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WANTED: Good Source of Electronics Theory

Discussion in 'Electronic Repair' started by EADGBE, Apr 19, 2008.

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    EADGBE Guest

    I am a home hobbyist who enjoys restoring vintage stereo equipment. I
    have a good working knowledge of electronic components and what they
    do. I have decent test equipment and good soldering skills, and most
    of the time I am able to track down and repair a particular problem.

    However, I do feel that I need to know more about the theory behind
    electronic circuits in order to improve my troubleshooting skills.

    Sometimes I find myself looking at a schematic and thinking, "Boy, I
    wish I knew more about what is supposed to be happening in this

    Can anyone point to any good books or online sources where I can learn
    more about how to analyze schematics and recognize what is supposed to
    be happening?

    CASE IN POINT: I am attempting to repair a tape deck. One channel
    (the right channel) of the headphone amplifier doesn't work. The tape
    deck has perfect output through the line out jacks.

    Here is the schematic for one of the channels of the tape deck's
    headphone amp:

    I have reproduced the schematic in my own handwriting because the
    downloaded schematic was too fuzzy to scan. I *think* I have copied
    everything correctly.

    I cannot figure out why this particular circuit isn't working
    correctly. I have checked each component--and even replaced a few
    transistors even though all transistors tested fine--but nothing is

    I have tested all of the electrolytics with my ESR meter and they all
    check out OK.

    I have checked all resistors and none of them have opened up or "gone

    Notice that there are some "typical" expected voltages indicated at
    certain points. These "typical" voltages have come right out of the
    service manual. All of the voltages marked "OK" are...well...OKAY.
    But the three voltages marked with a star (*) are NOT okay...they are
    all just a few millivolts each--around 20 to 30mV.

    All capacitors are electrolytic with the exception of the 4.7pF
    capacitor, which is ceramic.

    All resistors are 1/4 watt. If there is no "K" after the number, then
    that is the amount of actual ohms it has.

    For the record, I replaced Q204, Q206, and Q208 but still got exactly
    the same results.

    What am I overlooking?
  2. hr(bob)

    hr(bob) Guest

    Do you have an oscilloscope?? If you do, it is a simple matter of
    tracking the signal on the good side and comparing it to the "lack of
    signal" in the bad side, stage by stage.

    The training source given by the 2nd poster looks to be great!!!.

    Bob Hofmann
  3. Eeyore

    Eeyore Guest

    There's something amiss with the biasing of the output emitter-follower
    stage. The voltage on the base of Q208 should be about 1.2V less (not 0.2V
    less I think that 6.8V should be 5.8V) than the voltage on the base of
    Q207. That suggests possibly zero or very low current in Q205's collector
    circuit. Suggest you measure the voltage across the 12k connected to
    Q205's base-emitter.


    EADGBE Guest

    Could you repeat this part, please?

    There is no Q203 in this circuit.

    Thanks for your help!

    EADGBE Guest

    You are ABSOLUTELY CORRECT. I took another good, hard look at the
    service manual schematic (very fuzzy), and the base voltage of Q208 is
    indeed +5.8V.

    How did you deduce this?

    I'm not sure about the 12K resistor across Q205's base-emitter. I
    will take a voltage measurement across that resistor and post it here.

    EADGBE Guest

    FWIW, the base voltage of Q205 is +11.09V. This was measured at the
    junction of the base terminal and the 12K resistor.

    EADGBE Guest


    I'm not sure, but you might be reading my schematic incorrectly.

    There SHOULD be +7V at Q205's collector...but right now, there is only
    20 - 30mV.

    The +7V figure is what the service manual indicates is the "proper"
    voltage for Q205's collector.

    Regarding Q206, the actual voltages for the base and emitter are both
    currently reading just a few millivolts, just like the collector of

    The indicated voltages for Q206 are, once again, merely from the
    service manual, and the figure for the emitter was written
    incorrectly. The proper voltage for the emitter of Q206 is +5.8V.
  8. Eeyore

    Eeyore Guest

    Because there should be 2 Vbe's difference betwen those 2 points (to turn on the
    complementary emitter follower stage).

    That will be helpful.

  9. Eeyore

    Eeyore Guest

    That suggests a Vbe of 0.81V in which case Q205 *ought* to be well
    turned on and conducting strongly but it isn't.

    Replace Q205.

  10. Eeyore

    Eeyore Guest

    NO. Those are the service manual values. Q205's collector is actually almost at ground
    potential, i.e. not conducting.

  11. Eeyore

    Eeyore Guest

    In order to troubleshoot *discrete* transistor circuits that may be a tall
    order. They don't even teach this stuff much at college or Uni any more !

  12. e-micro

    e-micro Guest

  13. EADGBE

    EADGBE Guest

    OK, the plot thickens somewhat...

    Some of you have told me that, in my non-functioning amplifier
    circuit, the most likely culprit was Q205.

    I have replaced Q205 and the circuit STILL doesn't work. It is
    exactly the same as before.

    There is only 131.1mV on the collector of Q205, despite +11.09V on its
    base and +11.59V on its emitter.

  14. Eeyore

    Eeyore Guest

    And yet you continue to refuse to *directly* measure it's Vbe which is
    what's important.

    One other thing. Do you know how to test a transistor ?

  15. EADGBE

    EADGBE Guest

    According to the datasheet for Q205, the Vbe is:

    MINIMUM: -0.58
    TYPICAL: -0.62
    MAXIMUM: -0.68

    These figures were arrived at using the following test conditions:

    Vce = -6.0V
    Ic = -1.0mA

    As I have said, I am a hobbyist. Not sure how to do "real world"
    testing for Vbe or why it's important. I do know how to test NPN and
    PNP transistors using the diode function of a multimeter, that's it.
  16. Mr.T

    Mr.T Guest

    And from your own measurements there is only 0.50V Base-emitter, so the
    transistor is not correctly biased.
    Possibly a fault in a preceding stage, or even an O/C resistor or possibly a
    dry joint.
    Keep looking and measuring other voltages.

  17. EADGBE

    EADGBE Guest


    That was very good advice. In a way, I'm trying to do the same thing,
    but from a "functional" standpoint as well.

    In other words, I know what the expected voltages should be, but I
    can't help thinking that if I knew how this circuit actually amplified
    the signal, I would be in a better position to troubleshoot it, hence
    my original request for good sources for learning electronic theory.
    (Some of the sources that have been recommended so far seem to be
    excellent, by the way.) Where exactly does the signal go?, what
    should happen when it gets there?, etc. I believe that these are
    questions that I should be able to answer.

    So far, what I have is a collection of components that all seem to
    test perfectly OK, but don't work, for some as-yet unknown reason.
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