Bias Marantz PM80 MkII

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Bill71

Member
Joined 2017
#1
Hi,
I should calibrate the idling current of a Marantz PM80 MKII.
From manual service in class A should be 180 mV after 30 minutes, unfortunately, however, by turning the trimmer to the maximum I get about 140 mV on the tester.
I have clearly disconnected the load of the speakers and placed the volume at minimum and the sliders in the center position.
Would anyone know how to solve this problem?

thank you very much
 
#2
We have had this issue before there appears to have been a modification applied to these amplifiers and comes to light when you compare the MK II service manual with the MK IISE manual . If R749/750 are 12K then the class A bias setting voltage is 180mV if R749/750 are 47K then the class A setting voltage is 90mV . The class B voltage is the same for both at 18mV
 
#3
thank you very much
I checked, the resistances in the amplifier are 47K. In the Marantz PM 80 MKII manual the resistances are 12K.

The manual to refer to is Marantz PM80 MKII Special Edition so the model without tone controls?
thanks again, very competent
 
#5
Hi,
the class A amplifier does not sound better than Class AB.
I have a doubt.
Should I replace the 47k resistor with a 12k resistor?
In the diagram of the Marantz PM80 MKII service manual, R749 / 750 are 12K
Thanks for your help
 
#6
I would leave the bias setting/resistors as they are . The idle current was reduced for a reason , probably reliability . I have never been happy with the use of the class A term in association with this amplifier and see the bias switching as more a broadening of the A/B region .
 
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