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2325 p700 board issues

Discussion in 'Marantz Audio' started by oldguy68, Jan 9, 2018.

  1. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    Just going through a 2325 p700 board checking resistors and found that R739 on both boards is 10 ohm. The schematic is 4.7 ohm. Not sure if this was factory or second party add. Will this cause dc offset or bias problems?
     

     

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  2. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,119
    Location:
    uk.. the middle bit
    if bias is ok i would leave as is
     
  3. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    I have gone through the p700 and bias will not set.
    I have replaced transistors, caps, resistors, pots as per suggestions from AK site.
    DC offset is zero. Bias is between .1 and .2 mv and will not adjust to 15mv.
    Stumped.
     
  4. quaddriver

    quaddriver 120 What's per channel Subscriber

    .1 or .2mv is .0001 or .0002 on the DMM. meaning something is open and that is reading leakage. did you mean to type that as typed?
     
  5. Steven Tate

    Steven Tate AK Subscriber Subscriber

    Messages:
    2,955
    Location:
    N. Richland Hills, TX
    If turning the trimmer causes little or no change, be sure the test points are correct. I'm not aware of any bias service manual errors on the 2325 like we see in other units. But no change with trimmer adjustment always makes me double check test points. I've had it wrong more than once.
     
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  6. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    I am testing at J702 and J708. The pot does not change readings when adjusted. It is a bourns multi turn pot.
    I am rechecking components again. Voltages at test points are within spec except for J702 and J708.
    Removed the pot and it meters ok.
     

     

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  7. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    I put the old pot back in and can adjust to 1.7mv. DC offset is at 0.
    Voltage readings at
    J702 110.3 ??
    J703 -60.8
    J704 -60.8
    J705 -60.7
    J706 +60.7
    J708 +111.1 ??
    J709 +60.8
    All readings done with outputs removed.
    Any ideas?

    Bias on left channel is 15mv
    Dc offset is 0
     
  8. quaddriver

    quaddriver 120 What's per channel Subscriber

    it works with old junky pot, but not with multi turn...toss ole mr meter on both pots and report back how they behave

    if you didnt do the work, are all the OTHER resistors on the bias circuit the same as spec in the SCM?
     
  9. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    The pots test good off the board the board.
    R743 and 744 are 1.5 on the schematic but are actually 3.3ohm.
    Saw this same thing on another post.
    I changed the 10 ohm R739 to the 4.7 as listed on the Schematic to try it.
    All of the other resistors are as per schematic
     
  10. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    After checking again I put it all together fired it. Has been going for 12 hours with no issue. Sounds great.
    I will check bias and offset again tonight.
    This is my first time working on a marantz. Not being a skilled technician the AK Marantz forum was a great help.
    Also Blue Glow you tube video's were a big help
     
  11. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    Still puzzled as the bias is at 1mv. I have changed all the transistors, resistors, and electrolytic and film caps as well as new outputs. Also tried different pots.
    I just can't see what the problem is. Will operation at 1mv on bias for right channel have any detrimental effect? Any input would be very appreciated!
     

     

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  12. rcs16

    rcs16 Super Member

    Messages:
    4,071
    Location:
    Baldwin, Ontario, Canada
    Do you have original bjts installed?
    Looks like you need some fine tuning in the bias generator ckt (H715) to increase the voltage drop across H706,707 collectors. The Vbe multiplier voltage drop is controlled by the current generated by the bias network connected to H706,707. Changing any comps will affect it.
    See if changing R470 to a 2K multiturn pot will provide enough bias to meet spec, you want around ~30mA of bias in the o/p bjts. you have 0.22ohm emitter R's so that is 6.6mV across each R or 13.2mV across both, spec is 15mV as you mentioned.
    Make sure the bias pot is at minimum setting to start with and always a good idea to power up on dbt when you are doing this fiddling until you get it where you want.
     
  13. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    Back at it again.
    I installed a 2k Bourns 1w pot. No voltage reading across emitter resistors.
    DC offset is at zero. Bias is at 1.2 mv. Turning the pot from left to right gives me a loss in bias that I have. The emitters test ok.
    I am using 2n4923 for H714/715.
    H708 ksa940
    H709 ksc 2023
    H706 ksa1220 legs crossed
    H707 ksc2690 legs crossed
    H704 ksa1013
    H705 ksc2383
    H701/702 ksa992 hfe matched
    H703 ksa992 hfe matched to 702/702
    H720/721 changed to 1n4148
    H718/719 changed to 1n4148
    H716/717 changed to 1n4747
    All resistors done except r734/735 thermistors
    I am not sure what to do next.
    What next??
     
  14. OFrank

    OFrank New Member

    Messages:
    30
    Hello Oldguy68, I was wondering if you had ever resolved your bias problem? The max I get on the right channel is about 3mv, the left is set correctly.
     
  15. oldguy68

    oldguy68 New Member

    Messages:
    29
    Location:
    Alberta
    I was able to get an adjustment of about 11mv. Felt it was close enough and it has been singing seamlessly ever since. The multi turn bournes trimmers or the originals made no difference. In the end I left the old trimmers in for the bias.
    I did manage to get some original transistors for H706/H707 H708/H709 and H714/ H715. Unfortunately I got the last of the stock that he had but I feel that this helped with the settings.
     

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