A customer brought this powered speaker forrepair. It is a Wharfedale Titan 12’ subwooferAmplifier.
The complaint: No power.
I opened the unit checking the usual things such as the fuse, EMI filter coils, voltage on filter caps etc. These all checked out fine, and I measured 320VDC over the two filter caps.
The unit uses a SMPS. The smps and class-D Amplifier, are both on the same board.
I then connected my analog meter to one of the secondary output lines going to the amplifier. There was no voltage here, and nothing on any of the other output lines going to the pre-amp board.
I then proceeded to check if it is a shutdown problem, by switching the unit on and watching to see if the needle on my analog meter jumps up for a brief moment. No movement at all.
This made me believe I have a primary –side fault.
I checked the two, main switching Mosfets and they were good.
From the below photo you can see this unit has a small smps controller board soldered to the main pcb. This is called as SIP Module (System In Package). Click here for more information about SIP Module.
This board consists of the main controller IC, some transistors that control Over voltage protection and Over current protection, feedback from the opto-coupler, and various resistors and capacitors. I unsoldered this board.
I found two transistors and two diodes shorted. I believe they might have been damaged by a high voltage power surge.
I replaced these components, and also proceeded to check the start-up resistors on the main board. I also checked the ESR of the capacitors on the whole primary side, just in case. All these tested fine.
I was tempted to replace the IC as well , but decided to give it a shot without replacing it. Soldered the board back, powered up with Variac and bulb limiter, and IT WORKED!!
I tested with some audio and speaker connected again, and it operated as expected.
By following some systematic procedures, I was able to nail the fault down to a specific area, and then located the faulty components in that area.
I highly recommend Mr. Yong’s books. They will help you as they have helped me.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs.
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