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Problem with transistors? Or something?

Discussion in 'General Electronics' started by Teemu_K, Jan 3, 2004.

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  1. Teemu_K

    Teemu_K Guest

    Transitors pass all the voltage from collector to the base and cause a
    short. Transistors are perfectly ok, they're not broken (I tested
    them). The voltage is rather high but they should handle it although
    they're (almost) working on their limits. (The amount voltage passed
    from collector to base seems to increase while the test circuits
    supply voltage is increased though.) There seems to be no faults in
    the circuit build. What could cause this?

    The circuit is a basic amplifier. Transistors collector connected
    directly to supply voltage (65 V), no resistor. Transistor used is
    BC546 complemented with BC 556 which has emitter connected to supply

    I'm amazed. What should I look for to get the circuit to operate?
    Everything tells me it's supposed to work. Supply voltages are OK
    without the transistors. With them the circuit shorts.

    I know this is rather tricky question without describing the circuit
    entirely but if someone has experience of problems or symptons like
    this, (in any circuit), I'd be thankful of some hints and suggestions
    of what type of fault to look for.
  2. scada

    scada Guest

    It is unclear to me what you have. You need to post a schematic. If I
    understand you corectly, you are using a darlington amp with the collector
    of T1 going to B+, then the emmiter of T2 going to B-! Well as soon as there
    is current introduced to the base, it will short the supply!
  3. Teemu_K

    Teemu_K Guest

    Yes I was sure it would be. Note my line "without describing the
    No. The circuit doesn't have negative and positive supply voltages.

    It's impossible to post the schematic to everyone or to describe it in
    detail (or draw it with ANSI graphics), howerer...

    You, (or anyone interested), can check the circuit from

    to be specicic.

    Its the 60W guitar amp's preamplifier's 1st and 2nd stage. First is, I
    believe it is (I'm a beginner), a darlington. Second one is almost
    identical without the complementary transistor. When simulated with a
    software, or calculated, it works fine. In real life... no.

    After the R21 the voltage should be around 59 V because the main
    supply voltage in circuit I built is about 63 V, not the 60V. This
    shouldn't do any harm howerer. Without transistors all the preamps
    voltages seems to be ok but when transistors are put on their places
    the Q1 (BC546) shorts through its collector to base. The supply
    voltage decreses to about 32 V and resistors R21 and R8 (both 330 ohm)
    have to handle at leat 3 W of power. Even 5W resistors heat too much.
    No sound. No signal present after R1 or R2.

    I really don't know what I should look for to get it working.
    Everything seems fine. The transistors should be ok and I tested them.
    I'm not sure whether the author of the page really built the circuit
    him/herself but the poweramp operates and the preamp seems logical so
    I gather it should work, and I'm mostly sure that ..I.. have made an
    error. Just don't have any clue, that what it could be. No there is no
    short from collector to base,.. except when the transistors are put on
    their place. All the teething problems in the circuit should be fixed
    by now..

    BTW. I send him email. No answer. Otherwise I wouldn't be bothering

    Now I'm looking for the clues of what should I be looking for to get
    this circuit operating. Leak current??? From where? Why?

    I repeat my words...

    "if someone has experience of problems or symptons like this, (in any
    circuit), I'd be thankful of some hints and suggestions of what type
    of fault to look for."

    PS. The poweramp is fine and really sounds good. So if anyone is
    interested in building a simple, good sounding amplifier take a hint.
  4. Robert Baer

    Robert Baer Guest

    First, i see no darlington configuration in this section (Page65.htm).
    Q1 is a simple amplifier, Q2 is to bias Q3/Q4 to class AB1 or AB2; R3
    is adjusted for zero signal standby current thru Q3/Q4.
    Q3/Q4 is a "standard" complimentary push-pull output stage.
    I have no idea as to the setting of R3; before powering up, set it to
    zero (not 2K which very likely will "short" out a wimpy power supply).
    Slowly (!) increase the post setting to adjust the standby current to
    specified value

    I would not call the Q1/Q2 configuration a (complimentary) darlington
    due to the resistive coupling.
    The circuit is not unreasonable.
    ....and the current is not going from collector to base in the
    If the Vcb of Q2 in the preamp is near or at zero, check C3 (shorted
    as well as for polarity).

