltages are good, measure the output stage bias, 10mVDC on J798/J799 after warmup.
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