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Old 24th June 2008, 08:04 AM   #1
h@kan is offline h@kan  Sweden
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Default LM3875 overheated

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?
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Old 24th June 2008, 08:38 AM   #2
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Misplaced part,or a bad solder joint?

Double check all of your changes!
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Old 24th June 2008, 09:42 AM   #3
ratza is offline ratza  Romania
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Can it be caused by the lamps? Maybe a very close wire next to them?
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Old 24th June 2008, 10:10 AM   #4
h@kan is offline h@kan  Sweden
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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!
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Old 25th June 2008, 09:34 AM   #5
AndrewT is offline AndrewT  Scotland
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another modified project powered up without passing through that mains light bulb tester!!!!
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Old 25th June 2008, 10:24 AM   #6
h@kan is offline h@kan  Sweden
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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?
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Old 25th June 2008, 12:58 PM   #7
AndrewT is offline AndrewT  Scotland
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build a mains light bulb tester and keep it available for starting up EVERY new or modified project.
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Old 25th June 2008, 03:23 PM   #8
owen is offline owen  United Kingdom
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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...
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Old 25th June 2008, 09:07 PM   #9
r8833 is offline r8833  Canada
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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
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Old 26th June 2008, 07:06 AM   #10
h@kan is offline h@kan  Sweden
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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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