Help on Mission 700AS Subwoofer Amplifier!!!

dantey · 16th February 2004, 04:49 AM

This subwoofer was working up till a week ago.
Then recently, when I switched on my receiver, the subwoofer turned on and started buzzing. i took off the grill to see the cone pushed to it's outward limit. kinda like what you would see when you apply a DC voltage direct to the speaker(but with a humm/buzz).

I have examined the amp. Seems like all the transistors are good, including the output stages. I noticed that three resistors were discoloured therefore, somewhere along the line too much power is passed through them. The subwoofer hasn't blown a fuse but if left on, starts to smell like burnt electronic parts.

Does anyone know what is broken in this situation? Or maybe some logical steps to check with?

One thing that I did notice is that the transformer outputs about 71V total, which is bridge rectified to 64V DC, split between two capacitors at 32V and passes through a discoloured resistor. I believe it's a 2watt resistor. Measured the wattage at ~0.80W(32V across 1.2kohm) but the resistor is hot to the touch, borderline to being too hot . Is that normal?

thanks,
Daniel

ChocoHolic · 16th February 2004, 06:54 PM

Hi Dantey!
Be careful with the speaker itself.
If the cone is pushed to its limit you can be sure that there is
a remarkable DC signal! This causes a lot of heat in the voice coil
and since it is not moving it cannot dissipate much heat without
burning. The smell which you described might have come from the
voice coil....

Can you measure the DC-level at the output?

Cheers Markus

ChocoHolic · 16th February 2004, 07:03 PM

I am wondering how to get 64V DC from 71V AC...
71V AC would normally result in about 100V DC!
Or did you measure the 64V behind the resistor (64V+32V = close to 100V).....

Probably the 1.2KOhm do not supply the power stage.
0.8W will make a small resistor (1W package or 2W package)
already quite warm. But just from this we cannot be sure that
the 32V across it are correct....

If no transistors are defect (really sure?), then there might be something wrong with some internal ground connections, or the feedback loop opened?

Cheers
Markus

dantey · 17th February 2004, 02:05 AM

i checked the DC voltage on the output and it's 45V!!!!
that's across a 3.8ohm speaker load.
The 71V AC is from the transformer. 120V from the wall output and 71V AC measured from the two transformer output wires that aren't ground. the 64V DC is measured from the bridge rectifier connected to the 71V AC transformer wires.

Where do i go from here? How would I know if the feedback loop is bad?

dantey · 17th February 2004, 02:18 PM

do you think my rectifier could be a problem? should it be putting out 100V DC instead of 64V DC?
I applied a 9V DC battery to the rectifier and it gave me something like 8V DC at the output.

ChocoHolic · 17th February 2004, 05:47 PM

I don't think that the rectifier has any issues.
But you should definitely disconect the speaker for your
ongoing tests, or if you want to "hear" what's going on you
should add an 100 Ohms / 5W resistor in series to the speaker!

How did you check the transistors, that you are sure they
are alright?
Did you also measure the emitter resistors (or source resistors if
you have Mosfets) of the output stage? If one of them is burned and shows a high resistance then you could supect that at least one of the transistors is defect even if they did not look defect.

(The feedback loop can be found by searching a resistor or passive network which comes from the output and is connected to the input.)

Bye
Markus

dantey · 28th February 2004, 06:14 AM

i figured out the problem!
it ended up being the relay. only 1 out of 2 of the switches was working so one of the rails was getting voltage while the other wasn't connected.

daniel

Cybergent · 28th February 2004, 03:31 PM

But I´m pretty sure, that your speaker is already severely damaged.
If you operate a speaker under such conditions the voicecoil gets hot and either melts or burns.

sam9 · 28th February 2004, 03:41 PM

How did you determine that the output transistors are undamaged ? Having destroyed a few, I know they don't look different. If your DMM has a diode test you can apply it to the base-collector pins and the base-emitter pins. If these check out, check the rersistance between the base and collector - it should read in mega-ohms. Lower even kilo-ohm readings will indicate damage.

dantey · 28th February 2004, 09:10 PM

i think the speaker is fine. i never had it on for more than a second or so.
i tested the transistors with a transistor tester on the multimeter.

