Solution for 2270 high bias - early serial number

Discussion in 'Marantz Audio' started by hirscwi, May 9, 2017.

  1. hirscwi

    hirscwi AK Subscriber Subscriber

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    Indianapolis
    Yes, the unit in question is champaign faceplate. s/n 2801
     
  2. dgunnr

    dgunnr Marantz Owner

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    151
    Location:
    Southern NJ, USA
    Thank you and good luck getting your questions answered. Dennis
     
  3. runningdog

    runningdog AK Subscriber Subscriber

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    Sonoma, CA
    This chart helped me solve an issue with a 2245 amplifier, which is similar to the 2270. It had widely drifting bias, which started after replacing the original H760 bias transistor with a MJE243G. And bias could not be set correctly at R763. After viewing Tom's list, I decided to try the KSC3503. It's now possible to adjust the bias at R763, and it is reasonably stable now. With the MJE243G it was drifting from 2.0mV to 8mV.

    This 2245 is an early champagne face with the older style 2245/2270 amp board. Tomorrow I'll try this on the other amp and see if this fixes the same issue with it.

    Thank you Tom!
     
  4. sssboa

    sssboa Active Member

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    367
    It's becoming puzzling for me.
    Meanwhile I bought Hitachi D669 that I am pretty sure are fake. You can't see it in the pictures but they don't look or feel like NOS at all. Like knocked off yesterday rather. I have never seen such shiny and thin (flat) legs on transistors either.

    d669a.jpg d669b.jpg
     
    Last edited: May 17, 2017
  5. Trojan4Life

    Trojan4Life Active Member

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    398
    Could be fake or could be parts which failed at final test.

    You are wanting to match Vbe? You can buy a Atlas Peak DCA55 which will measure Vbe. You'll have to buy several transistors to test, or find a modern equivalent, they measure closer than the older transistors because the manufacturing tools have changed in many ways and the process has been well developed. A couple of replacement transistors have been suggested in earlier postings, as well as advising not to buy transistors off the internet because of counterfeit or known bad or fake parts.

    Alternately, you can check with a local community college and see if they have a curve tracer, learn how to use it and measure Vbe, find a friend who can loan you a curve tracer or DCA55 or meter that can measure Vbe.

    Lastly, you can make up a circuit to measure Vbe or pay Little Diode and get some genuine good parts from Jolly Old England across the pond.
     
  6. sssboa

    sssboa Active Member

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    367
    So it is the matter of setting the idle current, right? I can set with no problem the 35V primary voltage, then DC Offset fairly close to zero.

    Now the Idle Current, however I set it it goes up and up seems to never stop. Same in my 2270 with the round compensating/bias transistor 2SC984 and my other 2270 with the flat one 2SC496. Should it be 5mV for 2270 as stated in manual? I think I saw somewhere it should be 10mV rather.

    And what if it's much too high? Will it overheat the amp? Cause more distortion?
     
  7. Steven Tate

    Steven Tate AK Subscriber Subscriber

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    The setting depends on where you measure. If you measure between J754 and J760 as recommended by many, the setting is 10mV. Are you giving it lots of time to stabilize? I find that it takes 20-30 minutes to stabilize. I keep an eye on it during that time and constantly adjust it to something under 10 mV. When it rises above about 15 mV, I turn it back down to 9. Eventually, it should stabilize. If not, there is trouble somewhere. One suspect would be the bias transistor H760. For better insight into this, read this thread where expert member Ecluser has an in depth analysis. He believes the design is bad and recommends moving H760 from the main heat sink to the smaller driver heat sink, and has good theory behind his recommendation. Good reading for anyone having this problem.

    http://www.audiokarma.org/forums/index.php?threads/marantz-2270-bias-wont-go-below-15mv.322206/
     
  8. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    There are two different methods depending on your amp board and I believe it's to do with units of serial number being below 3901 or above it but I'm not quite sure of the number. Take a look at the service manual error sticky thread, I believe the info is in there.

    Overly high bias will generate excess heat due to increased current draw. I'm nmo amp guru but I don't think distortion rises but perhaps the current will rise far enough to just run away and cook the amp, taking out the outputs most likely.
     
