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
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
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
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
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
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
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?
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?
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
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
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.