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More Than 20 Bad Components Found In Amplifier Circuit Board
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.
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Note: Please check out his previous repair article below: