Bias? Marantz PM6010 KI

[robgilmo]

After reading this,

Adjusting the Bias Of Your Amp [English]

I am tempted to check my Marantz PM6010 KI , is it likely to have drifted off spec?

I wont over bias it like TNT advise.

If it has drifted is it worth changing the trim pots? Or cleaning the existing before adjustment?

I have found a service manual for the 6010OSE but not the KI, would the bias be the same on both versions?

The amp works fine but it wouldn't do any harm to check would it?

 

[Mooly]

There is no reason to suspect it should be incorrect. If you are determined to check it then the first thing to do is simply compare L and R channels. Remember that by the time you have read one and moved on to the other they will have drifted so bear that in mind. If they are similar then that's a good indicator that they are probably both correct. If the absolute values seem different to those quoted then make sure that you are replicating the factory test procedure to the letter with regard to mains voltage and temperature.

My advice... by all means check but think very very carefully before altering.

 

[robgilmo]

Thanks Mooly, it seems a fairly straight forward operation,

IDLING CURRENT ADJUSTMENT

Before switching the power ON, set the master volume control to the minimum position and the balance volume to the center positions. Also set semi-fixed resistors R755(Lch) and R756(Rch) on PCB P701 to the center positions.
Each of the cement resistors R767(Lch) and R768(Rch) on the PCB P701 is provided with three test points. Connect a digital voltmeter, set for the DC voltage input, to the test points at the two extremities of the three test points of R767 or R768.
After the setup above, switch the power ON, and adjust semi-fixed resistors R755(Lch) and R756(Rch) on PCB P701 according to the digital voltmeter reading. The target setting value is 10 mV(50 mA) for both the Lch and Rch.

http://2.bp.blogspot.com/-6C-uBVLsdcU/U9ZkYblSiQI/AAAAAAABDWs/f9pN01kqbQg/s1600/d.bmp.jpg​

http://electronicshelponline.blogspot.co.uk/2014/07/marantz-pm6010-ose-schematic-circuit.html

But if its unlikely to have drifted then thers no real point in pulling it off the rack. The last time I looked in there it was quite dusty and Ive had a dirty mode select switch in the past that needed cleaning as I was losing one channel , so after reading the article on TNT I wondered if the bias pots could also have deteriorated and thrown it off spec.​

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[Mooly]

What I'm afraid of is you thinking its drifted because you measure a different result to that quoted in the manual and then altering the pre-sets in an attempt to set it correctly.

The method in the manual is basic... it may even have been set on a distortion analyser at the factory... and the method in the manual (that you have posted) gives no indication of temperature. Is the bias checked and adjusted 'cold' or after allowing things to stabilise for say 60 minutes, or after running at say 1/3 power for 10 minutes. Also any DC offset will alter the measured result if you have speakers connected. By all means measure and learn and see what's what but don't automatically assume a problem if you measure something different. Dust on the pre-sets shouldn't be a problem because the wiper is in firm contact with the resistive track... its when you move them that its more likely to be an issue. Typically you would turn them fully one way and then the other a few times to clean the track up in combination with a cleaner (assuming the pots are 'open' and exposed. Many are enclosed and dust proof anyway.