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Old 15th January 2012, 02:09 PM   #11
hemanth is offline hemanth  India
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Hi Andrew,

Thanks for reviewing.

As I'm new to this, could you please redirect me to some good source for best practices?

Could you please point out some obvious errors and things that are broken?

Thanks.
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Old 15th January 2012, 05:15 PM   #12
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I'm too lazy to look at pictures so i'll just comment on the data you gave us re DC offset - the one with 31 volts indicates that the amplifier is stuck the positive supply rail. This means that either the LM3886 itself is shorted, or something in the input or feedback path is shorted to the positive rail. Check DC voltages on pin 10 and pin 9 of the LM3886.
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Old 16th January 2012, 08:56 AM   #13
Grega is offline Grega  Australia
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hi hemanth
have you insulation between the heatsink, the LM3886 and the screw that goes thru the LM / heatsink.
i notice you are using a metal case - which, if its grounded may be causing you grief. i am hoping insulation exists but can't see it in your pictures.
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Old 16th January 2012, 11:08 PM   #14
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From the third pictures it seems that the LM3886 don't adhere perfectly to the heatsink and I can't see any thermal compound between LM3386 and heatsink... I'm right?
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Old 17th January 2012, 02:12 PM   #15
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He is using a LM3886TF which doesn't have a metal tab as it is enclosed in heat conducting plastic. Isolation with mica is not required but heat sink compound is necessary / recommended. The LM3886T has a metal tab which is having negative voltage.
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Old 17th January 2012, 02:13 PM   #16
hemanth is offline hemanth  India
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I have some insulation between the enclosure and the heat sink. As the assembly is not yet completed, I do have the thermal compound which needs to be used between the heat sink and LM3886.

Today I checked the DC voltage between the pin 4 and pin 7 on the LM 318 and it showed 24.3V, then i checked the DC voltage between the pin 9 and pin 10 on the LM3886, it showed 0.6mV. Are these values fine or something screwed up?
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Old 17th January 2012, 02:59 PM   #17
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Quote:
Originally Posted by AndrewT View Post
having seen the pics you showed in post1, I cannot believe that Linux gave your assembly the OK.
I couldn't find any obvious error on the assembled *boards* only - didn't actually comment on the complete assembly. I am the supplier of the bare boards and components, and Hemanth did contact me, but my suggestions haven't yet been sufficient to locate the problem.
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Old 17th January 2012, 03:19 PM   #18
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Quote:
Originally Posted by hemanth View Post
I have some insulation between the enclosure and the heat sink. As the assembly is not yet completed, I do have the thermal compound which needs to be used between the heat sink and LM3886.
You don't need insulation, the LM3886TF Siva supplied are already insulated.

What you need is good thermal conductivity between the LM3886 and the heatsink, aka a flat, polished heatsink surface, thermal compound and a tight fastening.

Without that the LM3386's temperature will raise rapidly until it triggers the SPiKe protection...
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Old 17th January 2012, 06:08 PM   #19
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Quote:
Originally Posted by hemanth View Post
then i checked the DC voltage between the pin 9 and pin 10 on the LM3886, it showed 0.6mV. Are these values fine or something screwed up?
I asked for pin 9 to ground and pin 10 to ground, not pin 9 to pin 10. Generally, all measurements are made respective to ground. If you get more than a few mV then there could be a short between one of the inputs and one of the supply rails (check with buzzer on your multimeter).
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Old 18th January 2012, 01:23 PM   #20
hemanth is offline hemanth  India
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Thanks for all the inputs.

I checked the DC
1. Pin 9 and ground, it is showing 0.44V
2. Pin 10 and ground, it is showing 1.2mV
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