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SRMC 17th April 2003 04:48 AM

Can a bad cap kill an LM3875?
I built my first Gainclone from Thorsten's inverted LM3875 schematic and one channel works perfectly. However, the first time I powered up the other channel the chip sizzled and fumes came out approx. 3mm above pins 3 and 4 leaving a nice little crater. I have checked to make sure that I have it wired correctly and didn't wire any of the polarized caps backward (been there done that). At this point I can't figure out what went wrong, but I want to learn from this. Could a bad cap on the V- cause this or did I do something wrong?

The cap in question is an old surplus 1000uf 25v cap (all I had left after blowing up my last 50v Panasonic FC) and I don't know how old it is. I have a 260va Plitron w/ dual 18v secondaries and dual bridge rectifier that also powers the other channel so I don't think that the power supply is the problem.


Hoping to soon have a Gainclone with both channels working

carlosfm 17th April 2003 01:08 PM

25v cap?
Hi Steve,

With that transformer you shoud at least use 35v caps.
The cap may blow up because of over-voltage.
Also, are you shure you mounted the cap with the correct polarity?
Remember, on the V- polarity the positive lead of the cap connects to the circuit ground.

JOE DIRT® 17th April 2003 01:17 PM

SRMC.....when anytime you are building an amplifier...first have the outputs of the power supply fused and not connected to the circuit.......apply a light load with a power resistor and then check the voltages.....let it run for awhile....keep a lower fuse value in and connect the amp if all checks out.........another thing is to use a signal injector and make sure the correct path is little short and you will blow the power amp...hope this Helps!

Cheers!!The DIRT®

Peter Daniel 17th April 2003 01:18 PM

It doesn't seem that cap was damaged, so it's probably OK and correctly connected. Maybe the insulating pad wasn't working correctly and you had negative rail shorted to ground?

SRMC 17th April 2003 02:48 PM

Thanks for the help. I have fuses after the rectifier on the power supply output (3a) but I'm sure they are way too high. I used the same mica insulators under both of the chips with a small amount of heat conducting compound that is supposed to be electrically nonconductive ( My fear is that although I was using a stepped insulating washer that should have kept the screw centered in the mounting hole and prevented it from touching the chip, the screw attaching everything to the heatsink/case may still have shorted the negative rail to ground.

Thanks again.


SRMC 18th April 2003 02:41 PM

Problem solved!
It looks like the prize goes to Peter! It turns out the V- was shorting to ground. I found that I left a tiny burr when I drilled the hole to mount everything on the heatsink, and with the thin mica insulators I was using it still was enough to contact the back of the chip.


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