    Seems to me you may have a problem from the over-biasing in the
    amplifier stage that i first mentioned.
  5. Ken Taylor

    Ken Taylor Guest

    Did you use a printed circuit board for the construction? Are you sure you
    have the right transistor in Q1's position? What manufacturer of BC546 are

    For information - since you're a newbie :) - the 'B+' is your 60V and the
    'B-' is the 0V. You have to have + and - supplies, but whether they are +
    and - in respect to earth is dependant on the design.


  6. Rich Grise

    Rich Grise Guest

    Actually, I think they're darlingtons, as per the parts list; they're
    just drawn like that for simplification.
    And, of course, I'm sure you'll agree, that'll be twice as much for
    darlingtons as for singles. :)
  7. Teemu_K

    Teemu_K Guest

    Need to check it out. The capasitor is alright, not broken, ceramic
    doesn't mind of polarity. It was my first suspect but turned out too
    be ok. If the Vcb turns out to be what you said, (zero or near it),
    what could cause it? How can it be fixed? There should be around 27 V
    Could this over-biasing cause transistors in the pre-amp to act as if
    they were shorted?
  8. Teemu_K

    Teemu_K Guest

    I will test the pre-amp circuit with a lower supply voltage and see
    what it does...

    There seems to be no faults in
    The Vcb at Q2 is abot 0.88 V. That's the voltage drop over transistor.
    It ain't right and was just what I already suspected. The cap is ok.
    Why does the transistor let all the voltage pass through?
  9. scada

    scada Guest

    I believe the setup description should read adjust "R3" and not "R9" to give
    1/2 the supply voltage at the +C7. This is the zero crossing point of the
    Push-Pull output stage, the waveform needs to "Ride" on top of a zero volt
    reference point, that way the output sits at zero with no signal, this
    prevents distortion in the output wave.
  10. scada

    scada Guest

    To explain further:
    By setting the level at +C7 to half the supply voltage, the output can swing
    + & - from the mid-way supply voltage (30V level: + to 60V, - to 0V). If for
    example the +C7 point were set at 50V, then a positive signal would clip at
    just above a10V peak creating distortion! By setting the +C7 level at
    mid-supply voltage the waveform can swing equaly + & -. This is the output
    waveform "0V" reference level. Also Q1 and Q2 are not configured as a
    darlington!Q1 is an amplifier, Q2 is used to bias Q3 & Q4 for the zero
    crossing set point.
  11. Teemu_K

    Teemu_K Guest

    I gather that you're talking about poweramp now. It works perfectly
    fine when set on the mid. It could clip but to my ears it's not
    audible. I don't own an oscilloscope.

    My problem is however in the preamp. Mistakenly the parts in the both
    of the circuits, pre- and poweramp, have been named the same way. I'm
    a beginner and don't exactly know whether the preamp's first stage
    Q1&Q2 is a darlington or whether it is not. It looks like one to me
    except that there's an negative feedback capasitor C3.

    The author and designer of the circuit sent post to me yesterday and
    suspected transistor problem. Somehow it just seems hard to believe
    since the transistors I used were new. Just bought and even tested
    when not found working alright.

    I made some new tests and found out that they only seem to operate
    normally in quite low voltages. At 15 V the signal was passed through
    transistors but it wasn't amplified. Rather lowered. The transistors
    should take 65 V so it is obvious that they're not working properly.

    Now the question is: is this caused by something in the circuit or are
    the transistors just simply broken. What are the odds that six new
    transistors are broken when bought? (See I had substitutes to switch
    with if the ones first fitted wouldn't operate.)

    I'll buy some new transistors soon and try with some new models too.
    If they don't work then it is sure the problem is in the circuit,
    however I already think it is in it. It's obvious that I have made a
    fault since the author got it work and it's clear to see it should
    work. Just don't know what to look for...
  12. Robert Baer

    Robert Baer Guest

    Transistors do not "break".
    I strongly suspect that there is absolutely wrong with the transistors
    that you bought.
    Double check that the specified part types are placed in the circuit
    as indicated by the schematic.
    Then also double check all other component values by marking and/or
    color code; also measure resistors (power *off*) and calculate what one
    should see if alternate paths exist.
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