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CR-420 with left channel distortion

Discussion in 'Yamaha' started by skidmarks70, Nov 15, 2017.

  1. Chaulk61

    Chaulk61 New Member

    Messages:
    17
    The one in my cr 420 has writing on the cover, so I was curious if the one pictured may have been a replacement.

    At any rate, I sprayed all the switches/controls that I could access and if anything I made it worse.....the distortion is now more pronounced with the tuner ( which was the one input in which the noise was previously barely audible). Unless there is something simple that I may be overlooking, perhaps it is best I take it to a tech.
     

     

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  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,841
    Location:
    Fort Dodge, Iowa
    Deoxit won't do diddly squat. The relays contacts are pitted.
     
  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,841
    Location:
    Fort Dodge, Iowa
    Probably a good idea.
     
  4. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    Good to know. I'll just order a new one then. Thanks again for all the help
     
  5. cleverdick

    cleverdick Active Member

    Messages:
    202
    Location:
    Essex, UK
    Also, check for bad solder joints, particularly around the output stages. Plus - if it's anything like the CR-800, there will be 1.25" fuses underneath which can tarnish horribly. This will cause distortion, for sure.
     
  6. Chaulk61

    Chaulk61 New Member

    Messages:
    17
    I am still fiddling with this amp....cleaned contacts etc again and I checked the dc offset and it seemed fine, but when l went to measure idling current, and get readings of 0 on both left and right. How can this be or what am I doing wrong? Red to tp1 - black to tp2 - receiver on for 10 mins, speakers off, aux selected, vol at Minimum
     

     

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  7. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,893
    Location:
    uk.. the middle bit
    meter error ?
     
  8. Chaulk61

    Chaulk61 New Member

    Messages:
    17
    Don’t think so, meter seems to work fine for dc offset measurement on the Yamaha .....checked the idling current on another amp I have open beside it and I get a reading on that.
     
  9. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,893
    Location:
    uk.. the middle bit
    both channels suggests a power supply issue ..its something in common to both channels ..get the supply checked first .
     
  10. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,841
    Location:
    Fort Dodge, Iowa
    If your getting your +/- 29 vdc rail voltage. Then check that your emitter resistors R443-446 are still serviceable.
     
  11. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,893
    Location:
    uk.. the middle bit
    did i miss something ? i guess not ..
     

     

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  12. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    Well, I'm back and still having troubles. Life got in the way of playing with this for a few months.

    Anyways, I haven't gotten a new relay yet, but I was poking around and I learned that the voltages on TR401 all tank when the distortion kicks in. They're supposed to be ~28V, but at low volume with the speakers on they'll drop to ~20V and will decrease more if I increase the volume. If I turn the volume way down or turn off the speakers they'll come back to ~28V.

    I also checked TR403 and the collector follows TR401, the base is -0.14 which is about right, but the emitter measures -0.8V when the schematic indicates +0.6V. I don't know what all this means and maybe I still need a new relay, but I thought I'd check in before ordering.

    As a side note, I bought a DS212 pocket oscilloscope and it's really fun to follow a signal through the amp!
     
  13. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    Reading JPromo's thread about a very similar issue with his right channel, I tested my output transistors and one in the left channel does appear to have a short. Smacking that transistor also seems to bring it back for a few minutes (it's also near the relay). I'm going to go ahead and order some new transistors for the left channel and probably a new relay just for completeness. I'll post again after I get the parts installed.
     
  14. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,841
    Location:
    Fort Dodge, Iowa
    Could even be a loose transistor pin socket or and cold soldered connection.
     
  15. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,841
    Location:
    Fort Dodge, Iowa
    How did you test the output ? Did you remove it to test it ?
     
  16. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    So there's some interesting results. After I pulled the transistors I tried testing them and didn't see the short. I went ahead and put in the new TIP41C/TIP42C for the left channel. It sounded great and the signal from my little oscilloscope looked great. For about 20-30 minutes. And then the TIP42C started smoking. Possibly bad R443/R445?
     

     

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  17. zaibatsu

    zaibatsu Well-Known Member

    Messages:
    597
    Where did you purchase your TIP41C / TIP42C?
    Sounds like either non-genuine parts, or if they were from a reputable supplier, potentially a thermal / bias runaway issue.

    You might have some new problems to fix if the output/s shorted.
     
  18. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    Mouser. Any thoughts on what to start testing?
     
  19. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,893
    Location:
    uk.. the middle bit
    i would first see if powering on with outputs removed can be done or if resistors needed in place of them to complete feedback circuit . then do some voltage testing to see whats going on .
     
  20. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    I've pulled the transistor and it looks like there was some arcing between the retaining screwing and the back plate. The little screw insulator just fell apart too. The base to collector is definitely shorted. Can I turn on the receiver without the transistor or would that damage something?
     

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