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KR-7400 with a constant hum in both channels

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by nickhead, Dec 3, 2018.

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  1. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    I picked this up recently. Has a hum in both channels. While troubleshooting I found that if I tapped the relay the hum would go away for a few seconds. So I reflowed the relay. No change. Ordered a new relay, put him in. Still has a hum, but tapping the relay now has no effect. Any ideas where to look now?

    Nick
     

     

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  2. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    Look at components close to the relay. You can use a chop stick to tap components close by, to see which one's offending.
     
  3. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    Yeah, I've been poking and prodding. Nothing seems to get rid of it now. This will be a fun one.
     
  4. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    Any broken wires at the posts?
     
  5. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    No broken wires. I had it fixed for about 15 minutes. I reflowed most of the power supply board as some looked suspect. Fired it up and no hum. Let it run for about 15 minutes. Went upstairs, came back to it about half an hour later, turned it on and the hum is back. I may recap that board and replace the 330 ohm resistors. Kinda weird that the problem went away after working on the PS board and earlier it went away by tapping the relay.
     
  6. M Chavez

    M Chavez Active Member

    Messages:
    201
    Cold and cracked solder joints
    The Kenwood specialty
    Don't get discouraged - I am fighting a dead channel issue on my recently acquired 8100 and I too started at the relay
    It keeps moving - NOT kidding
     

     

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  7. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    And traces that like to disappear...
     
  8. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    Here's an image of my hum... Somewhat awkward looking 120Hz bugger.
     

    Attached Files:

  9. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    I think a recap of the PS is in order. It takes sooooo much longer to bug hunt these issues than it does to fix'em! :rant:
     
  10. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    I know these are famous for bad solder joints, but is it also normal for the traces to peel off the boards?
     
  11. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    Normal? From the factory? No. When reworking a board? Yeah...and it can be due to a variety of factors such as, how much heat a trace/pad was exposed to while in use, how much heat a trace/pad was exposed to when reworking the board, what environment the unit was exposed to for 40+ years, did the factory bend the traces of a component flat against the pad, making it difficult to desolder cleanly? And I'm sure there's more...
     

     

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  12. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    And I'm pretty sure the PS board is printed wrong. I recapped half of it last night, powered it on on the DBT and looks like a short. Double checked everything, all looked good, decided it was time for bed. This morning at work I'm looking at the service manual and it looks like CK4 and CK8, which I replaced yesterday, are showing on the pcb image as + to ground. I'm guessing the board and manual have that wrong. When I put in the replacement caps I used the + on the board as my guide. I'm betting when I get home tonight and turn those caps around my short goes away. I don't pick up a lot of Kenwoods, but I think this may be my last. Between the troubles with solder, traces and misprints I'm about to pull out the little bit of hair I have left!
     
  13. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    LOL! Hell, the Marantz forum has its own sticky regarding service manual screwups! Kenwood has their share as well. Here's a thread which has some PS board pics. Perhaps this might shed some light on cap orientation.
     
    nickhead likes this.
  14. slimecity

    slimecity Super Member

    Messages:
    3,069
    Location:
    New Zealand
    I think you have moved past this - but when you tapped on the old relay - you disconnected the contacts and hence the speakers cut off and the hum went away.

    If its in both channels yea I would agree that the cause is probably in the PS, particularly old reservoir caps.
     
    Last edited: Dec 5, 2018 at 7:31 PM
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  15. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    First let me take back my comment about the manual being wrong. It wasn't, those caps are on the negative voltage rails, so they should have + to ground.

    Working through the recap I've already replaced Ck4,6,7 and 8. Need to order the rest. But now I have a short somewhere that I can't find. Pins 3,7,8,9,11,12,13 and 14 are all grounded. I also reflowed most of the board. I can't see anything wrong, all my solder connections look good and I can't see any single problem that would cause all of these pins to be grounded. I'm beginning to hate this receiver.
     
  16. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    Oh. Those are all from the windings on the transformer. This just got bad.
     

     

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  17. M Chavez

    M Chavez Active Member

    Messages:
    201
    I learned early on with my handful of Kenwoods
    Flux flux flux flux and did I say FLUX
    Going and coming if you want to keep your traces intact
     
    nickhead likes this.
  18. nickhead

    nickhead Member

    Messages:
    92
    Location:
    Elizabethtown, KY
    Disconnected the transformer works fine. That's good. Still have many shorts. I removed all of the caps I put in, still shorts. I'm lost now.
     
  19. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    Have you checked the diodes in the bridge rectifier Dk1-4? And as slimecity mentioned, the main filter caps could be suspect. Right off the bat, pins 3, 8 & 12 are wired to the ground plane, so you can eliminate them as an issue. Your issue is power supply related, so at least it's narrowed down.

    Have you tried harsh language..? ;)
     
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  20. slimecity

    slimecity Super Member

    Messages:
    3,069
    Location:
    New Zealand
    There arent many things that can cause a dead short. its usually the outputs. If its not the outputs, look next at the rectifier bridge/diodes or main PS caps - its far less common but it happens.

    The only other thing I could think it a screw/wire/connection hitting the chassis where it shouldn't be doing so.
     

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