Kef PSW3000 Subwoofer Repair
A client dropped off his KEF PSW3000 subwoofer complaining that it had no sound. I powered up the unit and could see the power-on LED on the front of the wooden box illuminate, but could hear no clicking of the speaker protection relay inside.
I opened up the unit and could see no physical damage or heat damage on any of the pcb’s. The unit has an encased internal SMPS, which supplies the ±15 supply as well as the high voltage rails for the Power amp section. These voltages were all present and stable.
I measured the voltage on the speaker output before the protection relay and measured about 53V. This means that the amp was giving out DC on its output. This explains why the relay was not engaging. I then powered the unit off, discharged all filter capacitors, and continued to measure all of he semiconductors in-circuit with my multimeter in the diode test mode. Strangely all transistors and diodes on the amplifier section tested correctly.
I then decided to connect a 100Hz sine wave signal to the input of the preamp and trace the signal through the various boards with an oscilloscope. Using the service manual, I found that the signal was reaching the amplifier pcb, meaning that there was no fault on the preamp board.
I eventually found one opamp that was outputting around 14V. Looking at the schematic indicates U204. It was only when I removed the opamp, that I could see a clear horizontal crack in the IC.
I inserted a DIP8 socket and replaced the opamp with the same type. After doing this the relay engaged and I had sound from the speaker output. Next I noticed that the fan circuit wasn’t working. The fan itself tested fine.
Looking at the schematic it was clear that the fan runs actively along with the power amp output, meaning that the louder the audio sent to the speaker, the faster the fan turns. I found transistor Q202 at fault. After replacing it, the fan started working again.
One last thing I did was to take my FLIR thermal imaging camera and scan the boards for areas/ components that were getting too hot.
I found that the two pre-driver transistors were getting too hot for their package types. These were Q204 and Q206. They are TO-92 package transistors. They measured around 51deg C and rising.
A bit of research showed that on earlier models such as the KEF PSW2000 this was a common issue and cause of failure. Many users indicated that the subwoofer would end up failing after some time of operation, only to find that these transistors tested fine when cold.
I decided to use some larger package transistors to replace these, namely the BD139 and BD140, and mounted them on a piece of L shaped aluminum.
My only concern was that I would need to run some wires from the PCB to the actual transistors, risking oscillation.
I threaded the piece of aluminum where the mounting screws for the transistors and the back plate would attach.
I then mounted the transistors with SIL pads for isolation and also put some heatshrink tubing on all of the transistor pins.
Luckily there was no oscillation present, confirmed with an oscilloscope. I then added some silicon adhesive to support the wires where they entered the board.
I tested the subwoofer for about 3 hours. All good. Another happy client.
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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You can also check his previous repair article below: