How To Repair Amplifier No Sound

By on November 12, 2016
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how-to-repair-amplifier-no-sound

A customer of mine brought to me the TAC Integrated Stereo Power Amplifier With the Model of AV-355 for repair. The complaint was no sound but have power. Usually the first thing I do was to connect a speaker at the output jack to see if the sound really not present.




Another test is to check the output DC voltage at the speaker output jack. It should not have DC voltage and from the test I got 35.6 DC volt which mean the internal power transistor had already shorted.

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Alright, once the casing was removed I could see two circuit boards  (main and pre amp) and a big toroidal transformer-see the photo below:

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From the visual inspection, I found two  e-caps bloated on top of the capacitor casing, broken jumper wires and a burnt resistor-look at the white arrows in the photo below:

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I  knew if a burnt resistor were found this means the main power amplifier transistor will be shorted. True enough, when I placed my digital multimeter to the transistor pin (on board) I got very low ohm on all the legs (pins). From experience,  once you have found a shorted power transistor, you have to check all related components around that circuitry. If you want to learn how to accurately test electronic components you can check out Jestine’s Ebook HERE.

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After spending some times to check the circuit board, I can’t believe that there were actually 15 bad components found which include the burnt resistor.

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After the replacement with new components the stereo power amplifier with no sound problem was finally fixed. I did not post the video thus you would not hear the sound but the sound worked!

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suranga

This article was prepared for you by Suranga Bandara who owns an Electronics repair shop in Anuradapura, Sri Lanka.




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Note: You may check out previous post about fixing active speaker in the below link:

http://jestineyong.com/broken-circuit-board-fixed/

 

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22 Comments

  1. YH Wong

    November 12, 2016 at 3:38 pm

    Wow, a lot components are changed there, Suranga. Is it easy to purchase components in Sri Lanka ?

    Likes(1)Dislikes(1)
  2. PARASURAMAN

    November 12, 2016 at 4:27 pm

    Oh! My God! What a tedious work, but, with a positive result! Well done! Did you get exact replacements or you had to find matched?

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  3. Yogesh Panchal

    November 12, 2016 at 6:27 pm

    Excellent!
    keep up suranga.

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  4. Lad

    November 12, 2016 at 7:20 pm

    Congratulation.
    Regarding those shorted transistors, did you measured them in PCB first or did you desolder them first and only then you measured?.
    I still learn to repair electronic devices but sometimes it happens that if I measure a diode in PCB( being soldered) it seems to be shorted but after desoldering and testing again it is OK. So I asked here because you are better at repairing.
    Thank you for reply.

    Likes(6)Dislikes(0)
    • Parasuraman S

      November 13, 2016 at 3:16 pm

      Whenever there is a base to emitter or base to collector or emitter to collector resistor is connected, we will not get proper reading. So, we need to remove the transistor and check. Same thing can happen with diodes also, when a resistor or resistance is existent (through other return paths) across the measuring points. You have to just remove one end. But in the case of SMD, we have to remove the component and check.

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  5. Bulent NUR

    November 12, 2016 at 7:44 pm

    Thanks for sharing nice job, whats difficulty degree 1-10?

    Likes(2)Dislikes(1)
  6. Lewis Spedding

    November 12, 2016 at 8:00 pm

    Thank you Suranga. I find this fascinating as I used to repair like yourself. I am currently repairing my old VCR so thank you for your trouble

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  7. Bhupendra

    November 12, 2016 at 8:45 pm

    Brilliant job Suranga,you really done a complicated work. I am appreciate you.

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  8. Tito Kanshulu

    November 12, 2016 at 10:25 pm

    Good work Suranga though a bunch of bad culprits. Did you replace with similar components or you had used the equivalents?

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  9. Kevin

    November 13, 2016 at 5:25 am

    i find this information helpful and could help me trouble shoot some devices too.keep it up.

    Likes(1)Dislikes(1)
  10. Anthony

    November 13, 2016 at 4:22 pm

    Good work Suranga. A lot of dust there too which doesn't help things. Thanks for sharing.

    Likes(1)Dislikes(0)
  11. Joop

    November 13, 2016 at 10:01 pm

    Good Job. Thanks for sharing.

    Likes(1)Dislikes(0)
  12. Albert van Bemmelen

    November 14, 2016 at 5:18 am

    You took quite a risk to replace that many defect parts Mr.Suranga Bandara.
    But in the end it was a very successful repair, which only a Pro can fulfill! Apart from all the time it did cost also the cost of replacing all defect parts are always a little gamble. But afterwards another Customer is happy and that is all what counts!

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  13. Robert Calk

    November 15, 2016 at 2:20 am

    Nice job, Suranga.

    Likes(1)Dislikes(0)
  14. Lawrence

    November 15, 2016 at 5:02 am

    Wow after replacing al thoz bad components it worked. It was a risky worth a try. In sum cases afta replacing al thoz components dey r high chances of da same components to blow up again wyl testing. Man u gt patience. Dis is da reason y myself n amplifiers we are not frenz. When dey blow dey blow a chunk of components. I prefer tv's than amps. Good job buddy.

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  15. Henry

    November 15, 2016 at 1:42 pm

    It seems like a complex but in the end successful repair. Well done Suranga.
    I would however, appreciate some more info here, and particularly to the following statement:
    "Another test is to check the output DC voltage at the speaker output jack. It should not have DC voltage and from the test I got 35.6 DC volt which mean the internal power transistor had already shorted".
    Could anyone elaborate on that please. What exactly is the link between the DC voltage on the speaker output and the power transistor being shorted? Maybe some example with a small section of a schematic diagram.

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    • Parasuraman S

      November 17, 2016 at 7:14 pm

      Depending upon the construction of amplifier, the final output is taken from a push-pull stage many times. Many of us, who involve in Amplifier repair, would have noticed that there are DC blocking capacitors used for coupling. The Transistors are critically biased in such a way, that if there is even slightest variation, the entire array of transistors will stop conducting. This is achieved by a feed back circuit in the output stage, that cuts the biasing circuit entirely. In such a circumstance, DC will be present in the output stage, i.e, speaker outputs. More or less same thing happens, when any or a bunch of transistors fail. While the professional discreet amplifier section is designed for making it noise and distortion free to the maximum extent, any failure or even slightest leakage in a capacitor, can trigger auto cut off of the biasing circuit. Just download a few professional amplifier circuits, complimentary symmetry or push pull type, then you can understand this yourself. But trouble shooting such amplifiers is a pain, no doubt, as we cannot isolate the stages, without temporarily modifying the circuit for the sake of testing!

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  16. Henry

    November 18, 2016 at 9:19 am

    Thank you Parasuraman for that clarification. It helps. I did look at a few on line amplifier circuits and complimentary symmetry arrangements, and now have a bit better understanding. Many of those circuits showed a coupling DC blocking capacitor connected in series right before the speaker output. That should really stop any DC voltage reaching the speaker. However, I guess there are other solutions to the coupling capacitor location. Thanks again.

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    • Parasuraman S

      November 18, 2016 at 7:42 pm

      Another factor that helps in trouble shooting is, if we find + voltage in output, negative rail or biasing is open and vice versa!

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  17. Raju varghese

    November 21, 2016 at 1:33 pm

    Nice suranga , complicated job
    well done.

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  18. Humberto

    November 24, 2016 at 9:02 pm

    A great repair Suranga. Congratulations.

    Likes(0)Dislikes(0)
  19. Phil

    August 18, 2017 at 7:21 am

    Wow!!!, months of looking for someone who has mad skills, man im glad I found you!! Thank you for your intelligence. ?

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