  9. sssboa

    sssboa Active Member

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    367
    Yeah, good, J760 is easily accessible. I may have some success. But when I turn on from cold I have like 1mV and then it goes in 20 minutes up to the desired 5/10mV
     
  10. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    ^^^ That's quite normal.
     
  11. sssboa

    sssboa Active Member

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    I wonder if KSC2690 would work theoretically, Vbe aside. I don't remember why I bought these some time ago, I like it as it is plastic encased all around without collector at the back like most.
     

    Attached Files:

  12. sssboa

    sssboa Active Member

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    367
    My other 2270 with the round tube like bias/temp. compensating transistors 2SC984 goes over the target with trimming resistors turned to the limit.
    But max of 13mV when should be 10mV is no tragedy, is it? Marantz never gets hot really, if you had other brands you know what I mean.
    From my experience with other amps setting correctly the primary voltage (35V here) is most important for heat.
     
    Last edited: May 19, 2017
  13. rBuckner

    rBuckner Luv 2 Restore Subscriber

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    I don't think so, as long as it's stable.
     
  14. sssboa

    sssboa Active Member

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    367
    Yeah, I think I will stay with the original cylindrical transistors for now.
    BTW, this mounting outside the original bracket seems so lame, but hard to figure out anything else or maybe there are better ideas around

    2270biasmoddone.jpg 2270biasmoddone.jpg
     
  15. hirscwi

    hirscwi AK Subscriber Subscriber

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    Randy is correct (as is usually is), the bias (idle) current is dependent upon serial numbers.

    o s/n<3901 – J753 and J754 – set R763 for 5mV

    o s/n >3901 - J754 and J760 – set R763 for 10mV
     
  16. catrafter

    catrafter Marantz Specialist Subscriber

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    I add longer leads to the replacement bias transistor, cut the old transistor bracket and reattach it to the heat sink with the screw going through the hole of the new bias transistor. Don't forget the mica insulator and thermal compound.
    Sorry for the poor pictures.

    Tom
    2270 temp comp bracket (2).jpg 2270 temp comp bracket (3).jpg 2270 temp comp.JPG
     
    rhodus and MBuras like this.
  17. sssboa

    sssboa Active Member

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    367
    Yeah. This is good. Thx. You mean the new transistor goes between the bracket and the radiator with the screw through its hole? You don't really need to cut the bracket, do you? I reckon you cut off the part holding the cylindrical transistor, I don't really get what and why you did to the part holding the diode though :) that it looks so ... different :)

    BTW. I see these white paint marks on the trimmers for marking their position. I have them too in one 2270. I thought it had been made by a user but it seems it was in production. And in mine the marks are where the trimmers are turned to the limit for smallest voltage possible so they had to realize there was a problem with the bias being too high and never too low.
     
    Last edited: May 21, 2017
  18. catrafter

    catrafter Marantz Specialist Subscriber

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    Location:
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    Yes, the transistor goes between the bracket and the heat sink (radiator). I cut the bracket off because it is not used with the new transistor. I bend the 'tab' so that it presses the diode against the heat sink as the bracket is now the thickness of the new transistor body away from the heat sink.

    The white paint on the trimmers is to secure them from moving after they were adjusted.

    YMMV
    Tom
     
    runningdog likes this.
  19. sssboa

    sssboa Active Member

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    367
    I used Peak as well. I the picture I put the transistors in ascending order of Vbe I got. I measured 2 times. I am not sure if all the transistors here can be used anyway. My 2SD669 is from Ebay, the other transistors from Mouser and Digi-key.
    2SC2690 = 0.66V
    2N4923 = 0.68V
    MJE243 = 0.70V
    2SD669 = 0.72V
    KSC3503 = 0.74V

    transistors.jpg peak1.jpg
     
  20. iceman57ad

    iceman57ad New Member

    Messages:
    2
    Location:
    Chapel Hill NC.
    Hi my 2270 has the idle current problem. I have had this 2270 since 1978 when I bought it for $65 dollars
    From an acquaintance. Now that I have replaced the caps and some transistors including the output on 1side. With the idle current at 18.4 to 18.9 on both channels and barely knowing anything about electronics.
    I find there is no serial number plate on this unit. HOW DID I MISS THAT FOR SO LONG?
    I ask does anyone know how to tell if this is a low s/n unit or higher?
    Jerry
     

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