16th February 2004, 04:49 AM	#1
dantey diyAudio Member Join Date: Feb 2004 Location: GTA	Help on Mission 700AS Subwoofer Amplifier!!! This subwoofer was working up till a week ago. Then recently, when I switched on my receiver, the subwoofer turned on and started buzzing. i took off the grill to see the cone pushed to it's outward limit. kinda like what you would see when you apply a DC voltage direct to the speaker(but with a humm/buzz). I have examined the amp. Seems like all the transistors are good, including the output stages. I noticed that three resistors were discoloured therefore, somewhere along the line too much power is passed through them. The subwoofer hasn't blown a fuse but if left on, starts to smell like burnt electronic parts. Does anyone know what is broken in this situation? Or maybe some logical steps to check with? One thing that I did notice is that the transformer outputs about 71V total, which is bridge rectified to 64V DC, split between two capacitors at 32V and passes through a discoloured resistor. I believe it's a 2watt resistor. Measured the wattage at ~0.80W(32V across 1.2kohm) but the resistor is hot to the touch, borderline to being too hot . Is that normal? thanks, Daniel

28th February 2004, 03:31 PM	#8
Cybergent diyAudio Member Join Date: Jan 2004 Location: Germany / Bavaria / Augsburg	But I´m pretty sure, that your speaker is already severely damaged. If you operate a speaker under such conditions the voicecoil gets hot and either melts or burns. __________________ Best Regards, Cybergent

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16th February 2004, 06:54 PM	#2
ChocoHolic diyAudio Member Join Date: Dec 2003 Location: Munich	Hi Dantey! Be careful with the speaker itself. If the cone is pushed to its limit you can be sure that there is a remarkable DC signal! This causes a lot of heat in the voice coil and since it is not moving it cannot dissipate much heat without burning. The smell which you described might have come from the voice coil.... Can you measure the DC-level at the output? Cheers Markus

16th February 2004, 07:03 PM	#3
ChocoHolic diyAudio Member Join Date: Dec 2003 Location: Munich	I am wondering how to get 64V DC from 71V AC... 71V AC would normally result in about 100V DC! Or did you measure the 64V behind the resistor (64V+32V = close to 100V)..... Probably the 1.2KOhm do not supply the power stage. 0.8W will make a small resistor (1W package or 2W package) already quite warm. But just from this we cannot be sure that the 32V across it are correct.... If no transistors are defect (really sure?), then there might be something wrong with some internal ground connections, or the feedback loop opened? Cheers Markus

17th February 2004, 02:05 AM	#4
dantey diyAudio Member Join Date: Feb 2004 Location: GTA	i checked the DC voltage on the output and it's 45V!!!! that's across a 3.8ohm speaker load. The 71V AC is from the transformer. 120V from the wall output and 71V AC measured from the two transformer output wires that aren't ground. the 64V DC is measured from the bridge rectifier connected to the 71V AC transformer wires. Where do i go from here? How would I know if the feedback loop is bad?

17th February 2004, 02:18 PM	#5
dantey diyAudio Member Join Date: Feb 2004 Location: GTA	do you think my rectifier could be a problem? should it be putting out 100V DC instead of 64V DC? I applied a 9V DC battery to the rectifier and it gave me something like 8V DC at the output.

17th February 2004, 05:47 PM	#6
ChocoHolic diyAudio Member Join Date: Dec 2003 Location: Munich	I don't think that the rectifier has any issues. But you should definitely disconect the speaker for your ongoing tests, or if you want to "hear" what's going on you should add an 100 Ohms / 5W resistor in series to the speaker! How did you check the transistors, that you are sure they are alright? Did you also measure the emitter resistors (or source resistors if you have Mosfets) of the output stage? If one of them is burned and shows a high resistance then you could supect that at least one of the transistors is defect even if they did not look defect. (The feedback loop can be found by searching a resistor or passive network which comes from the output and is connected to the input.) Bye Markus

28th February 2004, 06:14 AM	#7
dantey diyAudio Member Join Date: Feb 2004 Location: GTA	i figured out the problem! it ended up being the relay. only 1 out of 2 of the switches was working so one of the rails was getting voltage while the other wasn't connected. daniel

28th February 2004, 03:41 PM	#9
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	How did you determine that the output transistors are undamaged ? Having destroyed a few, I know they don't look different. If your DMM has a diode test you can apply it to the base-collector pins and the base-emitter pins. If these check out, check the rersistance between the base and collector - it should read in mega-ohms. Lower even kilo-ohm readings will indicate damage.

28th February 2004, 09:10 PM	#10
dantey diyAudio Member Join Date: Feb 2004 Location: GTA	i think the speaker is fine. i never had it on for more than a second or so. i tested the transistors with a transistor tester on the multimeter.

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