Question about US Acoustics USX-4085 Car Audio Amplifier

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No sound out of it, took it apart and found one of the Irfz44n mosfets burned up and the resistor right beside it burned too, What would cause this?Thanks

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Hello rjhunting,

Most often the output MOSFET's and associated circuitry will burn out when the amp draws excessive current. And excessive current draw is most often caused by operating the amp into an impedance that is too low. Of course, depending on the age and actual use, the amp could have failed from normal use. The US Acoustics USX-4085 is a relatively good quality low-end amp that was manufactured with excellent components.

All of their amps manufactured after 2001 carried a 3-year warranty. But when operated in "tri-mode", with normal speakers and a subwoofer, the subwoofer minimum impedance rating was 8 ohms. And the normal speakers should be operated through a high pass capacitor while the sub needed a 15.9 mH low pass filter inductor.

The MOSFET's and resistors can probably be replaced at a qualified electronics shop. If you are somewhat handy with a soldering iron and have a controlled temperature soldering station, you can do it yourself. Parts are available from: and

Hope this helps.

Posted on Sep 28, 2009


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The problem is that the driver circuit is also damaged. Try powering up the unit with the output transistors disconnected. Measure each pad and report the results here. You should see something like 0.6,B+,0.0 on each set of three. Some will be negative rather than + (-0.6,B-, 0.0).

Keep us posted.

Jul 03, 2008 | Car Audio & Video


Bantam BC6 battery charger analyzer Break Down fix

**Edited 1/18/10 - Additions at bottom**
I had purchased a used, working well, Bantam BC6 battery charger/discharger/analyzer... quite a fancy & high-tech unit if I do say so myself.
Well about 3 weeks ago when I was doing a multiple charge-discharge test on a battery pack, as part of the discharge cycle, the unit beeped, stopped the discharge, and displayed "Break Down" on the screen. Looking that up on the bantam website and/or related forums, the recommendation is that this thing should be sent back to Korea for repair... not happenin.

So, what I assumed, is that the main heat-source in that whole unit was a transistor labeled IRFZ44N, and that it had somehow gone bad.
On ebay I typed in that part number and got a package of 5 coming this way.

With a few minutes of soldering activity I had the old, dead transistor removed from the board, and soldered in the new IRFZ44N transistor.

What to do & What to look for:
Disassembly - something like 5 screws located on the right-hand face(side where positive & negative charging cables and li-ion charging interface come out from) need to be removed and then the bottom of the unit can be slid out from the rest of the case. After the bottom is removed, you should notice two similar looking objects sticking up from the bottom of the board... these are both transistors. One is a 5 Volt voltage regulator which is located towards the center of the board. The other is THE transistor that you are looking for, the IRFZ44N - it is located near the circuit board edge and is closest to the panel you unscrewed first(assuming you unscrewed the right panel).
Orientation is critical in the re-installation/soldering of the new transistor... the back & shiny face of the transistor should be towards the black/negative charging receptacle; the black & bulging side of the transistor should be towards the red/positive charging receptacle.
Once the bottom is slid back over the newly installed transistor, try to make certain that the back of the transistor is in contact with the blue-ish thermal pad, and that is in contact with the metal case bottom after you slide it back in.... that is this transistors only way to dissapate the heat.

If this transistor fails and the unit breaks down again, I can picture myself setting up a more active method for cooling it... a fan or larger heatsink or something.

**1/18/10 Edit Addition**

Well it was initially embarrassing to discover that although the BC6 appeared to be functioning flawlessly, given a full day of discharging a battery pack... it was obvious that it was not discharging the pack(the voltage never dropped).
With a little Ohm-meter testing it was discovered that a 0.5 Ohm resistor which connects to one of the legs of the IRFZ44N burnt out; the resistor was reading over 10,000 Ohms. The resistor which burned out had R500 on it, and after testing the two R100 resistors next to it with an Ohm-meter and finding out that they are 0.1 Ohms, I made the assumption that the R500 should be 0.5 Ohms.


The picture shows the location of the burnt out .5 Ohm resistor(circled in yellow - right side of picture) and the part number & specs of the mouser-ordered replacement part(yellow rectangle around the area).

The dimensions of the resistor(original one and the one I replaced it with) are: .125" x .25"; the height doesn't really matter much, but it was .065" or less. The printed label on the new resistor is R50F - not that it matters that much, the 0.5 Ohms and 1 Watt power handling matter more than the label.

Since I haven't invested in any desolder tools, I ended up using two soldering irons, at once, to melt the solder on both sides of the dead resistor and get it off the board before things cooled.

Now, and with a few days of testing for verifications purposes, I can say the new 0.5 Ohm resistor has allowed this very useful BC6 battery analyzer to function like it originally did and always should.

