Marantz 2252B No bias - Keeps blowing C739 Capacitor

Discussion in 'Solid State' started by TubeJunkie, May 14, 2018.

  1. TubeJunkie

    TubeJunkie AK Subscriber Subscriber

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    I have a Marantz 2252B that has no bias. It keeps blowing C739 (1000uf 16V). I've replaced it with a Nichicon 1000uf 25v, but it keeps popping. It's probably something I did. But before today, it had very low bias, but not blowing that cap. It sounded good, but not in good health with low bias. Today I changed out Q743, 744 with KSC945ybu transistors, and was careful with the E,C,B orientation. after that C739 started blowing. I had changed bias and DC trim pots already. I'm just getting a little frustrated on this one. Figured I'd ask for help, while I step away from this thing for a day. Thanks to all thats willing to jump in. LOL
     

     

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  2. gort69

    gort69 AK Subscriber Subscriber

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  3. TubeJunkie

    TubeJunkie AK Subscriber Subscriber

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    1D0DF7A0-F414-4671-9B36-A9270F408443.jpeg 8699F07F-FCA6-4C3B-8991-029E54E654AB.jpeg Hi gort69 Thanks for chiming in. I was aware of the different pin out of ksc945y. I used shrink tubing on the Base (center pin) and bent it over to make the correct connections. I rechecked my work, and it’s correct. I have now installed the two original 2sc945 transistors. Put in a new cap in c7393 (1000uf 25v). After that I found one red wire on the main big filter caps was broke off ( could’ve broke as I was disassembling for above mentioned rework) . Powered up the unit again with Dim Bulb Tester. As I was checking bias and DC offset, C739 (1000uf 25v) popped again.

    I’m suspecting my attempted repair of some bad solder pads and circuit traces. See attached pics. These traces are part of the bias and DC circuit. I know it looks butchered, but it was kinda like this when I got this 2252b. I checked continuity, and all seems good, but maybe I’ve made a wrong connection?
     
  4. Binkman

    Binkman Addicted Member

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    perhaps suck off some of those previous solder blobs, could be cold joint one somewhere.
     
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  5. D_Zoot

    D_Zoot Member

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    As far as C739 goes, that cap is part of a small DC power circuit that supplies DC to the "Stereo" lamp, If I'm looking at the right cap anyway. It's not much of a circuit, diode, resistor and that cap. If the cap is popping, either it's in backwards or it's seeing AC. Check the direction of the cap, the silkscreened + mark may be wrong, that isn't unheard of. The positive side of the cap should be connected to resistor R789. Also check diode on the other side of the resistor, if the diode is shorted the cap will be seeing AC and that would make it pop.
     
  6. D_Zoot

    D_Zoot Member

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    Also, if I'm looking at the right schematic, Q743 and Q744 are in the protection circuit, and wouldn't be related to a bias or offset issue. How are you checking bias and what makes you suspect a bias problem?
     

     

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  7. TubeJunkie

    TubeJunkie AK Subscriber Subscriber

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    Thanks for the info guys. I checked the polarity of C739. I had it installed correctly, with positive side connected to R789 (which is just a jumper wire). I also pulled that diode (Q749) Original was a W06B, which I had previously replaced with a 1N4004, and it tests good. I have previously had this unit powered on with no problem blowing C739. It just started this.

    This unit has had low bias since day one. I’m checking bias per the service manual. Meter set to DC mv, positive meter probe to outside leg of R763 & negative probe to outside leg of R765.
     
  8. mbz

    mbz AK Subscriber Subscriber

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    The pix in post 3 is not real clear, difficult to locate the area in the sm.
    The pads in the black circle look like a solder bridge, the other ends appears to go off to a resistor, diode,,, so shorting this out does not make sense.
    Maybe add some component numbers ie, 1 or 2 transistors or are they trimmers??

