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Precision Acoustics Subwoofer Repair!

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Techyboy

New Member
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.
 

creakndale

New Member
Thank you for posting your detailed troubleshooting sequence. I have 3 electronic devices that need repair. I expect the problems to be with capacitors.

creakndale
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I don't mind constructive critism or even destructive for that matter. The guy has no scope, no audio generator, nor a Variac. It was also determined that the driving system has DC on it's outputs.

Initial problem was "thumping". Probe slips, blew up the amp. Agreed, I was sidetracked.

There is no bias pot. At the last fix, bias was too high and was disabled, and there was hum. Then the outputs blew again because of no heat sink.

Output stage is disabled, until I/we make some sense out of the MUTE signal even if it means increasing the start up delay.

This is the guys first repair and it's a learning experience. An AMP is not really something to learn on. I commend him. I've made the off that when he gives up, I'll fix it. He also said that it's not a good amp.

Two of my worst repairs:

My first: I built a 100 W "Leach AMP" and made a mirror image of the board. I was able to salvage it.

Another: A 500 W amp that was used in a disco. The outputs were shorted by a piece of wire and the components had puddled on the PC board. It had gone to 5 shops before it came to me. I made changes to the amp so it would not happen again.

An another. the amp kept coming into a shop for repair for the same problem. It was given to me to fix. Added parts (varistors on the supply) and it didn't come back again.

I got sick and tired of a piece of equipment blowing up at work, so I finally went at it big time. This thing had a 3 phase 50 Amp supply and generated 15 kV at 1.5 amps that was shunt regulated by a tube. The regulator circuit kept dieing. This thing had a 1 Meg 200 W resistor for a bleeder and had a voltage divider made up of about 7 power resisistors of about 500K each. I replaced what was out of spec and got out the Locktite 222 and it hasn't blown since. It's been at least 15 years now. Basically it's the power supply for a large electron gun which can melt metals. I did virtually the same thing to a 100 kV 0.1A X-ray power supply and it's been quieter than it's ever been. We got a used 3-gun version and all of the wires were either cut or removed from terminal strips. All they had to do was to pull a connector and undo about 6 bolts for the High voltage and ground. That system needed 70 Amp 3 phase. No one thought I would be able to get it to work. In the end, the 3 hearths were defective by design and really need to be replaced at about $3500 each. The system was still useable.
 
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Techyboy

New Member
I was able to fix the sub using only a DMM because that is all that was really required, at least in my case. My criticism of your approach stems from your lack of using a logical sequence or progression to diagnose the problem. Somehow, this gentleman had a working sub except for the thumping noise upon power up. Now he has a unit that doesn't work at all and from the sounds of it has almost replaced half the components on the board(with your guidance) and he is no closer to a solution than he was 2 weeks ago.

Shorting components out with live power applied is totally unnecessary, and i'm frankly surprised that you would even suggest he try this considering he is a newbie. Hell, even if you're experienced there's no need to use this approach. Last i heard, you were also mentioning about cutting traces on the PCB to further isolate. Complete insanity!

All these suggestions and carpet bombing replacement method is an extremely poor and ineffecient way to troubleshoot and repair any electronic device. Using your approach and the type of logic you use, if for example his problem was at the output(dead) you would be telling him his problem is at the input stage.

You are guessing and really have no idea even where the original problem may have been and you are continually guessing now. If you don't know, why don't you be honest and tell him "Hey i have no clue dude?"

I can tell you from my own experience, i only change components that are bad, i don't guess. The least you can do for this gentleman is issue him an apology for the poor advice given.

Lesson learned from all this......be wary of who you listen to on anonymous forum/message boards. "Confucius say because someone know Ohms law and can calculate voltage dividers does not make good tech"
 

lemofus

New Member
Hi,

Thanks for the advice, I do have a few points to clarify though. I was fully aware of the risks involved when I started fiddling with it in the first place. It was a "make it of brake it" type situation, I was not content using it as it was. It is a cheapish unit, nothing fancy, and I am using this as a learning experience. I have worsened things quite a bit, by being clumsy, accidently shorting things whilst doing measurements etc; since then I got some better tools (ez hooks, good multimeter, much better soldering iron - incredible just how much simple things like that can help) and have learned to be more posed and careful during any tests. I completely understand the risks of following some "unknown" person's advice, via the internet, going back and forth with tests and measurements etc. But I am really grateful for the time and help. Granted, so far I have made things worse, but I'm learning a heck of a lot; I knew nothing and I am getting comortable testing transistors, understanding a bit better how the circuit works, what the components are and do etc. And this is priceless! I won't be an electronics tech in this lifetime, that's for sure, but it's always good to learn.

I guess rule number 1 should be "If you don't know what you're doing then don't try", but there again how would any of us learn anything in life? I broke my fair share of things in the past, from bike parts to computers, but this is how I got comfortable with them. I guess I enjoy the DIY and the learning experience.

I appreciate the advice; if you do have any ideas about what to look at, what I'm doing wrong, what should be done etc, please feel free to share those; the great thing about the forums is that we can have multi-party discussions and meet people with all sorts of backgrounds and experiences. I try to describe my issues are accurately as possible, take pictures, send schematics etc - please let me know if you do have any questions or would like me to do some specific measurements with the tools I have.

Of course my last resort would be to give up, get a new amp for the sub, or bin the lot; but it's just not as much fun and at the moment I have a fair bit of time on my hands...
 
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