Hum in output of powered subwoofer

[spooney]

Not sure if this is the correct forum so please forgive me if I am wrong in posting here. I believe my issue lies somewhere in the power supply of my powered subwoofers amplifier. For a little background I have a ten inch Triad powered subwoofer that I recently acquired for the large sum of 5 dollars. The original owner stated that the subwoofer no longer played audio but had an annoying hum through the subwoofer driver. After inspecting the subwoofer I found that description to be spot on. Regardless of input no audio is produced other than that nasty hum. Unfortunately there is no model number on the sub. All I know is that it is a ten inch driver in a 13 inch square cabinet,built in 98-99,and there is a sticker on the board that says "Triad 9.1 Amplifier" . Google searches for the amp model number haven't uncovered anything useful. There is dc voltage present on the speaker leads(4-5 volts) and the rail voltages look a little odd. I have +63 volts and -40 volts on the rails. One of the large filter caps has a slight bulge to it but I am not sure if my entire issue is with the cap or if something else caused the stress on the cap and caused it to bulge. I am not new to electronics as I have repaired several car audio amplifiers but when it comes to home electronics I have very little experience. I was hoping somebody here might be able to point me in some directions or give me some advice as I'd like to get this thing booming again.

[DSP_Geek]

Is the bulging cap on the -40 v line? If so, that's your problem. Note that many DC voltmeters won't respond to the ripple component of an unfiltered signal, so -40 v is about what I'd expect from reading a rectified signal.

One of the rectifier diodes might be leaky as well, which could've hosed the cap. A scope would go a long way to solving this problem.

[spooney]

i've got a scope. I am ordering new caps as the ones in this amp were both bulging although one more so than the other. I didn't get a chance to check and see if that cap that was really bulging was on the -40 volt line before I removed it. When you say I may have a leaky rectifier diode are you referring to the bridge rectifier connected to the transformer? I will be sure to order up another one of those as well if thats what you are referring to but I think I did verify that it measured correctly....it escapes me right now. I did most of this late last night.

[DSP_Geek]

Yes, I meant the bridge. Just in case, however, you might want to check the idle current draw through the caps alone if that's possible, then hook up the amp and verify that. It may not be possible if everything's on one circuit board, unfortunately.

[spooney]

Most of the components are all on one board except for the transformer and the preamp board.

[jitter]

The hum is probably caused by too much ripple current. Like DSP_Geek I bet it's on the same line as the bulging capacitor was. It may just be the capacitor that's causing it (too little capacity to smooth out the ripple) or, as has already been suggested, the capacitor failed because a bridge cell is faulty.

When you say I may have a leaky rectifier diode are you referring to the bridge rectifier connected to the transformer?

A bridge rectifier is essentially four diodes in one housing, so yes.
You could use a DMM to measure the usual 0.6-0.7 V diode drop voltage when measured in forward direction. If all four measure that voltage, it's most likely OK.
A scope would reveal excessive ripple on the faulty line.

[spooney]

When I receive the new caps I will put the oscilloscope on it and see if there is anything crazy going on still. I may have to check back in with you guys to get this licked. Thanks for everybody's help.

[spooney]

Installed the new caps and all is well with the subwoofer. Not a bad sounding unit at all.