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Bias adjustment

Discussion in 'Electronic Repair' started by Arvid Puschnig, Dec 6, 2003.

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  1. Hi,

    some time ago I started repairing my old SONY amplifier TA-AV501R. It
    took some time getting the spare parts, but now all DC voltages look
    good. Since I had to change the power transistors, I now have to adjust
    the bias voltage.

    Procedure from service manual:
    ....
    2. After the power is turned on, complete within 30 seconds to adjust
    RV500 and RV550 so that the bias voltage becomes 1 mV.

    I tried this, but the bias voltage increases more and more even after
    the 30 seconds.
    Would it be right to adjust the bias voltage so that it becomes 1mV 30
    seconds after turning the power on?

    Is it right, that the bias voltage increases more and more even after
    this time?

    Schematics:
    http://www.sbox.tugraz.at/home/a/arvid/TA-AV501R/Power.jpg

    @Tweetldee:
    Thanks for your support so far!

    arvid
     
  2. Asimov

    Asimov Guest

    "Arvid Puschnig" bravely wrote to "All" (06 Dec 03 03:11:06)
    --- on the heady topic of "Bias adjustment"

    AP> From: "Arvid Puschnig" <arvid at sbox dot TUGraz dot at>

    AP> Hi,

    AP> some time ago I started repairing my old SONY amplifier TA-AV501R. It
    AP> took some time getting the spare parts, but now all DC voltages look
    AP> good. Since I had to change the power transistors, I now have to
    AP> adjust the bias voltage.

    AP> Procedure from service manual:
    AP> ...
    AP> 2. After the power is turned on, complete within 30 seconds to adjust
    AP> RV500 and RV550 so that the bias voltage becomes 1 mV.

    AP> I tried this, but the bias voltage increases more and more even after
    AP> the 30 seconds.
    AP> Would it be right to adjust the bias voltage so that it becomes 1mV 30
    AP> seconds after turning the power on?

    AP> Is it right, that the bias voltage increases more and more even after
    AP> this time?

    AP> Schematics:
    AP> http://www.sbox.tugraz.at/home/a/arvid/TA-AV501R/Power.jpg

    AP> @Tweetldee:
    AP> Thanks for your support so far!

    AP> arvid

    I hope by bias voltage you mean the voltage across an emitter
    resistor of an output power transistor and not the DC balance...?

    1mv? That's an awfully small voltage, I usually see a higher mv...
    Perhaps the emitter resistors are a really small value?

    Start from zero and turn it up slowly until it becomes stable. It is
    normal for the idle current to increase as the output transistors warm
    up. However at some point it should stabilize and if it doesn't there's
    some problem with the temperature feedback strategy.

    Some amps stability factor design depends on the emitter resistors to
    self limit the idle current and some use a diode, thermistor, or
    transistor in direct contact with the heat sink for feedback. Maybe you
    forgot to reposition some thermal feedback device when changing the
    output power transistors?

    .... Resistance Is Futile! (If < 1 ohm)
     
  3. :

    AP> Schematics:
    AP> http://www.sbox.tugraz.at/home/a/arvid/TA-AV501R/Power.jpg
    Yes, as you can see on the schematics bias voltages are taken from R527
    and R578.
    That's why I had to borrow an 4.5 digit voltmeter...
    That's right, 0.22Ohm!
    I thought so too, but the service manual notes that this adjustment
    must be completed within 30 seconds. Even after 60 seconds voltage
    increases. Perhaps there's no temperature feedback?
    It's very strange that this behaviour occurs on both channels, so this
    would mean that there's a problem within the temperature feedback in
    the left channel as well as in the right channel.
    I am sure there's no diode, thermistor or transistor in direct contact
    with the heatsink or the power transistors.
    Do you know, why this adjustment has to be finished within 30 seconds?
    The only thing I can imagine, would be that the resistors get too hot,
    if bias voltage would be too high, but I don't think that it will harm
    them if I start with the lowest value and turn it up slowly until the
    1mV get stable, even if it takes some minutes?

    Thanks so far,
    arvid
     
  4. Asimov

    Asimov Guest

    "Arvid Puschnig" bravely wrote to "All" (07 Dec 03 01:39:12)
    --- on the heady topic of "Re: Bias adjustment"

    AP> That's why I had to borrow an 4.5 digit voltmeter...
    AP> That's right, 0.22Ohm!

    0.22 ohms is actually a relatively large value, I've seen 0.1 ohms and
    smaller used. 1mv across 0.22 ohms means an idle current of 4.55mA which
    is actually a rather small idle current as many amps use 20 to 60mA.

    AP> I thought so too, but the service manual notes that this adjustment
    AP> must be completed within 30 seconds. Even after 60 seconds voltage
    AP> increases. Perhaps there's no temperature feedback?

    With 0.22 ohms the idle current will probably not runaway. I think the
    30 second constraint is to take advantage of the transistors being at
    room temperature. If the adjustment were made when warm then the idle
    current wouldn't increase according to the expected temperature graph.
    This could result in too little idle current when hot and possibly
    exhibiting more distortion than the specifications. That's the only
    reason I can think of.

