Wharfedale WH-D8 active Subwoofer repair

[Grablicht]

Hello, everybody. My subwoofer won't turn on anymore. I checked the fuse. I opened it up and checked every resistors and capacitors on the power board and even the MOSFETs. None of them are blown. I checked the amplifier board but none of the resistors and capacitors are blown too. Can the IC driver of the MOSFETs be blown without destroying the MOSFETs? Any hints would be much appreciated to fix this sub.

 

[Harald Kapp]

Check the power supply voltages. Are they present and correct?
How did you check the components?

 

[ZL2TPQ]

Hi Grablicht,

I've just finished fixing a WH-D8 with a similar fault description.​

On the power supply board, next to R1 (Large 100k) there is C11. Check it with an ESR meter. Mine showed many Ohms, but the ESR should be less than 1 Ohm.
If you havent access to an ESR (Effective Series Resistance) meter just replace the capacitor.
The R1 Resistor runs quite hot which warms C1, which is not good for an electrolytic capacitor, so keep them apart as far as possible.

R1 is connected to the rectified DC at about 330V and supplies the VCC pin on the oscillator IC. This voltage was around 8 or 9V in the fault condition. Not enough for the IC to run. After the capacitor was replaced, the voltage rose to 14.6v and allowed the IC to run and drive the power supply. The power supply has a auxillary winding that is rectified, regulated with a zener and continues to supply the chip vcc.
What I don't understand is why the designer did not arrange the R1 to disconect once the power supply started. It would save the constant heating.
Here's hoping that you have the sane fault.
Keith B

 

[Delta Prime]

What I don't understand is why the designer did not arrange the R1 to disconect once the power supply started. It would save the constant heating.

If you have a schematic of the circuit, you wish to understand, I could tell you why? & or why not!

 

[Harald Kapp]

Old thread from 2021 -> closed