Hi,

I have a budget powered subwoofer wtih a 100W amplifier. Right out of
nowhere, the sub started to have a constant, low-pitched hum that ruins the
whole home theater experience.

My thought is that one of the power output transistors has failed. What are
your thoughts and/or how can I troubleshoot this to identify the right
component? I have a decent Fluke meter and understand the basics of how to
use it, but I have been away from electronic repair for about 20 years.

Thanks for any help.

Reactor wrote:

Hi,

I have a budget powered subwoofer wtih a 100W amplifier. Right out of
nowhere, the sub started to have a constant, low-pitched hum that ruins the
whole home theater experience.

My thought is that one of the power output transistors has failed. What are
your thoughts and/or how can I troubleshoot this to identify the right
component? I have a decent Fluke meter and understand the basics of how to
use it, but I have been away from electronic repair for about 20 years.

Thanks for any help.



Assuming it's a fairly loud hum, I'd suspect one of the power supply
diodes has failed with a short, or the connection to the power supply
filter capacitor has opened (or has a cold solder joint) before I'd
blame an output transistor. If it's a low level hum, there's probably
either an open ground or a ground loop somewhere in the signal path.
Often these are introduced by cable TV connections, so it might be
interesting to know whether disconnecting the cable ends the hum --
if it does, then it's easy (and cheap) enough to buy or build an
isolator.

Just my 2 cents.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

Before attacking the output stage, do you have any sound with the buzz? If
so, disconnect it and see if it still buzzes. If it does when disconnected,
first check the main filter capacitors. Also, inspect the circuit board and
all connections for any cold solder connections.

If there is buzz when it is connected, then check to see where their is a
ground loop, or an opened ground, or if there is a defect in the
interconnect, or the unit driving it has a problem.

If there is a steady buzz and no sound at all, it is possible that there is
a defect in the output stage, and or if it is using a dual polar power
supply, one side of the supply may not be working.

If you find a bad output transistor, it is very common that the drivers, and
main bias resistors to the outputs are also damaged. They key is to
carefully check all the components involved in the circuits that are
effected. If there is a defective(s) part that is changed, and there are
other failed components, it is possible that the new part(s) will be damaged
when the unit is re-powered back up again. Power output stages in audio
equipment can be very tricky to service.

A common cause of output stage failure in many of these units are that the
users tend to drive them much too hard for their power rating, thus
overheating the output stage and power supply components.

--

Jerry G.
==========================


"Reactor" bruce DOT gettel AT myactv DOT net wrote in message
...
Hi,

I have a budget powered subwoofer wtih a 100W amplifier. Right out of
nowhere, the sub started to have a constant, low-pitched hum that ruins the
whole home theater experience.

My thought is that one of the power output transistors has failed. What are
your thoughts and/or how can I troubleshoot this to identify the right
component? I have a decent Fluke meter and understand the basics of how to
use it, but I have been away from electronic repair for about 20 years.

Thanks for any help.