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Old 15th March 2011, 01:02 PM   #1
Alexus is offline Alexus  Macedonia
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Default Problem with LM3876

Hello.
This is my first post here.
I have builded two amplifiers (two channels) with LM3876 using this schematic and pcb:
Code:
http://4.bp.blogspot.com/_HVX94ZdirXw/SuNiCjwsXjI/AAAAAAAAA54/HBHxhi0s3r4/s1600-h/lm3876-schematic-pcb-layout.JPG
The problem is, when I connect amplifier to supply (+/-28 VDC) for a short time, there is 7 DC volts at output.
The fuses don't blown, chip is electrically insulated from heatsink and don't heat up, supply are ok and tested with another test amplifier (not chip amp, but a transistor amplifier, and this amplifier works), all parts are brand new, but there is still 7 volts DC at output.

Click the image to open in full size.

Click the image to open in full size.

I measure the input for signal and there is 1,14 DC Volts. Note that cable for input signal is not yet connected, only supply cables are connected.
Now, I disconnect cables from supply to amplifier, and measure with Ohm meter: (a) from "+ to GND", and (b) from "GND to -", and (a) there is no resistance, (b) there is 63 Ohm resistance (look the picture below).
I measure second channel, and there is again 63 Ohm from "GND to -".

Click the image to open in full size.

All suggestion and help are welcomed.

Thank you.
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Old 15th March 2011, 07:25 PM   #2
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Hi,
Counting from left to right pin 5 is not solder and pin 5 is not too close to pin 7? It is because the picture?
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Old 15th March 2011, 07:30 PM   #3
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Hi,
Sorry I forgot that pin 5 in the LM3876 is NC. Disregard the questions.
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Old 15th March 2011, 07:37 PM   #4
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your picture and the (elektor-)layout from lm3876-schematic-pcb-layout.JPG (image) don't match
maybe you've done something wrong?
regards
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Old 15th March 2011, 07:59 PM   #5
Alexus is offline Alexus  Macedonia
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Quote:
Originally Posted by Juergen Knoop View Post
your picture and the (elektor-)layout from lm3876-schematic-pcb-layout.JPG (image) don't match
maybe you've done something wrong?
regards
Yes, I make some modifications, because author says that R6 and C6 is not needed there.

My modification is third below:

Click the image to open in full size.

If you compare both pcb, they are same.
I added extra track only at C2, because in original pcb is too wide and cannot fit smaller capacitor.
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Old 15th March 2011, 08:29 PM   #6
AndrewT is offline AndrewT  Scotland
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R6 & C6 are very definitely needed for a beginners amplifier.
Reading the Author's comment should have prompted you to ask then, not after you accepted his stupid advice.

You should not have 1.14Vdc at the input.

Measure various pins and component voltages and post a pic showing what you find.
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Old 15th March 2011, 08:40 PM   #7
Alexus is offline Alexus  Macedonia
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After searching the forum, I found this thread, and there is same problem but with different schematic:
LM3876TF: minus 5 to 7 volt DC at the output
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Old 15th March 2011, 10:46 PM   #8
mjf is offline mjf  Austria
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hello.
did you use 180k res instead of 18k?



...............sorry,the colours on my monitor are bad.

Last edited by mjf; 15th March 2011 at 11:11 PM. Reason: error
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Old 16th March 2011, 05:20 AM   #9
bbm3 is offline bbm3  United States
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Maybe a short here?
If not it looks very close.

Click the image to open in full size.
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Old 16th March 2011, 10:50 AM   #10
AndrewT is offline AndrewT  Scotland
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post a close up of the PCB, not the whole amplifier and the gubbins around it.

Use outdoor daylight, not flash.
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