Marantz 2275 DC Offset

Steven Tate

Steven Tate

CEO Flat Forehead Soc.
I have rebuilt the power supply and replaced the filter caps on my 2275. Next move is the power amps. Before jumping in, I measured DC offset and idle current on both channels. DC Offset was 185mV on left and 235 mV on right. The left channel adjusted near zero, bouncing between -0.3mV and +0.3mV. But the right channel jumps around between roughly -10mV and +10 mV. Quite a difference! Is this difference a problem? If so, where would I look on the right power amp board for a cause. Idle current set pretty close to 10mV on both channels.
Steve
 
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Standard procedure is to renew the differential input pairs with HFE matched and bonded ksc1845fbu or KSA992FBU ones.
Did you do that yet?

Bob
 
I did that on my 2325, but haven't pulled the amp boards yet on this one. I guess I will include that in my plans for these boards. Thanks!
 
First thing I thought was diff pair but Bob beat me to it! When I see bouncing dc offset I always double check that there is no cables running to the AUX input where I always do my amp setup, AUX is selected and that the volume pot is all the way down. Sometimes I've violated all 3! It could be a leaky e-cap or tantalum too. Hopefully that's it!
 
rBuckner said:
First thing I thought was diff pair but Bob beat me to it! When I see bouncing dc offset I always double check that there is no cables running to the AUX input where I always do my amp setup, AUX is selected and that the volume pot is all the way down. Sometimes I've violated all 3! It could be a leaky e-cap or tantalum too. Hopefully that's it!
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It was set to AUX with volume at zero and no input cables. So I passed that test. :D I will be pulling the right amp next and replacing the basics as well as the diff pair. I'm glad I checked it so I will have something to compare to. I have been at this point before AFTER rebuilding a board and wondering what it was like before. Lesson learned. Thanks! :thumbsup:
 
Note that one often wants a tight Vbe match on diff amps and current mirrors. Just doing an Hfe match won't get you low offsets.
 
That's as good a method as any. Otherwise, you end up connecting power supplies and such on the bench, and then fight the temperature difference problem between devices. The numbers change dramatically with temperature and IMO it's best to put a copper or aluminum clip between them for matching purposes. They should also be thermally connected in the amp, though I don't know if they are.
 
ConradH said:
That's as good a method as any. Otherwise, you end up connecting power supplies and such on the bench, and then fight the temperature difference problem between devices. The numbers change dramatically with temperature and IMO it's best to put a copper or aluminum clip between them for matching purposes. They should also be thermally connected in the amp, though I don't know if they are.
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Great. I'm going to do a little more research to try to understand this better. Thanks!
Steve
 
Just as a point of trivia/interest, take half that matching circuit, twist it around a bit, and you'll see it's nothing more than a common base amplifier, give or take a few bits. The common base configuration is used more for RF, so most aren't familiar with it.
 
In Marantz amps of this era, Hfe matching will get you close to center setting of the offset pot. In other designs Vbe matching may indeed be better.
 
I've been working on a setup, with Conrad's help, to match Vbe, but don't have it up yet. In the mean time, I rebuilt the right power amp with Leestereo's suggested parts list. I replaced C701 with a Panasonic polypropylene, all of the electrolytics, a couple of the ceramic caps with C0G types, the diff pair with KSC1845FTA's Hfe matched exactly, and replaced H703 with a KSA992. I still have right channel DC Offset all over the map, roughly -30mV to +30 mV same as before. Left side settles in at about 1 mV. Any ideas on where to look next?:dunno: Sound is reasonably good, so no major distortion or anything, but this clearly isn't right.
Steve
 
Hopefully I'm not repeating someone else here but maybe try this. Insert a cable or better yet a RCA plug (even better if it's shorted out) into the offending channel's Main In jack to isolate the preamp output from affecting the readings. Maybe the preamp is jumping around?

P.S. A cable could run from the Main In to the other channel's Preamp Out to have a terminated connection too that is known stable.
 
rBuckner said:
Hopefully I'm not repeating someone else here but maybe try this. Insert a cable or better yet a RCA plug (even better if it's shorted out) into the offending channel's Main In jack to isolate the preamp output from affecting the readings. Maybe the preamp is jumping around?

P.S. A cable could run from the Main In to the other channel's Preamp Out to have a terminated connection too that is known stable.
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Bingo! I connected the DMM to the right speaker terminals and watched the DC Offset jump around all over the place. Turned the unit off and plugged an RCA cable into left pre out an right main in. Turned the unit on and it was barely moving off of 2.5 mV. Easily adjusted to about .5mV and it moved a tenth or two either side of that over time. Turned it off, unplugged the RCA cable, and it was stable! Dirty contact in the pre-mainswitch? I'm going to Deoxit the pre out/main in jacks and see how it does over time. Thanks, rBuckner, for a great tip!:thumbsup:
 
Woot!

Actually I was thinking the preamp may have been wafting about but hey, why argue with success?!
 
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