    I'd also remove the solder and check the tracks, the one marked in red does not appear great.
    Bridge.JPG
     
  9. TubeJunkie

    TubeJunkie AK Subscriber Subscriber

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    Thanks mbz for your input. I removed the solder in the pics, and compared the traces to my other good working 2252b, and that solder bridge is replacing a damaged trace. So, it's correct. I also checked continuity. That solder bridge goes from Q722 to R712 (DC trim pot). As bad as it looks, it's correct. I'm glad I have a second 2252b with nice boards to compare to. So, I'm still stumped. Going by the SM guide for adjusting bias, I get max 5.2 MV DC, when adjusting R731, and 1.5 MV DC max when adjusting R732.

    As for C739 (1000uf 16v) that keeps popping, it's still out of circuit (waiting on more to arrive in the mail), the stereo lamp still works. Does that makes sense to you? Thanks again for all your help and for putting up with me. LOL
     
  10. mbz

    mbz AK Subscriber Subscriber

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    C739 acts as a smoothing cap. With it removed you will be getting half wave rectified voltage, the lamps etc will still work but they will be receiving
    "pulses" which will not be good long term. Something is not right on the power supply side, suggest measure dc voltage between pins J713(black)
    and J721(red probe), this will measure the voltage the cap is seeing. Also suggest measuring the transformer secondary AC voltage. Measure AC
    between pins J712 and J713.
     
  11. mbz

    mbz AK Subscriber Subscriber

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    Regarding the bias issue, suggest setting the power supply voltage
    first, ie, chapter 7, setting to 35Vdc adjusting R787. Likewise
    adjust the dc offset first (R711&712 say 0-10mV ideally).

    Does the amp respond to bias adjustment, ie does adjusting R731
    and R732 cause a change in the bias voltage (mV's) or no response.
     

     

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  12. petehall347

    petehall347 the brandy coffee man Subscriber

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    q749 correct way round ? what voltage at its cathode ?
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

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    35v is set measuring voltage between j722 and j728 . and turning r787 .. thought i would mention in case manual says otherwise .
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

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    switch to mono for now . it should take the load off c739 etc .. you could also remove f702 for more troubleshooting . doing that will switch off all lamps
     
  15. TubeJunkie

    TubeJunkie AK Subscriber Subscriber

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    dc voltage between pins J713 and J721 is 4.62 v.
    AC voltage between pins J12 and J713 is 8.33 v.

    Re-check/adjusted the power supply for 35.00 vdc - It holds 35vdc nicely. No fluctuation.
    Rechecked/adjusted DC for both channels to 0.00 mvDC. Bold holds nicely. No fluctuation.

    Both bias trim pots (R731 & R732) do respond to turning each. Both are new Bourns 1 kohm multi turn pots.

    The cathode (silver banded end) of Q749 (1N4004) is connected to R789 (0 ohm jumper wire)

    The voltage at the cathode of Q749 is 4.60 vdc

    Thanks again mbz
     
  16. petehall347

    petehall347 the brandy coffee man Subscriber

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    kill the stereo light. turn to mono. or remove lamp and look for a short .
     

     

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  17. TubeJunkie

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    818E1349-8C65-447C-AC6D-750B883C02DF.jpeg Will do petehall347 . I also want to mention, Q750 has a C1567 transistor, TO-126 package installed (looks old & original), but the SM states it should be a 2SC1885, TO-92 package. My other 2252b has a 2SC1885 in it. Has this component been replaced at some point? Seems like I remember reading another thread, stating that user found the same deal in his 22xx. ???
     
  18. petehall347

    petehall347 the brandy coffee man Subscriber

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    so .long as 35v is good i dont see a problem there right now .
     
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  19. TubeJunkie

    TubeJunkie AK Subscriber Subscriber

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    51414630-5F77-4B00-9AF9-7AD7A763926D.jpeg While looking for the problem with C739 blowing, I thought I’d investigate the Stereo lamp itself, since it looks like the previous owner had replaced it. It looks like an LED lamp. See pic. If this was replaced with an inappropriate value, could this be causing C739 to blow? By the way, I discovered the two soldered wires could’ve have been touching, due to no shrink tubing, or even electrical tape on them. Just one plastic sleeve slid over them together. Which would definitely cause problems.
     
  20. petehall347

    petehall347 the brandy coffee man Subscriber

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    try removing it or replace with correct lamp .
     

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