Here's a picture of the original R500 resistor...

on Jan 11, 2010 | Kitchen Appliances - Others

1 Answer

I have a beringer 5000 mixer and can not get no sound from it. it's like the speakers are not getting power

Oh, oh... got one on the bench now... There is a design defect that causes a short and destruction in the power amp portion.

Do a test: Plug headphones into the main 1/4 inch jacks. The sound won;t be very loud since these are line outputs, not for headphones.

IF you get sound out those, then one or both power amps are dead.

Worst case: if one amp is dead, several parts including three power MOSFET transistors are fried for any of the dead channels.
The failure of the amp can wipe out the main power supply. If you effect windows light up, then the small power supply is operating.
If the big supply is fried, there are two large MOSFET switching transistors that are gone. Often several smaller transistors and dides and resistors and one switching IC goes. MOST of these are surface mounted parts and would be beyond the scope of what you can repair.

I am trying to contact Behringer about the flaw but haven't heard back yet.

The nature of the flaw is this: The is insufficient clearance on an internal circuit layer from the edge that fits into a slot in the power amp heatsink. Any humidity that enters can cause the insulation to breakdown and arc to the heatsink causing a chain reaction.
I repaired one of these already before finding the cause, but this one had a smoking gun! I just yesterday milled out the slot deeper and wider and will put insulating tape to prevent further arcing. The arcing that had occured burned away the internal layer and the connections. I will replace thoose with hardwire.

Repair of this problem is time consuming to weed out all the blown parts and to safely bring the unit back to life.

The current unit I am working on took out ONLY the power amp. A 30 milliohm resistor went up in smoke making a real mess. All three power MOSFETS are gonzo and a zener as well... don't know what else I will find. The previous unit was worse as the power supply was wiped as well.

Feb 06, 2010 | Behringer Pmp3000 16 Channel Audio Mixer

1 Answer

Burtn out resistors on a quantum audio q 700.1

The resistors are probably burnt because of the audio mosfet transistors going out. you will have to test them with a meter. don't just replace the resistors, they will burn up again and maybe cause more damdage

Jan 29, 2010 | Car Audio & Video

1 Answer

HI I have a FSP200-61GT SPI switching power supply with a burned out Mosfet on the high voltage side. There is a (I think)a .5 or 1 watt resistor on both sides of the mosfet. But they test good, my problem...

I think it's usually about 820 - 1K Ohms... It;s the emitter resistor.In saying that we generally don't repair these SMPSU's anymore as they are so inexpensive these days like NZ$45.00 for a 350 Watter. Also 80% of the time the repaired supplies fail again shortly after due to the stress put on the other components, by the initial problem.Good luck.

Jan 27, 2010 | Computers & Internet

1 Answer

Apex GT2715 TV, burned resistor

Wire wound 190 ohm 10 watt should do the trick if that's all that's wrong with the set...accordianman

Feb 02, 2008 | Apex Digital GT2715DV 27 in. TV/DVD Combo

1 Answer

Dell Latitude C540/C640

Wow Buddy. I'm impressed with your work here.
I have seen many boards which were oem, which had what appeared to be 'add on' circuitry. It's common when a problem arises in a product, which they've already had a million of manufactured. Otherwise, maybe the NSA, CIA, or MA Bell doing a little snooping?
Ok just kidding.
Assuming the circuitry which was replaced was still there, I've experienced a couple of "no power" issues on these models.
Bad power board.
AC adapter socket unsoldered from board.
Dead short between motherboard and palmrest/case.
Bad memory.
Burned capacitors.
Best thing since it's apart already, just order another board and you'r good to go.
I sell the same board at my shop for 75-100.00 US.
If the processor's good, then you should be good to go.
Good luck

Jan 22, 2008 | Compaq Presario X6000 Notebook

1 Answer

Component for amplifier

All of the components with the IRFZ44N part numbers are the same. The rest of the information is the date/manufacturing code and isn't really important (although they should match if they were all used in the power supply). Digikey has them. The new lead free version is IRFZ44NPBF. The rectifier ->

Sep 17, 2007 | Audiofonics ADF-243 Car Audio Amplifier

1 Answer

Parallel connection of power mosfets with two of 5 smoking

The IRFZ46N are n channel high speed switches . They basicly drive the Mosfets. Sounds like you have some problem there my friend, and unless you paid alot for this amp. Its going to need probobly all the n channel switches as well as every mosfet + the bias resistors repalced. Before plugging in the amp jump out the fuse with a 100 watt light bulb.. If the bulb glows brightly when you apply power there is still a short in the unit and could be in the switching power supply section of the amp. If it glows bright and dims, The shorts are most likly gone. The bulb will minimize the amount of current the unit can draw and most of the time wont let the new parts blow out. Check out this service number and see if they can help you. Good Luck Main support number 978-823-0300

Jul 26, 2006 | Pyramid PB779X Car Audio Amplifier

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