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Old 2nd April 2014, 09:09 PM   #1
diyAudio Member
 
Join Date: Dec 2008
Default Help repairing Sub AMP

I have an old "inexpensive" 10" Infinity BU-80 subwoofer that I would like to repair.

It ain't much, but it used to woof a little.

I plugged it in the other day and just got some music modulated scratching out of it. Not very loud, just barely making scratching corresponding with the input.

So I pulled the plate off the back and "looked" around.

Thought I would replace the Caps on it and see what happens.

Then I noticed the little circuit that you see on speaker outputs. I have forgotten what it is called, but it's a cap and a resistor. The resistor went out with a splash at some point. I don't know if that is the root of the current problem, but it can't be helping!

Since the cap is "blown" I don't know what to get to replace it.

The main power caps have some tops that feel a little bowed but are intact. I thought I would replace those as well.

I shot some pix, I'll link the main larger pix as well.

1) What type and size cap should I get for the speaker output?
2) What brand caps would you recommend for the power caps? Something reliable and cheap.
3) Any other suggestions besides donate to a landfill!
4) Would that little blown cap, cause all the problems?
5) What is that circuit called on the speaker output? Cap and resistor between + and -.

Click the image to open in full size.

Click the image to open in full size.
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Bigger Board PIC Click Here
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Click the image to open in full size.
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Bigger Cap PIC--Click Here
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Old 3rd April 2014, 09:29 AM   #2
esgigt is offline esgigt  Netherlands
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Join Date: Dec 2013
Hi Les,

I hope my questions and remarks can be useful to you.

1) If you do not have it already, here is the Service Manual: http://akdatabase.org/AKview/albums/...0%20Servic.pdf

2) you wrote about the burned component at the amp's output. That's not a resistor but an electrolytic capacitor in the filter circuit. So, something might be very wrong, which is also indicated by what you hear.

3) "Bowed" electrolytic capacitors, have failed... This bowing is caused by heating up. And heating up might result in blowing-up... They must be replaced.

Can you tell me if there is DC over the speaker-terminals? And if not, is there AC output with the sub's input shorted?
I also like to know if the amp module (S53AMI) gets hot?

In the SM there is a reference to a Service Bulletin (#INF2000-01) which describes the repair. Also keep in mind that the output of the amp MUST be loaded when under test!

Don't forget to check the tracks on the PCB if they are not damaged....

Last edited by esgigt; 3rd April 2014 at 09:42 AM.
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Old 3rd April 2014, 05:01 PM   #3
diyAudio Member
 
Join Date: Dec 2008
Quote:
Originally Posted by esgigt View Post
Hi Les,

I hope my questions and remarks can be useful to you.

1) If you do not have it already, here is the Service Manual: http://akdatabase.org/AKview/albums/...0%20Servic.pdf

2) you wrote about the burned component at the amp's output. That's not a resistor but an electrolytic capacitor in the filter circuit. So, something might be very wrong, which is also indicated by what you hear.

3) "Bowed" electrolytic capacitors, have failed... This bowing is caused by heating up. And heating up might result in blowing-up... They must be replaced.

Can you tell me if there is DC over the speaker-terminals? And if not, is there AC output with the sub's input shorted?
I also like to know if the amp module (S53AMI) gets hot?

In the SM there is a reference to a Service Bulletin (#INF2000-01) which describes the repair. Also keep in mind that the output of the amp MUST be loaded when under test!

Don't forget to check the tracks on the PCB if they are not damaged....
Thanx! I'll look over this document very carefully. Looks like this is a common malaldy.
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Old 21st April 2014, 04:19 PM   #4
diyAudio Member
 
Join Date: Apr 2014
Is it possible to buy a replacement PC board?

I have the same BU-80 subwoofer. Its been great and never played it loud. It started a load humming noise recently. I unplugged the speaker wires and tried it on several different power plugs in the house. As soon as it is plugged in the unit just makes a loud humming noise.

It it worth fixing? I will take it apart and see if I can see anything obvious but I have no clue. I could probably figure out how to replace to entire board it they sold the replacement board.
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