Marantz dedicated amplifier squeals
I have an old Marantz dedicated amplifier model
PM 750 DC it says "console stereo amplifier" on the front. its pretty rare, I went looking at the marantz website and it isnt even listed, on the front the output display has two banks of LEDs that form a V pattern and a green power power LED that is at the bottom of the V. one half of the v is right channel, so on, anyways when you power it on the the right channel is emitting a high pitched squeal and the display shows full power except for the last LED is out, even though volume is at 0. the left channel is fine, Im running both speakers on mono off of the left now. I looked inside, nothing is burnt. can someone pinpoint where the problem is inside on the board? where do you send these old marantz amps for service? thanks
Jmeredith
PM 750 DC it says "console stereo amplifier" on the front. its pretty rare, I went looking at the marantz website and it isnt even listed, on the front the output display has two banks of LEDs that form a V pattern and a green power power LED that is at the bottom of the V. one half of the v is right channel, so on, anyways when you power it on the the right channel is emitting a high pitched squeal and the display shows full power except for the last LED is out, even though volume is at 0. the left channel is fine, Im running both speakers on mono off of the left now. I looked inside, nothing is burnt. can someone pinpoint where the problem is inside on the board? where do you send these old marantz amps for service? thanks
Jmeredith
Audio Players & Recorders - Marantz - PM7200 2-Channel Amplifier
Answers & Comments
Amplifiers have multiple stages. From your description there is a problem after the volume control and before the final stage of the amp that is causing oscillation. The typical cause of this is a combination of defective coupling capacitirs (parts that connect the stages) and leaky transistors in the feedback loops. This unit will require bench time to determine the exact cause. Typical parts cost for this type of problem is less than $10. Any shop that has experience with older audio gear should be able to resolve this relatively quickly. Where are you located?
Dan
The symptom sounds like a feeback loop. As long as that indicator is showing near maximum output on one channel AVOID ATTACHING ANY SPEAKERS TO IT!!!! You'll cook the tweeters.
Turn it off and manually operate every control, knob and pushbutton throughout its range many times to wipe oxides away from their conductors.
Set all EQ controls to their midpoints and re-evaluate.
We don't know if it's an input or output problem yet. Does it do that with all inputs?
So what was the solution?