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Technics SA5160 distortion on one channel

Discussion in 'Electronic Repair' started by Engineer, Jun 17, 2004.

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  1. Engineer

    Engineer Guest

    Greetings.
    I wonder if anyone has some insight into this problem.

    This receiver uses two STK0177 power amplifier "blocks", each with 10
    in-line connectors. The LH channel is highly distorted, the RH channel
    is perfect. The distortion is present on FM, AM and AUX (with sig,
    gen. input) so it's not the FM demodulator - it's definitely in the
    audio stages.

    The DC voltages on each of the two STK0177's power amplifier pins are
    as follows (with 115 VAC supply):

    Pin # RH channel LH channel (volts)
    1 +0.251 +0.229
    2 +0.002 +0.002
    3 +0.255 +0.229
    4 -26.90 -26.77
    5 -1.272 -1.288
    6 -28.29 -28.28
    7 +0.060 +0.20
    8 +28.23 +28.22
    9 +1.309 +1.257
    0 +26.95 +26.77

    (I hope the columns hold together!)

    I have not yet put a 'scope on each STK0177 line to watch for clipping
    or drive distortion (pin-out not known.) Nothing gets too hot and
    there is no perceptible difference in temperature between the two heat
    sinks (but they are thermally connected do some degree, so that's not
    conclusive.).

    Do people think this is a failed STK0177? Or is there another
    candidate component? Unfortunately, I do not have the schematic.
    Also, does anyone have a pointer or URL to the pinout and/or spec
    sheet for the STK0177? I did a quick Google but with no success.

    BTW, this unit is very difficult to service since all the electronics
    are on one large PCB and you can't access the bottom of this PCB
    without disconnecting just about everything and lifting it up (unlike
    other Technics models where you can take out the metal floor plate.)

    Thanks for all comments.

    Cheers,

    Roger
     
  2. I'm not at all sure the voltages are far enough off to suggest a problem
    with an output IC. Have you just tried manipulating the various controls and
    switches to eliminate them first?

    If you swap the output IC's, that would be the final test.

    That's an odd STK number though, you might have trouble finding one. I use
    East Coast Transistor parts for old STK's - they can usually suggest a good
    sub number if one exists.


    Mark Z.
     
  3. Asimov

    Asimov Guest

    "Engineer" bravely wrote to "All" (16 Jun 04 23:04:54)
    --- on the heady topic of "Technics SA5160 distortion on one channel"

    En> From: "Engineer" <>

    En> Greetings.
    En> I wonder if anyone has some insight into this problem.

    En> This receiver uses two STK0177 power amplifier "blocks", each with 10
    En> in-line connectors. The LH channel is highly distorted, the RH channel
    En> is perfect. The distortion is present on FM, AM and AUX (with sig,
    En> gen. input) so it's not the FM demodulator - it's definitely in the
    En> audio stages.

    En> The DC voltages on each of the two STK0177's power amplifier pins are
    En> as follows (with 115 VAC supply):

    En> Pin # RH channel LH channel (volts)
    En> 1 +0.251 +0.229
    En> 2 +0.002 +0.002
    En> 3 +0.255 +0.229
    En> 4 -26.90 -26.77
    En> 5 -1.272 -1.288
    En> 6 -28.29 -28.28
    En> 7 +0.060 +0.20
    En> 8 +28.23 +28.22
    En> 9 +1.309 +1.257
    En> 0 +26.95 +26.77

    En> (I hope the columns hold together!)

    En> I have not yet put a 'scope on each STK0177 line to watch for clipping
    En> or drive distortion (pin-out not known.) Nothing gets too hot and
    En> there is no perceptible difference in temperature between the two heat
    En> sinks (but they are thermally connected do some degree, so that's not
    En> conclusive.).

    En> Do people think this is a failed STK0177? Or is there another
    En> candidate component? Unfortunately, I do not have the schematic.
    En> Also, does anyone have a pointer or URL to the pinout and/or spec
    En> sheet for the STK0177? I did a quick Google but with no success.

    En> BTW, this unit is very difficult to service since all the electronics
    En> are on one large PCB and you can't access the bottom of this PCB
    En> without disconnecting just about everything and lifting it up (unlike
    En> other Technics models where you can take out the metal floor plate.)

    En> Thanks for all comments.

    En> Cheers,

    En> Roger

    With a lot of obvious distortion a scope will just confirm it but it
    could be useful to locate where it starts. If Pin 7 is the output to
    the speakers then 0.20 volts on the left channel is certainly cause
    enough for distortion. The +Vcc is also a little low compared to -Vcc.
    This unbalance indicates the IC is sourcing significant current from
    the positive supply into the speaker. Try and measure the same voltage
    without the speakers. If it goes way up then the IC could be bad.
    However, a leaky coupling cap can also unbalance the IC or high ESR
    can cause a parasitic oscillation though the latter isn't as likely.
    Good luck and be careful!

    A*s*i*m*o*v

    .... KPLA: Klingon Radio: All Klingon Opera, All The Time.
     
  4. Engineer

    Engineer Guest

    See below...

    Yes, all OK.
    Good point! But that is a VERY difficult job on this board (see below
    in my text.) I may have to do it, though.
    Thanks for that idea - will file it for future use.
     
  5. Engineer

    Engineer Guest

    See below...

    That was my plan.
    Damn, that's a typo! It's actually 0.020 VDC, a bit less than the RH
    channel.
    I don't know the pin-out but I plan to measure the voltage across the
    speaker termimals with and w/o a speaker connected, RH and LH
    channels.
    Cheers,

    Roger
     
  6. Engineer

    Engineer Guest

    (snip)

    Cheers,

    Roger
     
  7. Mr TUBEAMPS

    Mr TUBEAMPS Guest

    you can get the ic's from WES COMPONENTS
    phone - 0297979866
    or email
    thay are listed for $24.75

    john!
     
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