Boston Acoustics Soudware Subwoofer PowerSupply

[jinu_j]

Hi Folks

I am looking for help to trouble shoot my Boston Acoustics Soundware X subwoofer power PCB. The Circuit diagram is attached. I have checked the voltage across C3 (there is no capacitor as this is 240V board). The voltage i get is about 318V so i am assuming that the circuit behind that is working properly. But now I am not sure how to proceed. The circuit on the Left-hand side of the circuit is on a separate PCB that's soldered onto the board using the connector CN4 shown in the circuit diagram. So its difficult to trouble shoot components on that.

There are severe burn marks around the zener ZD3. The voltage across the ZD3 is 0.364V, also the capacitor on the mounted PCB looks blown but not sure if its blown or just the glue that changed color. What i am not sure is how to test the circuit post C3 point. There are so many grounds and I cant figure out how the circuit works. Also i cant figure out which ground to use for measuring. It does not look like all the grounds are the same (GND-P, PGND, SGND). Is my assumption correct that the PGND and GND-P is power Ground, where as SGND would be signal ground ?

Any help in troubleshooting the same would help. My concern is that is the zener and capacitor blown because of some other fault in the circuit or am i looking in the wrong place.

Thanks

[Per Hansson]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

It probably died because of the glue: it becomes conductive with age.
Follow the black burn marks to see how the current traveled to get an idea what got destroyed...

[jinu_j]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

Thats the surprising part, No other components is showing any burn marks. I think the zener got overheated and the glue around it burnt.

[R_J]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

Have you checked if ZD3 is shorted? Check D1 and R7, they supply the initial pvcc for Q2 which is acting as a 15v regulator to supply vcc for U1.
I suspect either ZD3,c13,c14 are shorted or R7 is open
PGND = Primary ground
SGND = Secondary ground
You must use Primary ground when checking voltages on the primary side such as pvcc

[jinu_j]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

Zd3 broke into half when i desoldered it. So guess thats damaged. The other components D1, R7,C13, C14 and R7 are fine.

Now await the replacement of ZD3 before i can continue testing.

[jinu_j]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

So finally got the parts. i replaced the Zener ZD3 and C13. Voltage at CN2 is still 0. But i am getting +20V and -20V at CN3

Voltage across C3 is 315V
Voltage across ZD3 is 19V
Voltage at HO is 167V
Voltage at LO is 6V
Voltage at VS is 157V
Voltage at CN2 :
pin 1 = 0V
pin 2= -20V
pin 3 = Grnd
pin 4 = +20V

Any advice on how to trouble shoot this further. Thanks

[momaka]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

What DC voltage do you get when you measure right across C8 and C16?
I'd find it strange to get 0V there, since +VH1 and -VH1 are coming from the same transformer as +V1 and -V1, which are probably OK, being that they are +20 and -20V respectively.

[jinu_j]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

Finally solved the mystery. It was dry solder on D2. Now getting 32V on CN2

[momaka]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

Finally solved the mystery. It was dry solder on D2. Now getting 32V on CN2

Good job!
It had to be something simple indeed. Just didn't seem reasonable to have proper voltages at CN3 and none at CN2, but all components be OK.

Curious: with the PSU taken care of now, did this get the subwoofer fixed?

[jinu_j]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

Good job!
It had to be something simple indeed. Just didn't seem reasonable to have proper voltages at CN3 and none at CN2, but all components be OK.

Curious: with the PSU taken care of now, did this get the subwoofer fixed?

Yes and No. By doing the subwoofer has started working meaning i can feel the vibrations in the woofer and the vibrations increase and decrease as i lower and increase the volume button in the subwoofer, but the output even at max is very much lower that what it was originally. Earlier in songs with heavy bass i could feel the punch but now its barely there.

This is the second time the power board has blown, the first time i took it to a reputed service center and they repaired it. I assumed they had done a good job. This was over 2 years ago. It is now when i opened it for repair that i realized that they replaced the original burnt board with a refurbished Power supply board from another model. My original board was model Boston Acoustics Soundware S. They have replaced with the power board of Boston Acoustics Soundware XS. One difference i noticed was in my original board the connector CN3 is a 3-pin connector whereas in the new board it is a 4-pin connector. So, the pin 1 is not connected to anything in my current setup. I don't know if this is creating the difference.

But for now, technically the subwoofer lives, but does not perform the way it should. I dont think i have the capability or equipment to debug audio signals and their amplifications.

[stj]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

check the psu output while playing some regae or other bass oriented stuff
i have found with some harmon-cardon crap that the psu can only just supply enough current and as the caps wear the current is restricted and causes the voltage to drop whenever the speaker tries to thump.

[jinu_j]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

check the psu output while playing some regae or other bass oriented stuff
i have found with some harmon-cardon crap that the psu can only just supply enough current and as the caps wear the current is restricted and causes the voltage to drop whenever the speaker tries to thump.

Can you advice on what and where to check. Are you referring to the voltage at CN3 and CN2 ?

[jinu_j]

Re: Boston Acoustics Soudware Subwoofer PowerSupply

Can you advice on what and where to check. Are you referring to the voltage at CN3 and CN2 ?

Just measured voltages at CN3 and CN2. There is no material drop in voltage when heavy bass is played. The voltage hovers around 19.7 (CN3) and 31.8 (CN4)