Bias and DC Offset Pots Marantz 2285B - diyAudio
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Old 8th August 2009, 11:35 PM   #1
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Default Bias and DC Offset Pots Marantz 2285B

I'm getting ready to start this one up after some repairs and want confirmation on the pot settings to ensure no blue

smoke. The schematic is here:

http://img90.imageshack.us/img90/8851/scanr.jpg

With both pots fully CW (arrow in schematic at top), I get maximum resistance for offset adjustment (R726) between Q715

and R710 and for bias adjustment (R727) between R722 and base of Q708. This is correct, right?

I have a picture of the pots themselves in this position here in case someone is familiar with how it should look, they

are fully CW as shown:

http://img4.imageshack.us/img4/1121/p1010391s.jpg

Also, the outputs have been changed to MJ21193G/94G. The original bias procedure measures across J741 and J742 for what I

believe is across 1.5ohm for a 30mV setting. This ends up 20mA. Do these settings hold for the new outputs also?
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Old 9th August 2009, 12:43 AM   #2
llwhtt is offline llwhtt  United States
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I think I would start with the offset adjustment in the middle unless you have something telling you different. You want 0 VDC on the wiper arm, leg 2. The bias should be set to max resistance or to the side that has two legs of the pot shorted together on the PCB. What does the service manual say if you have one.

Craig
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Old 9th August 2009, 02:03 AM   #3
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Quote:
to the side that has two legs of the pot shorted together on the PCB. What does the service manual say if you have one.

Craig [/B]
I believe what you are saying here is that the "arrow" (which represents the wiper, I think) in the schematic should be on the side with the two legs shorted together, or upper side for each on the schematic, which corresponds to full CW, as I've shown. This corresponds to more resistance between base and emitter, is this typical? I'll set the dc offset midway.
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Old 9th August 2009, 02:11 AM   #4
llwhtt is offline llwhtt  United States
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Do you measure 300 Ohms from end to end when set to that position, if so that should be correct. The middle of the DC offset pot will be close enough until things settle down.

Craig
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Old 9th August 2009, 02:17 AM   #5
llwhtt is offline llwhtt  United States
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Can't tell if there is an output relay or not, if there is and you had the DC offest max'd one way or the other the protect circuit might think there's a problem. Also if you have an o'scope, you should, make sure you monitor the output BEFORE the relay. This way you can see the output at a much lower voltage when powering up with a Variac instead of waiting for the relay at FULL voltage.

Craig
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Old 9th August 2009, 10:13 PM   #6
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Powering up was successful, used the dim bulb until the speaker relays kicked on and had simultaneous measurements on the DC and bias. I've got everything adjusted per the manual, the DC offset is very stable, but the bias starts at 30mV when cold then gradually drifts down to 21mV as things warm up and then stays there. The manual does not say whether to readjust the bias to 30mV at this point. All of the board-mounted heat sinks (drivers?) get pretty hot, but the outputs stay cold after 10 minutes. There is no load, of course, as per the manual, but, again, it says nothing about re-adjusting the bias after it gets warm. Obviously, if I set it to 30mV when its hot, it will be much higher when its cold, since it seems to drop precipitously during warm up. Is there any reason why I can't just live with 21mV bias after warmup, I'd probably feel a bit better about it. First ever amp rebuild here, replaced all caps and transistors.
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Old 9th August 2009, 10:17 PM   #7
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Quote:
Originally posted by DreadPirate
Is there any reason why I can't just live with 21mV bias after warmup?

I would go with that so long as there is no cross over distortion.

With my own designs I never set for a particular current but adjust until the crossover distortion goes.
Any more bias is just wasted in heat in the heatsink.
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