Amplifier problem

[foolange]

My home theater sub goes to full xmax and hums as soon as power is applied. I've tested the speaker on another amp with no issue. I'm fairly new to troubleshooting electronics but would like to repair the amp, if possible. My main goal is to learn from this project but also repair the amp spending as little as I can. If I have to replace the plate amp then it will just be a parts unit as it's not worth putting that much money into. Would anyone be willing to share some tips of where I should start looking? If this belongs in another area please move it. Thanks in advance!


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[b_force]

What kind of hum is it?
50/60Hz or 100/120Hz?

Is the hum also there with no inputs attached?

[Anks]

This is normally a sign of dc on the output as the speaker is being held in one extreme. Depending on how long this was like this (sometimes can only take a second) the speaker coil could also be damaged.

First thing to do it measure the leads going to the speaker without it connected for dc. Anything over 500mv isn't good and should be more like 10-20mv at most. Going of the description I would expect you will see one of the power supply rails on the output.

After you have done this it's time to work out what sort of output section is being used. It will either be discreet transistor or a chip amp more than likely.

Hope this helps.

[testian]

Also check the DC power supply rails.
The problem my creative subwoofer was a broken connection of the smoothing capacitor and this lead to a loud humming at the speaker.

What type of home theater sub do you have?

[foolange]

I'll have to check the label for the part number but it came from the Boston Acoustic MCS 100 home theater set. So it's not a high end sub. I picked it up for $10 knowing the shape it's in figuring I could use it for a test enclosure. Or reuse the 8" sub if it hasn't got permanent damage. I've just applied power once for a second and the hum shows up immediately with no source plugged into it. I bought it more for tinkering with.


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[Armadillo]

Q1: Next to the volume control, there is a LED, what colour is it when you just power the sub ON without signal input connection? Wait for a while, does the LED change any colour?
Q2: Is there a switch "ON / AUTO / OFF" on your unit? If yes, put the switch of OFF mode, is there still any hum?
Q3: If you are able to open the sub, jumper the centre leg of the volume control to GND [not -15v, or +15v but the GND], is there still hum?
While you are at it, clean the volume control with contact cleaner like "DEOXIT".
Q4: Connect the sub to a proper input [from your receiver or whatever], is there still hum?, Are you able to control the intensity of the volume by the volume control?