Fixing a Stereo Receiver

Discussion in 'General Electronics Chat' started by Buzzfly, Jul 19, 2009.

  1. Buzzfly

    Thread Starter New Member

    Jun 28, 2009
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    0
    I finally got around to taking my broken stereo receiver out of storage (The one from this thread) and have some pictures below to help out, I can take more if necessary, the first picture is the whole setup, the second is the problem area, or at least the area thats showing the effects. Any more help would be appreciated.

    [​IMG]


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  2. Wendy

    Moderator

    Mar 24, 2008
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    I would have kept this in the old thread, there isn't any benifit to starting a new one, and I had to access the old one to catch up.

    First, need make and model of this stereo. This will help find schematics, which are also a must.

    The beer on the other thread would be better spent for a technician friend, if such exists. Beer in the board could be a killer.

    The burned resistor is indeed an excellent clue, but without the schematic we are just waving our hands.

    Everything looks clean, which is good. I hate blowing out dust.
     
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    yes, a schematic is your first tool.

    The circuit symetry in the first photo suggests one of the stero channels feeding your output amp.

    I remember when we were young guys, if we had a blown fuse we'd wrap it with cigarette tinfoil and plug it back in. Now we've learnt that bringing power up in any circuit must be done with consideration to troubleshooting downstream sections.

    The beauty of the schematic is that you can not only logically deduct a problem, but you can plan a sectional powerup which minimizes strain to other circuit components.
     
  4. Buzzfly

    Thread Starter New Member

    Jun 28, 2009
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    0
    Heh, yea, sorry about making a new thread, I posted this at 10AM without getting any sleep the night before, wasn't really thinking. :rolleyes:

    The receiver is a Sansui R-50 and I've been looking on the web for the schematic but haven't had any luck
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    One thing that jumps out, the board has a certain symmetry. You might try to identify the burned resistor looking at it. I tried enlarging the first picture, but the detail just isn't there.

    I did a quick search, all the places I saw wanted to sell the manuals. For example, this vendor wants $15 for it.

    http://www.sansui.owner-manuals.com/R50-service-manual-SANSUI.html

    You get to download a PDF file for the privilege.
     
    Last edited: Jul 19, 2009
  6. millwood

    Guest

    I would venture a guess that the burn resistor is part of the vbe multiplier: the pot (vr07 adjusts bias), C153 is the filter cap for othe vbe multiplier, and behind that cover is your Vbe multiplier transistor.

    it is likely that the Vbe multiplier transistor burned open and forced all the current to go through the pot and the burn resistor.

    the heatsink is tiny so I doubt the receiver is any good soundwise.
     
  7. bluess57

    New Member

    Oct 18, 2009
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    I just stumbled upon this thread/forum, as I have exact same sansui receiver - same problem. I've found a schematic (.pdf file) if still anyone interested.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Take TR15 (and probably TR14) out and test.

    Good idea to also check the output Transistors, with a basic in situ test.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That looks like the power output stage(s). BTW: did you notice directly above the charred resistor there are a couple of 22 Ohm resistors that show signs of burning?

    As a blind guess, it looks like the output stage devices went south and these are bias balancing resistors that were getting too much current down the output stage when one or both of the output transistors blew.
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Tr15 and TR14(I'm really guessing the no as the board markings are obscured) are not theoutput transistors. They are the grey To122 or whatever transistors either side of the reisitors you have noted as slightly charred. The output transistors will be mounted on the heatsink.
     
  11. bluess57

    New Member

    Oct 18, 2009
    3
    0
    Here's part of the circuit, so far mines got shortcircuit across C-E on TR13, TR19, TR21
    On the other channel, someone else has previously done repairs and replaced the transistors with what appears to be the R-70 spec transistors.
    Not sure if that is good or bad? (ie. for R-50 model TR21 is a 2SA1102 for the R-70 spec its a 2SA1104)

    (BTW let me know if posting a schematic pic like this is a no-go)

    [​IMG]
     
    Last edited: Oct 20, 2009
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