LM3875 overheated

h@kan · 24th June 2008, 08:04 AM

I have a gainclone based on the lm3875 classic kit from audiosector with a seperatate tube buffer. I love it but the mids and hights sounds a little "sharp" so I purchased the premium parts resistors to see if there was any improvement.

When the resistor change was done I plugged it in and turned on the power. There was a moderate hum and suddenly it started to smell like burned plastic. After some investigation I have found that the lm3875 chip on one (just one) of the channels gets overheated.

A few seconds with power on and the chip gets VERY warm.

Any idea anyone?

DigitalJunkie · 24th June 2008, 08:38 AM

Misplaced part,or a bad solder joint?

Double check all of your changes!

ratza · 24th June 2008, 09:42 AM

Can it be caused by the lamps? Maybe a very close wire next to them?

h@kan · 24th June 2008, 10:10 AM

I have bypassed the tube buffer at this moment so that is not the issue. I will re-check the solder joints (again) ant tripple check the parts placement.

Thanks for your answers!

AndrewT · 25th June 2008, 09:34 AM

another modified project powered up without passing through that mains light bulb tester!!!!

h@kan · 25th June 2008, 10:24 AM

Quote:

Originally posted by AndrewT
another modified project powered up without passing through that mains light bulb tester!!!!

Sorry I dont follow you. What should I have done?

AndrewT · 25th June 2008, 12:58 PM

build a mains light bulb tester and keep it available for starting up EVERY new or modified project.

owen · 25th June 2008, 03:23 PM

Most likely is that the chip went into self-oscillating self-destruct.

This could be due to a missed solder joint, or a feedback resistor that is incorrect....

Take a good close look there.

Another possibility is a broken PCB trace - that could be harder to spot.

Have fun

Owen

PS the light-bulb trick is to wire a light bulb in series into the Live line on a powercord.

Any faults, the bulb lights up, and limits the maximum amount of current to the device...

r8833 · 25th June 2008, 09:07 PM

also check the positive speaker terminals ..at one time one of my speaker posts was touching the chassis causing the chip to overheat and oscillate

h@kan · 26th June 2008, 07:06 AM

First of all thank you for all your answers.

In the future I will definitly use a bulb-tester after changes in my system.

I will let you know if there is any progress.

24th June 2008, 09:42 AM	#3
ratza diyAudio Member Join Date: Dec 2007 Location: Sibiu, Romania	Can it be caused by the lamps? Maybe a very close wire next to them? __________________ Any solution is a compromise.

25th June 2008, 09:34 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	another modified project powered up without passing through that mains light bulb tester!!!! __________________ regards Andrew T.

25th June 2008, 12:58 PM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	build a mains light bulb tester and keep it available for starting up EVERY new or modified project. __________________ regards Andrew T.

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24th June 2008, 08:38 AM	#2
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	Misplaced part,or a bad solder joint? Double check all of your changes!

24th June 2008, 10:10 AM	#4
h@kan diyAudio Member Join Date: Apr 2008 Location: North of Sweden	I have bypassed the tube buffer at this moment so that is not the issue. I will re-check the solder joints (again) ant tripple check the parts placement. Thanks for your answers!

25th June 2008, 03:23 PM	#8
owen diyAudio Member Join Date: Mar 2004 Location: Shilton	Most likely is that the chip went into self-oscillating self-destruct. This could be due to a missed solder joint, or a feedback resistor that is incorrect.... Take a good close look there. Another possibility is a broken PCB trace - that could be harder to spot. Have fun Owen PS the light-bulb trick is to wire a light bulb in series into the Live line on a powercord. Any faults, the bulb lights up, and limits the maximum amount of current to the device...

25th June 2008, 09:07 PM	#9
r8833 diyAudio Member Join Date: Nov 2004 Location: canada	also check the positive speaker terminals ..at one time one of my speaker posts was touching the chassis causing the chip to overheat and oscillate

26th June 2008, 07:06 AM	#10
h@kan diyAudio Member Join Date: Apr 2008 Location: North of Sweden	First of all thank you for all your answers. In the future I will definitly use a bulb-tester after changes in my system. I will let you know if there is any progress.

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