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More Than 20 Bad Components Found In Amplifier Circuit Board

By Jestine Yong on August 22, 2018

how to fix and repair power acoustic amplifier

A customer brought this Power Acoustic Mono Block Amp in for repair and told me that it had stop playing and there was no sign off life to it (No Power). I ask him what he was doing when the amp stop working. (You need to ask the customer to narrow down the problem).

So, he told me he was driving his car and it just stops working.

As you can see from the picture below when he gave me the amp I saw the positive power supply terminal had been melted, which indicate a short, either internally or externally.

So, with that in mind I powered up the unit using my power supply bring up the voltage very slowly and monitoring the current draw on my PSU. The amp current at full 12 VDC was only 0.34 amps which indicate that there was no short internally, and indeed there was no power to the amp.

I proceeded to remove the six screws from the case and inspect the circuit board for any visual sign of damage or blown components and could not find any.

Below is the drive circuit for the power supply.

To check the power supply alone I remove the positive and Negative rectifiers that supply the output drive section to isolate it (See picture below). Car Amp has mainly two sections. Power supply and the output section.

When I remove the rectifiers and turn the power supply I Check with my Oscilloscope for the square wave at the gate of the PSU mosfets and the wave was present, then I check at the two input pins for the Rectifiers for a larger Square wave signal and that was also present. This proves that the power supply section was working fine and the fault was coming for the output section.

Then I began to check the output Mosfet for any shorts, but they also tested good. I replace the positive and Negative rectifiers and powered the amp once more and check the output Mosfet square wave and found that there was no square wave present. I began to suspect the output driver board was faulty (See above for Driver Board), so I probe the Driver IC pins for square waves and there was none. Check the power supply to the IC pins and found none. The next step was to check for supply to the Driver board and it was there. That means that the driver board had problem. Below is the picture of the driver board

Notice the colour marks on the transistors? It is to identify how many different types on the board. I began to check the transistors and found some of them shorted. I replace all the transistors since they must be match. I also check the surrounding components and found one resistor out of tolerance and two Resistor type capacitor that was also shorted. I change them to ceramic caps. I replace the driver board. See picture below for the bad components.

After I replace the Driver board and powered the amp, I could see the power LED turn Green which was a good sign. Tested the output drive signal. I was very happy to see this am came back to life. See picture below for output signal on my Oscilloscope.

Conclusion: Amplifier work can be quite difficult sometimes, but once you understand how the amplifier works it will become easy.

This article was prepared for you by Andre Gopee from Calitech Limited Trinidad West Indies.

Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.

By the way if you have any good repair article that you want me to publish in this blog please do contact me HERE.

Note: Please check out his previous repair article below:

https://jestineyong.com/usn-52-krautkramer-branson-flaw-detector-battery-charging-circuit-repair/

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amplifier repair

19 Comments

Parasuraman

August 22, 2018 at 1:14 pm

Excellent professional approach and remedy, with a lot of patience and tolerance! Great job there!

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- Andre Gopee
  
  August 24, 2018 at 11:03 pm
  
  Thank You. It really takes a lot of patience to repair these amps.
  
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Albert van Bemmelen

August 22, 2018 at 3:09 pm

Pro job! Finding equally Hfe matching transistor pairs must have been the hardest part. I understand that the positive power supply terminal was still useable afterwards although it had heavily melted.

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- Andre Gopee
  
  August 24, 2018 at 11:05 pm
  
  Thank You... Yes the positive terminal was still usable after a little cleaning
  
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- Andre Gopee
  
  August 24, 2018 at 11:05 pm
  
  Thank You. It really takes a lot of patience to repair these amps.
  
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Andrew F. Ali

August 22, 2018 at 8:17 pm

Nice job. We know how these amps run at max output in vehicles in Trinidad. I'm here in Point Fortin. Nice to see a fellow trini-tech in this forum.

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- Andre Gopee
  
  August 24, 2018 at 11:06 pm
  
  Thanks Andrew. Yes these amps we really run them hot.
  
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moshe jacobson

August 22, 2018 at 9:05 pm

A really excellent methodical approach.
thank you for presenting it.

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- Andre Gopee
  
  August 24, 2018 at 11:07 pm
  
  Thank you.
  
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Robert Calk Jr.

August 22, 2018 at 11:15 pm

Good job, Andre. Using the PSU/Isolation Transformer that has an ammeter is the fastest way to check for a shorted device. I still use my Series Test Light sometimes.

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- Albert van Bemmelen
  
  August 23, 2018 at 5:32 pm
  
  Hi Robert, I tried to buy such a handy transformer but over 220 Euro/dollar exclusive shipping and VAT is way too much for my budget. And the 230VAC version is at least twice as expensive as the 110VAC version.
  
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  - Robert Calk Jr.
    
    August 24, 2018 at 1:20 am
    
    Hi Albert,
    
    Build your own! You just need to decide how much current that you want it to be capable of, and go from there. There are probably many ideas for one that you can google. You also need a voltage/ammeter and you can add a wattmeter if you wish. Paul has a video about his you can watch: https://www.youtube.com/watch?v=51mjt9nFoeA&t=303s
    It's very dangerous to be working on devices without isolation!
    
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    - Albert van Bemmelen
      
      August 26, 2018 at 3:57 am
      
      I know about the safety transformer why I wanted to buy one Robert. Maybe I can build one. I need good powerful transformer(s) for that but they are very pricey. Recently I had a 2KVA transformer that was 230VAC in and 110VAC out. I gave it away because I need at least 2 identical ones to make a 230VAC to 230VAC safety transformer.The 2KVA transformer was too heavy anyway.
      
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      - Robert Calk Jr.
        
        August 26, 2018 at 9:51 pm
        
        Yeah, you definitely need a good transformer. It may be cheaper for you to build your own transformer. If you do build your own Isolation Transformer, I hope you write an article about it so we can see it!
        
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- Andre Gopee
  
  August 24, 2018 at 11:09 pm
  
  Thanks Robert. I normally use a 24 Volt light bulb in series to bring up the power supply.
  
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Mason

August 22, 2018 at 11:48 pm

Very nice logical approach to troubleshooting. Excellent job.
Thank you for documenting and sharing your repair.
Mason

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- Andre Gopee
  
  August 24, 2018 at 11:10 pm
  
  Thank You.
  
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Humberto

August 24, 2018 at 12:36 am

What a great repair Andre. But at last the equipment went back to life.

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- Andre Gopee
  
  August 24, 2018 at 11:11 pm
  
  Thank Humberto, the customer was very happy.
  
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