Marantz 2285b: Help with Idle Bias and DC Offset

[timtuff]

First, I would like to thank everyone on this forum. After about 10 years of suffering through the left channel fading in and out on my 2285b, I have finally worked up the courage to go through and clean all the pots following the suggestions posting here. Now both channels are working great and I have gotten rid of that scratchy sound that occurred when adjusting the volume. Without all the information available here, I never would have known what to do!

So now that I have cleaned all the switches and pots, I am looking to adjust the DC offset and the idle bias. So I got the service manual which lists the directs as follows:

1. Main Amplifier DC off-set alignment: Connect a DC voltmeter with 0.5 or 1 V range between the speaker terminals and adjust the trimming resistor R726 for zero DC output on the meter. Repeat the same procedure for the other channel.

2. Idle-current adjustment: Connect a VTVM between pin terminals J741 and J742. Next, adjust the trimming resistor R727 so the VTVM reads 30mV DC. Repeast for the other channel.

3. Check DC offset voltage aligned in the procedure 1 and readjust R726 if needed.


I hooked up my multimeter to the speaker jacks and got steady readings of 2.1 mV in one channel and 2.7 mV in the other channel. Great!

Then I hooked up my multimeter to the leads on R741 and R742 (as shown in the picture below) and get a reading of 8.2 mV. Oh dear. So before adjusting the bias pot I just wanted to double check that I am not doing anything wrong! I have nothing connected to the amplifier and the volume turned all the way down when doing the readings. Is it possible that there is a misprint in the service manual (30 mV is quite a bit away)? Am I attaching things to the wrong leads? I also haven’t changed any other components in the amplifier…is it a bad idea to try to crank up the idle bias before checking for any bad capacitors or something? (Doing that would probably be beyond my skill level though).

I have also noted that the idle bias and DC output resistor pots are mislabeled on this unit.
From: http://www.audiokarma.org/forums/showthread.php?t=238551 I take it that the idle bias are the two pots closest to the center of the board and the offset pots are the two farthest away from the center of the board.

 

[Jailtime]

R727s (bias pots) are closer to the center of the board. Offset pots (R726) are the outside ones. The pins for checking bias appear to be correct. Be careful when you adjust those trimmers, they are sensitive. And use good mini grabber leads, you don't want to short across those pins.

BTW, welcome to AK!

 

[timtuff]

R727s (bias pots) are closer to the center of the board. Offset pots (R726) are the outside ones. The pins for checking bias appear to be correct. Be careful when you adjust those trimmers, they are sensitive. And use good mini grabber leads, you don't want to short across those pins.

BTW, welcome to AK!

Thanks Jailtime! I know this is my first post, but I definitely have been lurking for the last few months - this is such a great site

OK I will be careful not to short across those pins - thanks for the warning!

So no reason not to try to crank that bias up to 30 mV?
haha maybe I am just a little nervous about killing something...

 

[Jailtime]

I wouldn't worry much. If it won't adjust to 30mV, that's when you have some problems.

J743 and 744 are for the other channel, if the SM didn't state that.

 

[1978_DREAMER]

Here's a pic of the trimmer ident's

Reading your posts, I wasn't sure if you couldn't find the R726 & R727 labels on the board and mine is apart right now so here's how they're marked;

You really helped me out with this post, my caps just showed up tonight and I was going to have the same testing to perform. Thanks! Jerry

 

[timtuff]

There they are! Thanks for pointing that out Jerry! I didn't see that at all.

Ok well I adjusted the bias trimmers up to 30 mV and it seems to be holding steady after 4 hrs, so hopefully it will stay up there over time. Sound-wise, all I have to say is wow - I knew it sounded a little muddy before, but I thought that was just because it was an old unit. It sounds much better now with a whole lot more punch! Thanks to everyone again for your help with this!

Maybe one day I will work up the courage to go through and replace the capacitors. The unit does seem to run a little warm, so maybe that will help. Anyway we will see. I don't want to learn/perfect my soldering skills on the marantz, so I will need to find something else to practice on first.

 

[Jailtime]

With bias adjusted correctly, the heatsink should be lukewarm after idling for around half an hour.

 

[timtuff]

Thanks again for the note Jailtime!

So when I hover my hand over the unit I feel this hot stream of air coming up between the P700 board and the board next to it with the relay (pretty much the right-center side of my picture above). I have let the unit idle for hours, and the black row of heat-sinks always feels cold to me. Indoor temp is about 66 F.

 

[timtuff]

Oh but the two heat sinks on the P700 board do get warm.
Also I don't feel this stream of heat around the main capacitors (i.e. the left-center side of the photo).

 

[Jailtime]

That board with the relay is the power supply board, they usually run warm. The filter caps should not get hot, that's a good thing if you don't feel heat there. Sounds to me like your receiver is OK.

I'd still consider having it restored if it's your daily driver.

 

[Pauljuhn]

What about R726 and R727 ?

I wouldn't worry much. If it won't adjust to 30mV, that's when you have some problems.

J743 and 744 are for the other channel, if the SM didn't state that.

Hi there,
Does this mean, R776 should be used for other channel?
And R777 should be used for J743/J744 ?

Thanks so much!

Paul