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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    The bias trim pots in my 2325 are super sensitive, to the point that achieving 15mV is impossible, even after repeated cleaning. The traces and joints look fine, so I've decided to swap in new pots, including for DC offset. What have other folks used? I'm considering the Bourns 3368 series.
     
  2. mbz

    mbz Well-Known Member

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    Not familiar with te Marantz 2325 however the replacement "must" be a multi turn type like the 3296 series. Just
    need correct orientation.
     
  3. runningdog

    runningdog AK Subscriber Subscriber

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    Some folks like the multi turn, some go for the Bourns single turns depending on the application. The 2325 uses 1Kohm pots. Here is the Bourns part number for a multi turn 1K ohm pot with the three legs offset:

    3296Y-1-102LF

    There are four suffix types: W, X, Y, and Z.

    W and X have their three legs in line, Y and Z are in a triangular pattern. The 'Y' pattern is usually the one to use on vintage Marantz amplifiers.

    W and Y have the adjuster on the top, and X and Z put it on the side. The X and Z won't work on a 2325, so you end up again with the 'Y' designation.
     
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  4. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Thanks, especially for the explanation about the leg patterns.
     
  5. Bert 1100

    Bert 1100 Well-Known Member

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    The 2325 uses bigger, wire-wound pots. I wonder if the power rating of the bourns (500mW) is appropriate ?
     
  6. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    Yeah, me too. Good point Bert!
     
  7. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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  8. catrafter

    catrafter Marantz Specialist Subscriber

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    Here's my take on this. If anyone has a better understanding of Vbe multipliers I hope they chime in, I wish I understood them better.
    I don't know what wattage the original controls are.
    Let's assume the circuit is operating normally.
    The Vbe multiplier circuit consists of H714 (connected as a diode), H715 and the bias control R740 (connected as a rheostat).
    The voltage on the collector of H715 should be about 0.6VDC, the voltage on the emitter should be about -0.6VDC with respect to ground.
    Let's assume that the Vf drop across H714 is 0.6VDC, it is probably more, but we are assuming worst case.
    This puts zero (0) VDC on the junction of H715 and the bias control R740. There is -0.6VDC on the other end of the bias rheostat.
    Let's assume that the control (1000Ω) is in the middle at 500Ω. Ohms law says Power = E(squared)/R, therefore, (0.6 times 0.6) is 0.1296/500 equals 0.259W.

    Tom
     
    Last edited: Aug 9, 2017
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  9. wlhd1610

    wlhd1610 Penny and her new friend Subscriber

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    Good job Tom !
     
  10. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    Power = E(squared)/R, therefore, (0.6 times 0.6) is 0.1296/500 equals 0.259W.

    I get .36/500 = 0.00072 in this scenario. That's on both sides of the pot though so the pot wattage = 2 * .00072 = 0.00144. I don't have the schematic here at work but isn't there 1.2V across it? That would make for 1.44/500 = 0.00288. Anyway you cut it then, the Bourns pot is fine.
     
  11. Steven Tate

    Steven Tate AK Subscriber Subscriber

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    All this math is hurting my brain. I'll put my finger on the pot and let you know if it's hot. :biggrin:
     
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  12. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Thanks guys. I appreciate the collective Marantz brain trust weighing in on this.
     
  13. catrafter

    catrafter Marantz Specialist Subscriber

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    Thanks Randy! Math is not my forte!

    Tom
     
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  14. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    The new 3296Y trimmers have been installed, but aren't responding/changing DC offset or bias when I attempt to adjust. They can only go in one way, so I know the orientation isn't wrong (unless the 3296Ys aren't the right trimmer in the first place). The only other thing I can think of is that the original trimmers have two outer legs that were soldered into the board, but the 3296Ys don't have them and so I left those spots empty. Color me puzzled.
     
  15. mbz

    mbz Well-Known Member

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    796
    Location:
    Melbourne, Victoria
    With original trimmers you got down to 15mV. If there is no response to the new trimmers there is a likelyhood that it's an installation
    issue somehow. Also no response to both DC offset and bias trimmers using same technique. Will chase up the artwork unless others
    more familiar...
     
  16. mbz

    mbz Well-Known Member

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    Service manual contains no artwork. Check trackside that there are traces going to the legs of the trimmers.
     
  17. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    I think those are 25 turn trimmers so they won't seem to do a lot. Keep cranking!
     
  18. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    I was thinking it could be that, but I turned them a fair amount and don't want to crank them too far. Should there be a light "click" felt with each full turn?
     
  19. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    Nope, just at the ends. Well not usually anyway.
     
  20. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Ok, then I've turned them through the range.
     

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