Push-pull amp question

Discussion in 'General Electronics Chat' started by Adriara, Feb 21, 2011.

  1. Adriara

    Thread Starter New Member

    Feb 18, 2011
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    Hello ,
    I am working on a class A amp ( MC7300 ). The left channel is warmer than the right one.All voltages on the final transistors are very much alike on both channels . I changed all finals and the pre-finals on both positive and negative heatsinks and got a much beter thermal behaviour ( it is heating much less ) , however it is still warmer than the right channel. I also matched the new transistors regarding theit Hfe.
    Could somebody , please , give me an idea on what to do next ? Or if i need to do other check ups on the transistors ?

    Thank you very much .
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    A class A amplifier will have a large quiescent bias current. Possibly there will be some method of adjusting this to compensate for ageing or component replacement.

    You should check the schematic (or if available the service instructions) for information on what the correct bias current should be, and for any adjustment procedure.
     
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  3. Adriara

    Thread Starter New Member

    Feb 18, 2011
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    Thank you very much for your answer , i do have the schematics but there is no mention about any adjustments ...in the preamp module there is a potentiometer for each channel used to adjust the bias. The adjustment is ok. I identified the problem to be located in the final transistors left group ( pair : negative & positive heatsinks ) , by really changing the left group with the right one ...so my problem is in the final transistors. The two preamp pcbs are identical and ok. I would like to learn if there are any other measurements i am supposed to perform on these final transistors , other than hfe...and which might determine a possible diference...thank you
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    As a first step, you might compare the supply current for the two sides: they should be similar. To really be sure of a correct set-up, you would need to know the correct current level, as well as the actual levels.

    Unfortunately, the transistors you have replaced may not be all that is wrong. It is quite possible that some problem exists with other components in the circuit. If you could understand how the bias current is determined, you might be able to see whether the measured values were reasonable.

    Apart from DC faults, is there any possibility that the amplifier could be oscillating, perhaps at too high a frequency to be heard? If you have access to an oscilloscope, you should check this out.

    Finally, if the amplifier uses a DC coupled bridged output, an excessive output DC offset could cause overheating and also may harm loudspeakers. Not knowing the detail of your amplifier, I do not know if this could be applicable.
     
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  5. Adriara

    Thread Starter New Member

    Feb 18, 2011
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    Unfortunatelly the repareshop is closed now (6:50 pm here )
    Yes i have a scope and i will prepare details , including schematics and some picture of the amp for tomorrow evening. I have to apologise for being a little slow with my messages as i am new on this forum and need to get used to the procedures ...
    I find extremelly kind of you to try to help me . Will get back tomorrow evening.
    Have a good night Sir.
     
  6. Adriara

    Thread Starter New Member

    Feb 18, 2011
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    Hello ,
    Please see atachment for the schematic , and let me know if more info is needed.

    Thank you very much.

    Best regards,

    Adriara
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Well, that rather beats me. You said that this is a Class A amplifier, but the voltage conditions given around the output stage suggest a fairly minimal Class AB bias - note the voltages around the 0.5Ω resistors R53 through R62 are all quoted as "0V". It would appear that the bias current is small. That said, a comparison of the voltages across these resistors and the voltages at eqiuvalent points on the other channel could be instructive.

    I note that there does appear to be a quiescent bias adjustment, namely the pot R42, 200Ω. You would however be well advised to find out what bias setting was recommended by the manufacturer before making any adjustments. One thing to note though: deterioration of this pot could lead to excessive bias current and hence overheating.

    I see that this amplifier incorporates an output matching transformer. it will be important that the output DC balance is good, or quite significant current might circulate here. Again, measure around R53 to R62 to check.

    On the whole, I think that it would be indispensable to have the manufacturers set-up and service instructions so that you have the best possible information to help you. Here, as you might say, I am at the end of my science. I would rather not make further guesses which might lead to expensive mistakes.
     
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  8. Adriara

    Thread Starter New Member

    Feb 18, 2011
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    Sorry for the class A , i was blocked on some theory i consulted before ...and lost contact with reality.I will study everything again , keeping in mind your advices.
    Unfortunatelly this is all i have from the manufacturer. The potentiometer is good ( i have even interchanged the preamp modules before) , and apart that the adjustment is smooth(i happen to be aware of this kind of problem when a dyrty or old and worn potentiometer gives an unstable adjustment ). Anyway : thank you so much for the time you spent with this problem.
    All the best to you.
     
  9. Adriara

    Thread Starter New Member

    Feb 18, 2011
    7
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    Hello Mr.Adjuster ,

    This is just to share with you the fact that i found the problem , it was the final audio transformer ( shorted windings ). I realised the feedback signal that comes back to the diferential amplifier , came a little distorted ( at a slightly higher volume became visible on the scope ) .After replacing the transformer everything works just fine.

    Thank you for trying to help me at that time when everything appeared so confuse.

    All the best to you ,

    Adrian
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    300
    Hello Adrian,

    Thank you for taking the trouble to let us know. It is good to hear that you got to the bottom of the problem in the end.

    Regards

    Adjuster
     
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