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Need help to identify resistor type and value on the power supply section of CR-840 receiver

Discussion in 'Yamaha' started by doitnow, Dec 6, 2018.

  1. doitnow

    doitnow Member

    Messages:
    93
    Hi all, while trying to adjust DC offset on my Yamaha CR 840 the probe slipped and followed by a small flame that took out a resistor in the power supply section. This resistor is located in between a big white TRW 390R 5W resistor and the two 4.7 uf, 50V electrolyte capacitors. I looked at the service manual from online source hoping it is listed on the manual, however, I can only narrowed it down to the 800's resistors section of the schematic but couldn't tell which one it is to determine the value and type of the said resistor.
    It is a small resistor about a 1/4 inch in size.

    All this took was about a second or two before I turned the receiver off and I have not turned it back on since then. Is there anything else that might have damaged during this mishap?

    Thank you in advance for anyone's inputs and helps to identify it.
     

     

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  2. Bob

    Bob AK Subscriber Subscriber

    Messages:
    2,384
    Location:
    West coast
    take a pix (no flash, under sunlight, close up- several if needed), and place focus
    on PCB marking/stenciling - we're looking for the component marking on the PCB,
    download service manual, they usually have a components layout and the schematic,
    post close-ups of both. (and do us a favor, save time, by circling the relevant areas).

    you would be extremely lucky if all that "broke" was the resistor. if you don't diagnose
    the problem (DBT, measure voltages as per schematic, etc) then you could start
    by replacing all the (electrically) surrounding components then retest.

    first off, buy a set of mini-clips that your probes insert into. you'll still need it for your
    original task. second, download the service manual. third pixs, fourth, post all.
     
  3. doitnow

    doitnow Member

    Messages:
    93
    Thank you Bob for the reply. Here is a picture of the burned resistor in question, the little one behind the big white TRW PW5 5W resistor . This is in the power supply section of the receiver. I had the Schematic downloaded but couldn't figure out for sure as to which resistor is the one I am looking for. It's just that I am not versed in reading schematics so any help would be greatly appreciated.

    I am crossing my fingers hoping that's the only resistor that took the hit to protect the others as the engineer intended. Yamaha CR-840 burned resistor in the power supply (1).JPG
     
  4. restorer-john

    restorer-john Addicted Member

    Messages:
    6,535
    Location:
    Australia
    It's quite possible you've smoked up something more than the resistor as it is connected directly to the pre-driver IC. Let's hope you haven't smoked that IC too.

    As for value, the same value resistor will be on the other channel's pre-driver IC, connected to the same pin number. I can't tell from your picture what pin that is (the device has 15 pins) but you will be able to see it.
     
  5. avionic

    avionic " Black Knights " Subscriber

    Messages:
    44,639
    Location:
    Fort Dodge, Iowa
    Non OEM part..
     
  6. doitnow

    doitnow Member

    Messages:
    93
    Restore-John: this resistor is in the power supply section, just right next to the pre-driver section so there's no other channel that I can refer to so please clarify, thanks.

    avionics: non OEM part for the big white resistor? I wouldn't know as I am a second owner to this receiver for about last 8-9 years and had never have it serviced since then. Should I change it out then? Thanks.
     

     

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  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
    44,639
    Location:
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    Just found a big time issue with the free HFE CR-840 service manual.It looks like its picturing the CR-820 circuit board. Its looks like the same goes for the schematic as well.
     
    Last edited: Dec 7, 2018
  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    44,639
    Location:
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    Pretty sure that they didn't use TRW. So now I wonder if its even the right part.
     
  9. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,113
    Location:
    uk.. the middle bit
    is that pin 11 it connects to ?
     
  10. avionic

    avionic " Black Knights " Subscriber

    Messages:
    44,639
    Location:
    Fort Dodge, Iowa
    Hopefully someone can do a visual.. Or someone has a good service manual.
     
  11. doitnow

    doitnow Member

    Messages:
    93
    avionic: the receiver was working fine since I own it except for some light bulbs went out when I first got it. I have just replaced all the capacitors in the power supply as a preemptive measures that recommended by many members of this site. After the capacitors replacement everything was working fine and sounded great to me. I let them worked in for a few days before I decided to adjust the DC offsets that led to this situation.

    So, was the big white resistor changed to non OEM part before it came to me? It is entirely possible.
     

     

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  12. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    2,217
    Location:
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    Here it is all you need to know :


    CR840-Resistor.JPG

    it look like a 390 ohm resistor and schematic will confirm it !

    CR840-R719-390-ohm.JPG
     
  13. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    2,217
    Location:
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    you are right the original part (R717 , 2 W resistor) has been replaced ! see my picture
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,113
    Location:
    uk.. the middle bit
  15. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
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    Location:
    France
    more recent , yes
     
  16. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,113
    Location:
    uk.. the middle bit
    still pin 11 ..
    am wondering where the short circuit physically was to burn that resistor .
     

     

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  17. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    please explain how you were doing this ? where were your probes ?
     
  18. doitnow

    doitnow Member

    Messages:
    93
    clinic-audio: is it metal film type or just carbon type 390 ohms resistor that I should replace it with and is it a fuse resistor, quarter watt.

    petehall347: I attached a picture from the back of the circuit board, the one circle in red is the resistor in question and the other is the IC701.

    Did I toast the IC701
     

    Attached Files:

  19. doitnow

    doitnow Member

    Messages:
    93
    petehall347: one probe (red or positive) to the test point (TR and TL for right and left channel) the other to the metal frame of the receiver, the multimeter was set to measure DC in 200 mv. range. Thanks.
     
  20. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,113
    Location:
    uk.. the middle bit
    now i see it as pin 10 ..
     

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