I wanted to post about a recent repair i did for a home theatre subwoofer. The complaint was a dead unit(no sound) except for the green led light turning on when cranking the power/volume control.
Once the unit was opened, you get a good sense of just how cheaply built/designed this particular unit is. Caps, pre-amp IC as well as power-amp IC all blotted out with some type of black ink/residue of some sort as well as glue smeared everywhere holding this together. Not a pretty picture, but i press on.
I decide to check the final output stage, so carefully scraping off the residue determined the part to be a LM1875T 25W Power amp 5 pin IC. On this part, there's a Vcc and -Vee. I check these values with the DMM and get a reading of 7.8V and -24.9V DC. Bingo. Vcc is too low to drive the amp.
I go to the power supply, and low and behold visually see the bulging caps on the secondary. I measure across the secondary 18V AC and what do you think the rating of the capacitors are rated at? Yes, a meager 25V which explains the bulging. I measure the zener regulator and reads only 8.3V confirming the cap is leaky.
Feeling i've fixed the issue i replace the 2 caps with a 4700uF 35V rating assemble everything back together, hook up my receiver to the sub, turn it on and surprise surprise..........it's still dead.
Open up again, plug in the AC, and measure on the power amp......Vcc and -Vee both read good at plus/minus 25V DC. I crank the volume and measure the inputs on the power amp.......read nothing. Looks like this unit has 2 problems.
Time to check the pre-amp stage. Again, scrape off the black residue and find the part# matching with the corresponding data sheet. Supply voltage is present but again no input voltage results in also no output. This particular unit has an adjustable low pass filter from 40Hz to 180Hz controlled by a potentiometer. I measure the pot and it reads open. Success at last. Open pot equals no input signal coming in. I remove the pot and determine its a 50K nominal value.
Change the pot, solder the wires back on, assemble the unit and plug it in and it's working.
The reason i posted this was hopefully to help others who may be having a similiar issue with their sub and give them a general guide as to how to approach the problem. This was accomplished without the use of a schematic and did not require me to jumper/short/bridge or replace parts using the carpet bombing method.
I feel for the gentleman in the other thread, who is learning by doing his own repair but is receiving very poor advice in how to diagnose his problem. It's definitely not easy helping someone over a PC without looking and measuring yourself, but some of the advice given just makes me shudder. I wouldn't touch that unit now with a ten foot pole.
That particular thread is 21 pages and counting.........
On a final note, i also wanted to mention a subwoofer i looked at in the past read completely open across the woofer itself. Since it was a small sub(6.5 inches) i ASSUMED the speaker was blown. I told the individual it was not worth the repair. 2 years later as it still sat in my basement, i looked at it again since i wanted to get rid of it and maybe sell the power amplifier section. This time i looked a little more closely and would you believe the problem was one of the wires coming off the voice coil was cold solder. Touched that up, and indeed started working again.
I've rightfully returned the unit to its original owner who was more than happy to take it back. Lesson learned...never ASSUME. I'm not perfect either, but at least i'm honest.
Once the unit was opened, you get a good sense of just how cheaply built/designed this particular unit is. Caps, pre-amp IC as well as power-amp IC all blotted out with some type of black ink/residue of some sort as well as glue smeared everywhere holding this together. Not a pretty picture, but i press on.
I decide to check the final output stage, so carefully scraping off the residue determined the part to be a LM1875T 25W Power amp 5 pin IC. On this part, there's a Vcc and -Vee. I check these values with the DMM and get a reading of 7.8V and -24.9V DC. Bingo. Vcc is too low to drive the amp.
I go to the power supply, and low and behold visually see the bulging caps on the secondary. I measure across the secondary 18V AC and what do you think the rating of the capacitors are rated at? Yes, a meager 25V which explains the bulging. I measure the zener regulator and reads only 8.3V confirming the cap is leaky.
Feeling i've fixed the issue i replace the 2 caps with a 4700uF 35V rating assemble everything back together, hook up my receiver to the sub, turn it on and surprise surprise..........it's still dead.
Open up again, plug in the AC, and measure on the power amp......Vcc and -Vee both read good at plus/minus 25V DC. I crank the volume and measure the inputs on the power amp.......read nothing. Looks like this unit has 2 problems.
Time to check the pre-amp stage. Again, scrape off the black residue and find the part# matching with the corresponding data sheet. Supply voltage is present but again no input voltage results in also no output. This particular unit has an adjustable low pass filter from 40Hz to 180Hz controlled by a potentiometer. I measure the pot and it reads open. Success at last. Open pot equals no input signal coming in. I remove the pot and determine its a 50K nominal value.
Change the pot, solder the wires back on, assemble the unit and plug it in and it's working.
The reason i posted this was hopefully to help others who may be having a similiar issue with their sub and give them a general guide as to how to approach the problem. This was accomplished without the use of a schematic and did not require me to jumper/short/bridge or replace parts using the carpet bombing method.
I feel for the gentleman in the other thread, who is learning by doing his own repair but is receiving very poor advice in how to diagnose his problem. It's definitely not easy helping someone over a PC without looking and measuring yourself, but some of the advice given just makes me shudder. I wouldn't touch that unit now with a ten foot pole.
That particular thread is 21 pages and counting.........
On a final note, i also wanted to mention a subwoofer i looked at in the past read completely open across the woofer itself. Since it was a small sub(6.5 inches) i ASSUMED the speaker was blown. I told the individual it was not worth the repair. 2 years later as it still sat in my basement, i looked at it again since i wanted to get rid of it and maybe sell the power amplifier section. This time i looked a little more closely and would you believe the problem was one of the wires coming off the voice coil was cold solder. Touched that up, and indeed started working again.
I've rightfully returned the unit to its original owner who was more than happy to take it back. Lesson learned...never ASSUME. I'm not perfect either, but at least i'm honest.