Marantz 2325 bias and DC offset trim pot replacements

Discussion in 'Marantz Audio' started by dbxdx5, Aug 8, 2017.

  1. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    There was a solder blob there, which I removed when I took out the old trimmer. That's what made me think Pin 2 connected there.
     
  2. llwhtt

    llwhtt Well-Known Member

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    Probably just excess from the wave soldering process during manufacture.
     
  3. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Makes sense. I'll fix the connection and report back. Thanks Craig.
     
  4. llwhtt

    llwhtt Well-Known Member

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    I can't hold my breath much longer!;)
     
  5. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Me either!

    But we'll both have to. I decided to order two new trimmers (still the 3296Y). I had snipped the legs of the originals after soldering them in the first time, and I felt the center one was a bit too short to work with for a decent splice to a jumper. Ideally, the uncut length of the replacements will be sufficient to just bend over and down into the solder pad for R710 (after first insulating with some heat shrink).
     
  6. llwhtt

    llwhtt Well-Known Member

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    I guess that's a good enough excuse. Those old Marantz pots sure are some oddballs, hard to decide what to replace them with and still look decent.
     
  7. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Agreed. I did some digging to try and figure who made them, but turned up zilch based on the markings "661" and what appears to be an "M". Possibly Murata?
     
    Last edited: Aug 23, 2017
  8. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Ok, so I finally have an update.

    I'll start by saying that I'm still smacking my head for not seeing that I had Pin 2 connected to ground. Sure enough when I put in another pair of trimmers and used a jumper from Pin 2 to the pad for R710, I was able to adjust DC offset without a problem. Thanks again to both Tom and Craig.

    There does, however, remain one DC offset issue that existed in the left channel before replacing the original trimmer and is still present with the new one, albeit a little less pronounced. I can adjust to around zero DC, but it won't hold steady, fluctuating by as much as 50 or 60 mV. By contrast, the right channel fluctuates by less than 1 mV. Idle current is right at 15 mV and Here's what else I've done to solve this:

    R713 - Replaced with 25 turn Bourns trimmer
    C703 - Replaced with a bipolar Nichicon ES
    C706, C703 - Replaced with low ESR Nichicon
    C710 - Replaced with a C0G type
    H701, H702, H703 - Replaced with KSA992 transistors
    H718, H719 - Replaced with 1N4148
    Resoldered a broken joint at the ground shield of the male RCA input plug. (I thought this was the culprit.)

    I'd appreciate any direction for next steps.
     
  9. llwhtt

    llwhtt Well-Known Member

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    Try shorting the amplifier input to ground, pull the pre out/main in jumpers if it has them short the main in to ground. There's some circuitry between the volume control and the main in so let's make sure where the problem is, preamp or power amp.

    Craig
     
  10. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    The 2325 has internal jumpers. If I run an RCA patch cord from left main in to right pre out and right main in to left pre out, that should also tell me if the problem is in the preamp or amp section, right? If the fluctuation remains with the left channel it's the amp board, if it moves to the right, it's the preamp.
     
  11. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    The fluctuation moved to the right channel when I cross wired the patch cord.
     
  12. Oerets

    Oerets AK Subscriber Subscriber

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    Could be from DC fluctuations being amplified from the preamp stage. Disconnect the audio input to the driver board and see if the DC settles down.



    Barney
     
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  13. mbz

    mbz Well-Known Member

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    This is a bit left field, ok to disregard. Maybe dc offset caused by one rails voltage bouncing/out of synch with the other.
    Maybe one of the zeners Z716, Z717. Maybe a resistor on the power rail causing same effect, R719, 720 OK, more interested
    in a 1-2W item...
     
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  14. llwhtt

    llwhtt Well-Known Member

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    So the problem is in the preamp, there are several ways to eliminate stages. You can try grounding inputs or outputs of the stages between the volume control and preamp out. With the volume control full CCW it still has the drift?

    Craig
     
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  15. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Yep. I've had the volume all the way down when I've been measuring the offset
     
  16. llwhtt

    llwhtt Well-Known Member

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    OK your problem "should" be between the volume control and the main out. The schematic from HiFi Engine is a real pain to follow. If you ground outputs of those stages one at a time you should be able to narrow it down. Do you have an O'scope? Probing the outputs of the stages should get you there also.

    Craig
     
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  17. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Thanks for taking a look at the schematic. I'll have to dive in and see what I find. But alas, no oscilloscope.
     
  18. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    If you recapped the tone control board, go back and look for a backwards electrolytic cap. That can cause wafting.
     
  19. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Thanks to much patient guidance from llwhtt, a.k.a. Craig, we pinpointed the problem as a bad tantalum at CE33 on the tone/preamp board.
     
  20. llwhtt

    llwhtt Well-Known Member

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    Where do I send the bill? LOL

    Craig
     
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