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CR-620 Stuck in Protection Mode

Discussion in 'Yamaha' started by Demiglaz, Oct 23, 2018.

  1. Demiglaz

    Demiglaz New Member

    Messages:
    17
    Gang, I'm the original owner of a CR-620. Never over-worked, and always treated well. The only maintenance performed on it during its life was to clean the pots with Deoxit in attempt to fix "scratchiness", which worked perfectly.

    Turned it on in August 2018 and, low and behold, the output relay never kicked in. Checked a lot of obvious stuff -- fuses, disconnected speakers, etc., with everything checking out. Still no relay response, so I think it's stuck in protection mode. The dial lamps and LEDs all light up. With the exceptions noted below, the entire board looks decent. Very clean, no burn spots, no bulging caps, etc. Solder junctions for the relay all look good.

    I combed the threads on AK and began to suspect TR802, which is a 2SD234 attached to a u-shaped heat sink. Upon inspection of the back of the board, I noticed that the 3 solder junctions for TR802 showed evidence of excessive heat -- degradation of the solder and some rosin surrounding the junctions. Noticed the same for C802, 1000uF 50V. So, based solely on physical evidence, I replaced TR802 with a TIP31A from Mouser, and I replaced C802 with a Nichicon PW Series, exact same values, and also from Mouser. Still no go. Relay will not kick in. I never tested the old 2SD234, but I did test C802 after it was removed, and it was definitely bad. I know, I know. Stupid to run around replacing components before they test bad. I guess I got lucky on C802.

    I absolutely adore this old CR-620, my first hi-fi component, and would love to save it. Any advice you guys could provide would be greatly appreciated. I have the full schematic (can share if need be). I'm a newbie at electronics. I have a decent multimeter and know how to use it, but I have no skill with testing components in place. I might be able to do some testing with guidance. I basically don't know where to go from here, and I don't have a trusted shop I can ask for help or repair. Again, any advice you could provide would be greatly appreciated.
     

     

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  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
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    Welcome to AK..First, you need to check for any DC voltage at the outputs of the amplifiers.
     
  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
    Location:
    Fort Dodge, Iowa
    Find R775 and 776 on the amplifier circuit. Connect your meters black test lead to the metal chassis ground. Then with your red test lead. Probe either end of both of those resistors. Ideally, there should only be near zero volts there. More than 1-2 volts or less on either resistor will trigger the protect circuit.
     
  4. Demiglaz

    Demiglaz New Member

    Messages:
    17
    Thanks for the welcome and help. Okay, will test R775 and 776. Do I test with the unit plugged in and powered on? Or, test with power off?

    Also, I can find R775 and 776 on the schematic, but not the board. How do I find on the board, please? There are some numbers on the board -- 700s -- so I think I'm looking in the right place. But lots of resistors in that area, and again, not all are numbered (just have an "R" next to them).
     
    Last edited: Oct 24, 2018
  5. Demiglaz

    Demiglaz New Member

    Messages:
    17
    UPDATE: I think I found R775 and 776. Behind the coils near the faceplate. Tested out as follows, with my multimeter set to 20V scale. Unit Powered OFF = one resistor showing 0.31 on both sides, and the other showing 0.0 on both sides. Unit Powered ON = one resistor showing 0.01 on both sides, and the other showing -1 (same resistor that read 0.0 with power off). In case it matters, I measured from the bottom side of the board at the solder points.
     
  6. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
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    Is that 1v or 1 mv. Makes a big difference!
    When making voltage measurements power always needs to be applied...
     
    Last edited: Oct 24, 2018

     

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  7. Demiglaz

    Demiglaz New Member

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    Hmmm. Forgive my ignorance, but I'm not sure about 1v versus 1mv. With Power ON, and with multimeter set at 20V, I get " 0.01 " out of both sides of one resistor, and " -1 . " out of both sides of the other. Please see photos below. In case it matters, the " -1 . " comes off the resistor on the left as you look at the unit from the front (faceplate).
     

    Attached Files:

  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
    Location:
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    Thats "over ranged" there is more than 20 volts at that test point
    [​IMG]

    Flip your range selector to the 200v range.
     
    Last edited: Oct 24, 2018
  9. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
    Location:
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    Me thinks you have a shorted output transistor or two. Or something holding the output at rail.
     
  10. avionic

    avionic " Black Knights " Subscriber

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    43,685
    Location:
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    Theres probably around 35v to 40volts there.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    Which resistor was it? The one closest to the side of the unit or closest to the center of the unit?
     

     

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  12. Demiglaz

    Demiglaz New Member

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    Okay, will test in 200v range and report back. It's the resistor closest to the side of the unit.
     
  13. Demiglaz

    Demiglaz New Member

    Messages:
    17
    Okay, with multimeter set to 200v, that resistor closest to the side of the unit comes in at 39.4v on both sides.
     
  14. avionic

    avionic " Black Knights " Subscriber

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    43,685
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    Unplug the receiver. Unscrew and remove the output transistors one at a time and test for shorted junctions.. Keep track of what transistor goes to which socket.
     
  15. avionic

    avionic " Black Knights " Subscriber

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    Do you know how to test a transistor with your meter?
     
  16. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Location:
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  17. Demiglaz

    Demiglaz New Member

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    I assume you are talking about the frame-mounted transistors on the side of the unit -- four in total? Two of which are behind angled metal plates? Please see photo below and confirm. BTW, thanks so much for the help and your patience with me. You are a saint. I found a link on AK for testing transistors out of circuit, and I think I can follow that. Never done it before, but the instructions seem pretty clear. I'll just need to find the data sheets for those transistors. Looks like each channel has two different resistors. Please confirm that I'm looking at the correct transistors to test.
     

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  18. Demiglaz

    Demiglaz New Member

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    17
    Whoops. Said resistors above when I meant transistors.
     
  19. avionic

    avionic " Black Knights " Subscriber

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    Not really necessary..See my link in post # 16
     
  20. avionic

    avionic " Black Knights " Subscriber

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    Those are the ones.
     

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