My 4 channel car Amp distorting on 1 channel (large pics)

[MikeHunt79]

First of all let me say, Hi everyone. First post and all. I must admit, i'm a bit of a n00b when it comes to a lot of this stuff, so go easy.

Anyway, here goes: My car amp was distorting when the volume turned up only a tiny bit, on 1 only of 4 of the channels... I think running it at 2ohm instead 4ohms that probably did it. So I took it apart, and apart from the burnt looking blackened mess in the bottom left corner, it looks great.


Now... I need to fix it. I reakon one of those chips has overheated, and that's why there's black stuff everywhere.... The chips by the black stuff say "irfz44n" on them, so I looked up the datasheet for them, and they are something to do with the MOSFET power supply. This is as far as my knowledge of amps goes i'm afraid.

I'm about to order some more irfz44n's, as they're not too expensive, hopefully replacing them should bring the amp back to it's former glory. Do any audio/electronics guru's have any thoughts on what else could be causing this, or what other parts I could get, since the postage is currently gonna cost more than the irfz44n's themselves. Anything would be helpful as I am just making assumptions and might be replacing perfecly good semiconductors here.

[JOE DIRT®]

MikeHunt...Invest in a meter before you replace anything or borrow one

there should be fuses inside the amp....did they blow also?
other than that I would wait before ordering anything and do some prelimanary testing

For the record most inexpensive 4 channel amps use the same power supply as their 2 channel clones do.... so they cannot handle the lower impedance......until you get into competition gear that have separate suppplies


DIRT®

[MikeHunt79]

Thanx for the reply!

Originally posted by JOE DIRT®
MikeHunt...Invest in a meter before you replace anything or borrow one

Yep, I've got a couple of meters (and I have access to one with an occilliscope), but didn`t really know what to measure and where to measure it. The amp still works ok, it's just that the "R1" channel starts to clip very harshly even at a really low volume. The other 3 channels (L1,R2,L2) sound great. Would it be worth putting a sine wave through R1 and looking at the resulting wave on the scope? Also, it's worth mentioning I tried running R1 and R2 at 2 ohms, I've always run R2 and L2 at 4 ohms, before I had these problems.

Originally posted by JOE DIRT®
there should be fuses inside the amp....did they blow also?
other than that I would wait before ordering anything and do some prelimanary testing

There are 2 30 amp fuses, and both are fine. The amp is even meant to have "thermal overload protection", but it doesn`t look like it's worked in this case. I'm tempted to try swapping some of those chips that look blown around (there's 8 of 'em), and see if it affects different channels. (?)

Originally posted by JOE DIRT®
For the record most inexpensive 4 channel amps use the same power supply as their 2 channel clones do.... so they cannot handle the lower impedance......until you get into competition gear that have separate suppplies

Ahh, ok... Thanx for the tip. I think i'll take it easier on this amp from now on, if I ever fix it.

[thylantyr]

Mr. Jedi in training

I prefer the brute force method of amp repair.

For a few dollars, replace the power transistors (ones mounted
to the heatsink) on the affending channel.

You can trace the wires and find the section of the board
that corresponds to that channel.

If the other channels work, the power supply is fine.

Order extra transistors because shippng charges may
be the biggest cost. /heh/

Then proceed to step 2.

Step 2.

Is it still bad after transistor swap? then you need to dig
deeper like South Park using the probe.. small signal issues
perhaps... worry about that next.

If you are electronics saavy, you can skip the brute force
method and DMM the power transistors after
you unsoldered them from the board - based on readings,
is the transistor bad ?

That amp driving 2 ohms seems a bit ambitious, only
one transistor per rail, 4 channels.. yikes...

[Alain L]

One of the output transistor is probably faulty. Just take a voltmeter and with the amplifier on verified the dc voltage between emitter and base of the output transistor. It should be around 0.7 volts. When you will find something different there si your faulty transistors.

P.S. Base and emitter are the exterior pin of the transistor. the one in the middle is the collector. be carefull not to touch or short to any others parts or pins.

[MikeHunt79]

Thanx for the replies guys. (would've replied sooner, but I've been on holiday)

I forgot to mention, this poblem has always been intermittent, I decided to take the amp out after a week of it getting worse.

I tried putting the amp back in, (it was a nice day, so i'd thought i'd test it) and it now seems ok.... the only thing I done was to move the transistor on the left (the one in the 2nd pic) further over to the left, since it looked like it was shorting out on the heatsink screw...

If it starts playing up again, i'llput some of these ideads into action.

[mwmkravchenko]

Your first pics showed some crap around the electrolytic caps (the blue ones)in the power supply. As a bit of precaution you could replace them too. They might have a leak.

Mark

[Rarkov]

What is the make of your amp? To be honest - for a product that is designed to be bounced around, it looks like an accident waiting to happen! Rats nests like that are fine in domestic amps but not cars. I was reluctant to P2P wire a gainclone for my Sister because she is taking it to France with her in hand luggage!

Gaz

[MikeHunt79]

Update:

Replaced all four of the power transistors, and for a minute or so after I powered it up, it started doing crazy things on the other channels, but it settled down after a few minutes of music... It's back to it old self again, which I'm really pleased about.

I'm really glad it's sorted, thanx for all the help and advice guys.

mwmkravchenko: All the cap have that black goo... I doubt it's electolyte, I think it's just there to stop the caps from vibrating.

Rarkov: Dunno the make, it doesn't say who makes it at all (not even on the PCB!). Do you have any tips on how I could sort out the rats nest... Use cable ties, insulation tape, or something like that? Also, what p2p wiring? (please excuse my n00byness) I'd like to prevent any short circuits or the like, as this amp is the only thing in my car powering my speakers (i have no headunit).

[Rarkov]

Yes, I'd clean up the wiring a bit, and create a nice wiring loom (tie them all together and make 'em neat). Try to avoid them sitting over hot areas like the output transistors. You could even make two looms. One for you power supply and another for low voltage stuff!

P2p stands for Point 2 Point and it means wiring a circuit without a PCB - just using the components' leads. My concern was the quality of the work on the PCB and the filter on the power input. However, as long as it doesn't shake around, it'll be fine.

Gaz