LM3886 - getting hot - low volume

[diarav]

I am in the process of building a 6-channel amp using LM3886. the schematic is from ESPs site http://sound.westhost.com/project19.htm . I made my own PCBs (3 channels on a single board).

5 channels are working fine (good output sound, heatsink just gets mildly warm, chips dont feel warm on touching..)

But one of them (same schematic, layout as the 5 above) actually gets hot within 15 to 20 secs of power on. While testing this the output volume suddenly dropped, and has been so since. I still have 2A fuses intact on the power rails.

I checked all components and they were soldered correctly. I dont see any short circuit, dry solder on the board.

Possible causes:
1) Possible damage to any resistor , capacitor during soldering (I am not very skilled !!).
2) Defective components as such.

Questions:

1) What is the first area / component to check/replace as the possible cause for this problem.

2) is my LM3886 chip possible damaged? I am still getting an output.

[kuldeepsingh]

is the pcb used is same as for the other 4 channels, just have a check on pcb tracks and whether any possibility of deburrs is there. And also check whether all components are well soldered.

[diarav]

Yes, the PCB layout is same as other 5 amps. I checked closely for solder shorting, discontinouty and did not find any.
Need to know what actually can cause the chip to heat up so fast.

[BWRX]

Originally posted by diarav
1) What is the first area / component to check/replace as the possible cause for this problem.
2) is my LM3886 chip possible damaged? I am still getting an output.

Before replacing anything answer a couple of questions.

When you tested the amps did you have a source connected to the input? If not, was the input left open while you were testing? Have you tried shorting the input to ground to see if the chip still gets really hot?

Did you use a load on the output while testing?

If you can't find any errors in your wiring or construction then there is a problem with one or more parts. Check the polarity of electrolytic caps to make sure they're installed properly. Snoop around with a multimeter and see if you can find any unusual voltages. Are the supply rail voltages at the chip's pin correct? What is the DC offset at the output of the bad amp? Things like that.

[diarav]

Thanks for the response Brian.

Chip gets hot even if input is open or connected - never tried shorting to ground.

Output was connected to 16ohms speaker.

Caps are connected with correct polarity.

Ah I did not check supply voltages? Perhaps will.

Question:
To check DC offset at output, does one short the input to the ground or leave it open?

Also I have currently disassembled the amp from my casing as I need to work on my casing, so it may be a while before I reassemble and test and post my results.

[BWRX]

Originally posted by diarav
To check DC offset at output, does one short the input to the ground or leave it open?

Short the input to ground when measuring DC offset at the output.

[fibbrilator]

Dear Diarav,

Did you solve the issue? Was it oscillating? just curious.

Regards

[sukyone]

Your Lm3886 Chip are LM3886T Or LM3886TF?
if LM3886T please Check TAB of chip is not connect to ground.