| h@kan |
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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| DigitalJunkie |
Misplaced part,or a bad solder joint?
Double check all of your changes!;) |
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| ratza |
| Can it be caused by the lamps? Maybe a very close wire next to them? |
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| h@kan |
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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| AndrewT |
| another modified project powered up without passing through that mains light bulb tester!!!! |
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| h@kan |
| 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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| AndrewT |
| build a mains light bulb tester and keep it available for starting up EVERY new or modified project. |
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| owen |
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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| r8833 |
| 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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| h@kan |
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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| h@kan |
Now it's up and running again.
I replaced the lm3875 chip that overheated and the problem was gone. The cause of the overheating problem, as far as I know, could be:
1. Their was something shortened the chip. When I looked closely their was some heat sink compound clogged between two pins and these pins looked "burned".
2. I used the same power button to the tube buffer and the chipamp. I dont know much about tubes but when hooking up the buffer to and oscilloscope it showed strange things at startup (5-8 secs or so). Maybe you are supposed to "heat up" the buffer before turning the chipamp on.
3. I dreamed everything and there was no problem (not likely since my better half complained about me trying to burn down the house).
By the way it got it self a nice pair of wings now :) |
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