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Kenwood Receiver troubleshooting

Discussion in 'Electronic Repair' started by [email protected], Mar 4, 2008.

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  1. Guest

    Hi all. Im a newbie to this particular group, one can usually find me
    on the Rec.Games.Pinball forums, but I happen to have a querry that
    seems would be better suited to this group for help.

    I have an older but still beloved Kenwood receiver, (I think it's a KR-
    V8070) Early 5.1 surround sound, etc. I had stored it for a while,
    but was using it to supply a low level signal for some woofer
    repairs.

    This particular unit actually has a switch on the back that allows for
    changing the stability of the output depending on what the impedance
    load of the speakers being used are. I thought I had switched it to
    4Ohms so that the individual 4Ohm woofer I was repairing wouldn't do
    any damage while I was powering it.

    However, it seems I didn't have it switched correctly, and as such,
    upon turning up the volume a bit, I ended up blowing the Amp. It
    immediately went into "standby" mode, and now no longer operates
    properly.

    I waited 15 minutes, and retried it, but the receiver is stuck in
    standby permanently.

    I have plenty of skill in electronics repair, and boardwork, (as I
    generally fix Solid State Pinball and arcade stuff) but when it comes
    to home audio, Im in the dark as to where to look for troubleshooting
    suggestions.

    The repair shop looked at it, said it was fixable, but they want $150
    to repair it, which isn't worth it to me.

    I'd just like to know what to look for, how to diagnose or
    troubleshoot this issue and maybe get the part(s) needed to repair it,
    and give it a shot.

    Can someone point this newbie in the right direction?
    Thanks
    Vinny
     
  2. jakdedert

    jakdedert Guest

    Obviously the problem is in the output section; the first place to look.
    Less likely is that the protection circuitry has gone awry. For
    either you should get a schematic; but some initial observation is
    helpful. Does the unit use discrete output devices (transistors), or
    does it use 'chip' amplifiers?

    If the chips amps, it's likely one has blown. Replace it and you're
    likely finished. It may have multiple identical chips you can sub to
    the affected channels (like from the the surround circuitry) for
    diagnosis purposes.

    If discrete, it gets a little more difficult. It's possible that the
    overload took out something further up the line, ie the driver stages.
    Still, you might get by testing the output devices and replacing if
    defective. Some testing/adjusting is in order if that is the case, even
    after the amp is working again.

    In either case, since this is a surround amp, you probably have working
    channels--if you bypass the 'protect' circuit--which you can use to
    compare your findings.

    Check the Repair FAQ for more detailed information. Post any specific
    findings here; and people with infinitely more experience than I are
    likely to give more detailed help.

    jak
     
  3. Guest

    Vinny
    I obviously know the difference between a transistor and an IC chip,
    but I wouldn't know where inside to verify what's what. Can you give
    me a suggestion on where to find a set of schematics so I could start
    to try and disseminate what's what inside?
    Ok, I follow this logic, (no pun intended of course) but again, not
    knowing what's what, I won't know what to try and substitute yet
    Again, not knowing what makes up the protection circuit, I wouldn't
    even know the first way to go about bypassing it for testing
    purposes. Again, I guess Im gonna need a set of schematics first.
    Be glad to, could you tell me where to find the FAQ here?
    Thanks for the suggestions Jak, I'll try to report back as soon as I
    have a chance to go further with the diagnosis on this.
    Vinny
     
  4. whit3rd

    whit3rd Guest

    It's not THAT hard. The protection circuit detects DC on the outputs,
    so you can look at the outputs with a voltmeter and it should be
    obvious at
    power-up which is the clunker. Sneak up the power with a variable
    transformer if it's too quick to shut down otherwise.

    The output stage is just an NPN / PNP power transistor pair, with the
    emitters connected (through a high current resistor) to the output.
    Trace the wire to find the transistors, find the one that has shorted
    (usually E-C diode check will show the short). Sometimes it's the
    driver stage, too...
     
  5. jakdedert

    jakdedert Guest

    Okay, sorry that went over your head. They're going to be the biggest
    semiconductors on the board...either amp modules or (I think) TO-220
    transistor packages. I'd have to look into my similar vintage Kenwood
    receiver...I think it's a discrete output (separate components).

    Start your education here: <http://www.repairfaq.org/>

    Check back. This is almost definitely fixable, although it might
    stretch you some. Your background should be adaptable to the task.

    jak
     
  6. Guest

    First, thanks for the suggestions. But what may not be "THAT hard" to
    you, Im afraid, is at the moment, a bit lost on me. I don't have a
    background in electronics.. But I do have a working knowledge and a
    fairly competent understanding of circuits and basic solid state
    components and I can work my way around a soldering/desoldering
    station and ohm meter.

    If I asked you to fix a 1992 WPC Williams Driver board based solely
    on a bad General Illumination circuit... what would you say to me?

    Well if you are familiar with the architecture of the boardset, and
    the circuits employed in running that then Im sure it'd be easy for
    you too. Right now Im not familar with either as far as home audio,
    and this particular receiver. Therefore Im asking for the help I
    have.

    I'll take the cover off again this weekend, and start to dig around.
    Maybe the perverbial lightbulb will turn on for me this time around.
    Thanks again for the suggestions.
    Vinny
     
  7. James Sweet

    James Sweet Guest

    If I asked you to fix a 1992 WPC Williams Driver board based solely

    While I have no experience with those, I would look over the board, follow
    the outputs back to identify the GI portion of the board, then look for the
    usual suspects such as cracked solder joints or obviously burned parts, then
    test any semiconductors with a multimeter. If it isn't raw AC from the
    transformer, then I would expect it to be a fairly standard linear power
    supply.

    The same is true with an amp, or virtually any other piece of analog
    equipment. Look it over and get a rough visual identification of the
    sections, make an educated guess as to which areas are most likely to
    contain the problem, look for obvious visual fault indications, and then
    start testing the individual components for faults. In this case, test the
    output devices, if those are not shorted, test the driver components and
    bias resistors. Usually the problem will be in one of those areas, but if it
    isn't, work your way back. If you don't find anything shorted, apply power
    and verify that the supply rails are present and that the voltage is
    something reasonable. The schematic for any power amplifier of similar
    construction (discrete vs monolithic output stage) should be similar enough
    to give you an idea of the circuit.
     
  8. Arfa Daily

    Arfa Daily Guest

    First, thanks for the suggestions. But what may not be "THAT hard" to
    you, Im afraid, is at the moment, a bit lost on me. I don't have a
    background in electronics.. But I do have a working knowledge and a
    fairly competent understanding of circuits and basic solid state
    components and I can work my way around a soldering/desoldering
    station and ohm meter.

    If I asked you to fix a 1992 WPC Williams Driver board based solely
    on a bad General Illumination circuit... what would you say to me?

    Well if you are familiar with the architecture of the boardset, and
    the circuits employed in running that then Im sure it'd be easy for
    you too. Right now Im not familar with either as far as home audio,
    and this particular receiver. Therefore Im asking for the help I
    have.

    I'll take the cover off again this weekend, and start to dig around.
    Maybe the perverbial lightbulb will turn on for me this time around.
    Thanks again for the suggestions.
    Vinny

    I can do a Williams 7 series CPU board ... There's one in my Cosmic
    Gunfight ! d;~}

    Arfa
     
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