    Personally I would adjust idle current according to distortion at the
    33% power level. The 1/3 power level results in the most heating of the
    output transistors and at that point dissipation is 25% of full output
    power for each device. This is the greatest thermal stress point. i.e. a
    100 watts rms amp run at 1/3 output causes 25 watts dissipation in each
    transistor. However at 100 watts output the dissipation reduces to about
    15 watts per device.


    AP> Do you know, why this adjustment has to be finished within 30 seconds?
    AP> The only thing I can imagine, would be that the resistors get too hot,

    Indeed the resistors were selected to dissipate much more than 4.5
    "micro" watts!!! I'll bet they are rated to dissipate 3 to 5 watts maybe
    even 7 watts. I don't think the resistors would get hot enough from the
    idle current to make a difference.


    AP> if bias voltage would be too high, but I don't think that it will harm
    AP> them if I start with the lowest value and turn it up slowly until the
    AP> 1mV get stable, even if it takes some minutes?

    Well, the technique of adjusting it to 1mv within 30 seconds seems okay
    if the design calls for it. Like I said I'd get the amp preconditioned
    at 33% power and then adjust the current for least distortion but it's
    probably more complicated because of the extra analyzing equipment
    required. Seems to me the designer did the leg work to make it simpler
    for the technician. That's a rare thing these days and was more common
    in the heyday of discrete component stereo.

    .... The current limits placed are based on resistance
     
  5. Thank you very much.
    I'll try the simple way to adjust bias voltage to 1mV within 30 seconds
    so that voltages for both channels increase in the same way. There's
    just one question left: Since voltages increase very fast within those
    30 seconds, should I try to "reach" the 1mV exactly after those 30
    seconds or would it be better to "reach" them earlier? (10/15/20
    seconds?)
    I will use 2 voltmeters to perform this (one per channel).

    arvid
     
  6. The volume control must be set to zero. The bias current in most of
    these types of amps is about 60 to 80 ma or so. Set the voltage
    accross the 0.22 ohm emitter resistors to about 15 to 20 mv. After
    setting this, check the DC offset is as close to 0 Volts as possible
    at the speakers, with no speakers connected.

    I normaly check the power output in to a dummy load, and use a
    distortion analyser accross the dummy load to look at the distortion
    through the bandwidth of the output stage at 1 Watt, 10 Watts, and
    about 20 Watts. The amp is fed with a low distortion audio sweep
    generator to test. This way, I can also set the bias for optimum,
    using as low a current I can get away with, and still have as low
    distortion as possible, and proper output power.

    It would be best for you to obtain the information from Sony. If you
    were to purchase the origional service manual, it should normaly be in
    there.

    Jerry Greenberg
    http://www.zoom-one.com

    --
     
  7. Now I set the bias so that voltages get 1mV within 30 seconds for each
    channel.
    Voltage for right channel increases a bit faster so 1mV is reached
    after 10 seconds, for left channel it takes about 30 seconds. When
    transistors are warm (after a few minutes), bias gets stable at about
    6mV and I set voltages to the same value, because of the different
    speed of increment. So voltages become 1mV within 30 seconds (10/30
    seconds) when transistors are "cold" and voltages are the same when
    transistors are warm.
    Do you think this would be right or doesn't it matter if voltages have
    different values for each channel when transistors are warm?

    arvid
     
  8. Asimov

    Asimov Guest

    "Arvid Puschnig" bravely wrote to "All" (10 Dec 03 14:40:02)
    --- on the heady topic of "Re: Bias adjustment"

    AP> From: "Arvid Puschnig" <arvid at sbox dot TUGraz dot at>

    AP> Now I set the bias so that voltages get 1mV within 30 seconds for each
    AP> channel.
    AP> Voltage for right channel increases a bit faster so 1mV is reached
    AP> after 10 seconds, for left channel it takes about 30 seconds. When
    AP> transistors are warm (after a few minutes), bias gets stable at about
    AP> 6mV and I set voltages to the same value, because of the different
    AP> speed of increment. So voltages become 1mV within 30 seconds (10/30
    AP> seconds) when transistors are "cold" and voltages are the same when
    AP> transistors are warm.
    AP> Do you think this would be right or doesn't it matter if voltages have
    AP> different values for each channel when transistors are warm?

    AP> arvid

    Your results seem quite acceptable. 6mV across 0.22 ohms is about 30mA
    which is a typical value for that type of amplifier circuit. Be
    reassured that a little difference is to be expected and quite normal.
    Consider that even no two speakers are completely alike.

    Keeping in mind that crossover distortion is most objectionable at very
    low listening levels, then it is quite appropriate to equalize the idle
    current after the temperature stabilizes.

    Some designs use a higher idle current so that the amplifier operates in
    Class-A mode all the time at low listening levels and then in Class-B at
    concert levels. Hence the design name of Class-AB.

    .... I worked hard to attach the electrodes to it.
     
  9. Hi,

    I want to thank all of you for your time and your help.
    My amplifier is now working again and even without the usage of special
    equipment for setting the bias voltages it sounds quite good. Now it's
    time to start the 24h test...

    thx,
    arvid
     
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