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problem wit a Radford integrated amplifier

Discussion in 'Electronic Repair' started by Gaetan Mailloux, Apr 7, 2007.

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  1. Hello

    I have an integrated amplifier Radford HD 250. I did replace its powers
    transistors and drivers.

    I don't have the diagram of this amplifier, on the board of drivers and
    pre-driver, there is three triming potentiometer; so from left on the
    right its; P6, P7 and P5. I presume that one among them is for adjusting
    the symmetry and one for the quiescence current of but I don't what are
    P6, P7 and P5 and how much millianpres is the quiescence current.

    Do you know someone that knows all it and who does the diagram of this
    ampli have, if yes, are you able to forward him my email ?

    Thank you

    Gatan
     

  2. sci.electronics.repair is the only appropriate group you posted to.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  3. Guest

    Gaetan:

    Same answer as on the other group. Without the shop manual, it is
    difficult to determine the precise quiescent/bias current required
    without a bank of instruments and enough time to allow the outputs to
    heat (or not) indicating the proper setting (or not).

    On the other hand and especially if you closely match the drivers and
    outputs, the 'factory' settings should be adequate for most
    situations. Keep in mind that (part of) the balance is between heating
    and distortion... so with a good design, you can afford to err towards
    the cool side without having the amp sound like crap, and that is
    also typical of factory settings. If you do not match the pairs, it
    should not be the end of the world, but it does give additional
    margins for error.

    What are the outputs and drivers? I am guessing either 2N3055 outputs,
    driven either by paired TIP-31/32 or 2N3054 drivers? And what, if any,
    symptoms is the unit displaying that has you in there doing these
    repairs?

    You have already managed the other typical sources of problems? Old
    electrolytic cap, power-supply problems, drifted resistors, and such-
    like?

    Related: I have quite a bit of vintage AR electronics about. Early
    production amplifiers and receivers used carbon-element pots for bias
    adjustments, two for each channel. Naturally, they drifted over time
    causing all sorts of ruckus usually ending with blown outputs (and
    sometimes drivers). As it happens, AR developed a wretched reputation
    due to these failures. Even though back in the day they had an
    indefinite warranty on their amps and receivers so-equipped, and would
    replace the pots with fixed resistors after a bit of checking for the
    ideal fixed value. Their shop manual reflects this process and how
    that value is determined. One can 'shotgun' it at a given value, but
    the precise balance will be lost even if 'safety' is achieved.

    This is why I hesitate to give you anything exact or to have you alter
    the factory settings without a manual.

    Peter Wieck
    Wyncote, PA
     
  4. Tim Schwartz

    Tim Schwartz Guest

    Hello Gatan,

    First, set the regulated supply to 70 volts, this is the pot on the
    power supply board. I also suggest that the amp be set to 120V, not
    110V at the transformer, as you don't want to exceed the rating of the
    main filter cap.

    P5 is supposed to be set for symmetrical clipping. I have found that
    the pot introduces so much noise that you are better off replacing it
    with a 47K resistor. The amp will clip symmetrically.

    P7 is the bias pot., though the instructions say P6 is for adjusting
    crossover spikes. The factory procedure is:

    With input of 70 mV into the tuner input (All other buttons off and
    they do not specify where to set the input level controls or volume.)
    and an 8 ohm load connected Adjust P6 (Crossover spikes) and P7
    (Distortion)

    "Connect Distortion measuring set (D.M.S.) to amplifier output and
    oscilloscope to the D.M.S. output. Reject fundamental and measure
    distortion. Adjust P6 for minimum amplitude of crossover spikes as
    observed on the oscilloscope and P7 for minimum distortion.

    The original Z170 and Z171 output transistors are long obsolete. I'd
    use MJ15003 and MJ15004 for outputs.

    Regards,
    Tim Schwartz
    Bristol Electronics
     
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