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marantz Pm7200 Phono Stage Problem
marantz Pm7200 Phono Stage Problem
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Old 20th August 2019, 08:32 AM   #1
reddish75 is offline reddish75  United Kingdom
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Default marantz Pm7200 Phono Stage Problem

Hi,

My Marantz PM7200 phonostage has half a volt difference at the amplifier outputs. This doesn't surprise me as the amplifier seems poorly put together but I would like to try and solve this.

The output looks nice and clean on both channels and voltage supply is spot on for both channels but am stuck as to where to look next?

Thanks
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Old 20th August 2019, 08:45 AM   #2
steveu is offline steveu  United States
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Assuming you mean the voltage at pins 1 and pin 7 of the NJM2114 are 0.5V different, it's probably poor JFET matching and not worth messing with. Note the 10uF/16V caps are there to remove any such offset.
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Old 20th August 2019, 10:37 AM   #3
poetry2me is offline poetry2me
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I looked up the datasheet of NJM2114, a bipolar OpAmp.


It is low noise and fast, but has significant allowable Input Offset Voltage (typical 0.2mV max 3mV), which, when amplified in a phono equalizer (very high DC amplification), can turn into 0.5V easily.


I agree, leave it as it is, as long as you have a coupling capacitor at the output.


Some people even want the offset to be at least some hundred mV in order to have the capacitors biased, for lower distortion. Just check if cap polarity is correct and turn around if necessary.



To be safe not to reopen the device too soon, you may want to recap and maybe add film caps in parallel to the electrolytics.


- Johannes
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Old 20th August 2019, 11:27 AM   #4
reddish75 is offline reddish75  United Kingdom
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Quote:
Originally Posted by steveu View Post
Assuming you mean the voltage at pins 1 and pin 7 of the NJM2114 are 0.5V different, it's probably poor JFET matching and not worth messing with. Note the 10uF/16V caps are there to remove any such offset.
Its the output of the power amplifier that is 0.5v different.
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Old 20th August 2019, 12:22 PM   #5
Boydk is offline Boydk  Denmark
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Thatīs how I read too, nothing wit DC diff. but one channel is o,5 volt (ac) louder than the other. You need an oscilloscope, and then feed the input with a mono signal. Then start measuring both channels in the phono amp from input to output to see, where the difference occurs. My best guess would be 1 of 3: One of the input fetīs is bad, one half of the opamp is bad, or one of the output caps (10uF/16v) has lost itīs capacitance resulting in deep bass missing in one channel.
Keep us updated
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Old 20th August 2019, 12:34 PM   #6
reddish75 is offline reddish75  United Kingdom
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Caps both replaced as I had those, no difference, will order a new lamp next as the fets are 369's
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Old 20th August 2019, 12:40 PM   #7
Boydk is offline Boydk  Denmark
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Not a bad price, if theyīre genuine:

2SK369 2SK369BL Genuine Toshiba Audiio JFET @ AmpsLab
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Old 20th August 2019, 12:42 PM   #8
Boydk is offline Boydk  Denmark
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And this is a pretty reliable seller from Ireland (Iīve bought stuff there)

Semiconductor: 2SK369BL (2SK 369BL) - N-CHNL/J-FET / 40V / 10mA...
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Old 20th August 2019, 01:10 PM   #9
madis64 is online now madis64  Estonia
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Quote:
Originally Posted by reddish75 View Post
Its the output of the power amplifier that is 0.5v different.
If the DC is in the output of the power amp (on speaker connectors) then whatever goes on in phono stage does not matter (as long as the signal is treated correctly) - the power amp stage has a capacitor on its signal input

PM-7200 power stage input.jpg

so whatever goes on before that is irrelevant regarding the current problem.

Last edited by madis64; 20th August 2019 at 01:17 PM.
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Old 20th August 2019, 01:39 PM   #10
Mooly is offline Mooly  United Kingdom
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marantz Pm7200 Phono Stage Problem
I must admit I read this when it was first posted and assumed it was an issue with DC offset of the phono stage... however

How are you testing and comparing the gain of the phono section? My first thought is that if you are feeding a test signal into the inputs then you may be feeding to much signal into the stage. That would require you to set the volume at an artificially low level and that is precisely where channel imbalances in pots are at their worst.
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