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NEC Amplifier Repair

Discussion in 'Electronic Repair' started by Steve Mackie, Aug 11, 2003.

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  1. Steve Mackie

    Steve Mackie Guest

    -NEC AUA-7000E Amplifier.
    -No speaker output.
    -Problem is intermittent.

    See http://www3.ns.sympatico.ca/steve.mackie/nec1.jpg and
    http://www3.ns.sympatico.ca/steve.mackie/nec2.jpg for reference pictures.

    Circled in red is a 24V relay that turns the left and right outputs to the
    speaker selector switch (circled in green). When the system is working fine
    I am reading ~26.4V to the relay coil, then the system is not working
    properly, there is no voltage present. The relay does seem to be operating
    properly. I couldn't read the ohm rating on the bobbin to see if it's
    correct, but I am getting continuity. Circled in blue is the DC input from
    the transformer circuit, I am measuring ~79V.

    The pink circle is pointing out the area where I am hearing a buzz/arcing
    sound when the system is not working properly. No buzz when everything is
    fine. None of the wires in that area seem to be shorting or bare. I haven't
    checked the blue transistor. I left the system on (while it was working
    fine) for about 10 minutes and the relay clicked off and on at the same time
    a buzz came from this area. While the system was not working properly, I
    just started tapping a few components. When I tapped the resistor (I think)
    circled in yellow the relay clicked on and stayed on. I left the amp on for
    about an hour after that and it worked fine, but the relay did click off and
    on once since then. This may have been a fluke, I will turn the amp back on
    tomorrow to see.

    Steve
     
  2. Alex

    Alex Guest

    checked for obvious things I hope, like dry joints.
     
  3. Ben

    Ben Guest

    When the amp is in fault condition, check that there isn't any DC voltage on
    the output of either channel.
     
  4. Steve Mackie

    Steve Mackie Guest

    Acutally, when the system is in fault I am getting no DC voltage at the
    either output. But when it's working properly, I'm getting ~3.2V out of the
    right channel and none out of the left.

    Steve
     
  5. David

    David Guest

    Sounds like you are looking mostly in the wrong circuit, the protect
    circuit.
    You need to troubleshoot the audio outputs as one of the channels is going
    into a fault condition, probably an excessive dc offset, which causes the
    relay to shut off.

    If you really have 3.2v DC on one of the channels even when the unit is
    working, you clearly need to troubleshoot that channel for the problem.
    Usually the protect circuit kicks on somewhere around 3 volts on many older
    amplifiers so your intermittent might just be due to the close tolerance of
    the dc already present on the one channel output. You should under proper
    operating conditions have generally less than 100mV or 0.1 volts on the
    speaker outputs. Make sure you do measure the amplifier to speaker outputs
    before the relay when it does kick off as once it is off you will not
    measure any dc on the speaker terminals.

    Since direct drive audio amplifiers are very easy to troubleshoot when you
    have two identical circuits, one that is working, you should have no trouble
    making voltage measurements and comparing between the two channels to find
    the problem.

    David
     
  6. Steve Mackie

    Steve Mackie Guest

    Makes sense. When it's not working and the relay is "off", I am reading
    between 4V and 13V in the right channel circuit going into the relay. When
    the relay clicks "on", I am reading less than 3V. When the relay clicks off,
    I don't get any voltage on the speaker terminals. Which is obvious because
    "Terminals---->Selector---->Relay----->Circuitry".

    The four transistors on the heatsink are all reading 39V in the centre (C).
    The left channel is reading about .5V on one side of the transistor (B) and
    0V on the other (E). The right channel is reading anywhere from 3V-12V on
    both sides of the transistors (B and E), depending on how long the amp has
    been turned on.

    Steve
     
  7. David

    David Guest

    A few special notes after looking again at the pictures.
    1. those output transistors are for a long time not available. The very few
    places that have them charge a LOT for them. Take extra care not to blow
    them and hope they are not bad.
    2. Those white ceramic resistors are the emitter resistors and any slight
    change in value WILL produce a DC offset. These are usually precision type
    2%, but not always. A poor solder connection will on one of these can cause
    a problem.
    3. Due to the age the electrolytic capacitors will need checked for both
    high esr and dc leakage.
    4. The multi-legged transistors are known to go leaky from age and could be
    the trouble.
    5. The easiest way to troubleshoot one of these is to remove the output
    transistors and insert 150 ohm 2 watt 1% resistors in place of the B-E for
    the transistor AND powering the unit up on a variac to limit the power
    supply voltages (works well on a stereo unit with one good channel). You
    need only make sure you have enough voltage to bias all the transistors in
    the output stage. Then you can make all voltage and current measurements
    without fear of blowing the output transistors.
    6. The Adjustment potentiometers if it has TWO per channel, one will be for
    output bias current and one will be for dc offset fine adjust usually. If
    it has ONE per channel it will be for output bias offset.
    7. Almost all Audio Amplifiers of discrete design are very similar in layout
    and function. Go find some basic amplifier schematics that are similar and
    you will have a good idea what to expect.
    http://sound.westhost.com/project03.htm

    David
     
  8. Steve Mackie

    Steve Mackie Guest

    Question: Can you check a resistor without removing from the board?
     
  9. Yes, with some qualifications. Parallel circuit resistance may make the
    reading lower, and if you're using a digital mutimeter, residual circuit
    voltage may make the reading higher, especially with resistance over about
    5-10 kOhms. I usually check higher value resistors out-of-circuit.

    Mark Z.
     
  10. Sofie

    Sofie Guest

    Steve Makie:

    Yes..... and No.
    Low ohm resistors can usually be confidently checked in-circuit, but many
    times anything higher that a couple hundred ohms may fool you and your meter
    because of the surrounding circuitry. It is a good idea to measure
    in-circuit resistors 2 times..... reversing the red and black test leads
    for the 2nd test..... if you get a big variation in readings then it would
    be best to unsolder and lift one leg of the resistor to make an accurate
    test possible.
    --
    Best Regards,
    Daniel Sofie
    Electronics Supply & Repair
     
  11. Asimov

    Asimov Guest

    to "All" (12 Aug 03 22:35:26)
    --- on the topic of "Re: NEC Amplifier Repair"

    SM> Reply-To: "Steve Mackie" <>
    SM> From: "Steve Mackie" <>

    SM> Question: Can you check a resistor without removing from the board?

    I do it all the time with an ordinary digital multi-meter. But there are
    more than one way to measure in-circuit. The simplest is with the
    circuit operating and then checking for the proper voltage drop.

    Another is to lift just 1 leg but I guess technically it is removing it
    even if just partially. Still another is to cut the resistor leg at the
    corner of the 90 degree bend but I guess this is also technically a
    little bit like removing it and it's not practical with smt's but then
    cutting a trace would do the same thing.

    One more way is to inject a very tiny known current and measure the
    resulting voltage drop with a sensitive meter. As long as no forward
    bias is applied to a semiconductor or a coil isn't in parallel shorting
    the resistor then the measurement should be pretty much correct, barring
    other resistors in the circuit. That is where theory comes in handy
    using Norton and Thevenin by shorting one end then the other, etc.

    Finally the really fancy way is to use a ground synthesizing circuit.
    Basically it applies a low impedance on either side of the resistor to
    isolate it from the circuit and then measures the floating resistance
    like an ohm meter. It's a bit cumbersome since it requires 3 or 4 wires
    and it may damage the circuit if there's too much current injected into
    it, plus the extra expense of the specialized equipment.

    There are probably many more ways to test resistors in-circuit but I'm
    hoping someone more experienced jumps in with their perspective.


    .... Real techs don't lick nine-volt batteries!
     
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