Clarion car stereo problem - bad Sanyo LA4743B chip?

Hi everyone. I'd like to first introduce myself - I'm a recent graduate from the University of Kentucky with a BS in EE. I'm now working as a Controls Engineer for Cummins, at the plant that produces most of the Dodge Ram diesel engines. I like to mess around with odds and ends, and have a problem with a car stereo amp:

A few years back, my Clarion car CD player went on the fritz. It sounded ok when the balance and fade were set to power only one speaker, but otherwise, it sounded like every speaker was blown. When hooked up to an external amp, however, the sound was fine. But something wasn't right.

My replacement stereo was stolen this week, so I drug the Clarion back out. I drove to my friend's place so I could run the preamps to a set of computer speakers to make sure it sounded fine, which it did. However, I noticed something disturbing - the unit's internal amp was extremely hot - after two minutes, the stereo's casing above the amp chip's heatsink became too hot to touch.

Tonight I took the thing apart and found the amp chip - a Sanyo LA4743B. It doesn't appear screwed up, and the other components seem normal.

Does this sound like it is indeed the amp chip, or might it be something else? Is there an easy way to test it out?

Any help would be MUCH appreciated!!!

Thanks everyone,
Brian
 
Do you have access to an infrared temperature "gun" at work? I have found those to be useful in isolating overheating components in the past. Of course, your finger will do fine as well. :)

Of course, just because the chip is hot doesn't mean it's dead. And if you're testing it on the bench, make sure it's properly heat sunk if you're playing loud music through it -- the case is probably the heat sink in this case, so SOME heat is normal.

I'm no amp repair wizard, but I have had good success in the past by applying an input signal with an audio signal generator, following the waveform through the amplifier with a 'scope, and seeing where it stops (or gets horribly distorted). Of course, for this to work, you have to get the amp to fail on the bench. Then, verify that the component you suspect has proper inputs (i.e. no shorted caps, proper power supply voltage, etc) and replace it if need be.

First thing to double check -- make sure that the load you're driving is 4 ohms or higher (I'm assuming that it's a single chip in that amp), that the supply voltage is correct, and in the case of bad sound (instead of just plain overheating), check that your bench power supply can deliver enough current.

There is a spec for the amplifier IC in question here: http://katalog.elektroda.net/download.php?id=37101

Incidentally, the spec sheet seems to indicate that the chip will work fine up to 80 or 100 degrees C, but I wouldn't expect it to dissipate much heat unless you're really blasting the tunes. If I'm reading the sheet right, it lists max power dissipation (Pd) as peaking at 40W when output is 10W, it actually drops after that. At small output values, the Pd is around 10W.

Wes
 
All of my testing was performed in the car, hooked up as it would be in real life. At the time the amp wasn't driving any speakers, but when it is, they're 4 ohms each. Yes, it's just one chip in the stereo.

I had a new question - does anyone know if the preouts are controlled by the amp chip in any way? I figure since they're called "preamp outputs" then they're not, but I dunno.

I'm asking because I'm considering just snipping both Vcc+ pins, cutting off power to the amp chip. I'm hoping it'll leave the preamp outputs untouched, and the stereo itself won't notice. It's easier (and will likely sound better) than replacing the chip.

Any thoughts or suggestions on this?
 
Looking at the datasheet it requires very little in the way of external components. You may want to check the power feed to the chip but more than likely the chip has had it.

It's highly unlikely that the preout stage will be affected by this chip, there may be some loading on it but thats it. You should be able to remove it and the rest of the unit will work fine.

http://pdf.alldatasheet.com/datasheet-pdf/view/40147/SANYO/LA4743B.html is a link to the datasheet.

It may be worth removing the chip and constructing a small test rig to test just the chip itself. If it performs fine out of circuit you know the fault is in the head unit. If you can get an oscilloscope, try seeing if any of the outputs has high frequency oscillation. If they do it's possible some internal compensation capacitors have failed (see the data sheet for more details on this)
 
I think I've 'fixed' the Clarion. I took the Dremel tool to the amp chip this evening, taking out the two Vcc inputs and the +5V standby, just for the heck of it.

My bench testing (consisting of a 12V, 1.25A AC adapter and attaching the preouts to my Aiwa stereo) basically confirmed that the amp is now disabled and the sound is not affected. When I attempted this setup before, any volume over 12 on the knob or any decent bass note would cycle power to the stereo. Now, I can turn it to full volume with no problems whatsoever. The multimeter confirms that there's no voltage coming out of the speaker wires, too.

Tomorrow I'll do a test in my vehicle to double check, but I'm pretty confident it's ok now. Well, ok for my needs, anyway.

Thanks everyone for your help!!