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Old 10th November 2006, 06:41 AM   #11
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Do you have any fuses? If yes, which values?
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Old 10th November 2006, 07:23 AM   #12
Lucas_G is offline Lucas_G  Netherlands
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Quote:
Originally posted by peranders
Do you have any fuses? If yes, which values?
Hi Peranders,

I don't have any fuses other than at the iec 230Volt inlet.
I know that some people like to add fuses at their DC rails, and maybe it would have been a life saver for my LM3886 chip, but I also know that many people think they are too slow to be a lifesaver for chips... Do you think I should add one?

Today I disconnected the PSU and Amp board and measured the voltages on the particular PSU that was connected with the blown LM3886. I measured 25 AC and 33 DC on the rails. Polarities were all OK.

So it remains a mystery to me. I doublechecked all soldering points, values etc.

There remain two possibilities:

1. Might it still be oscillations that killed the chip so fast? I did not install the feedback capacitor, only the feedback resistor (22,1K) and its loop is a bit long-legged...

2. Are there any fake LM3886 around? I bought these from some source in Asia (don't remember which) and it is labelled PM14BV above the LM3886TF inscription. Or did I just catch a faulty one?


If I do not find the clue to this, I guess I will have to try the second ampboard and see whether it will smoke too. If I would insert fuses, what values would do?

Regards,

Lucas

P.S. I hope BrianGT or Peter Daniel might look into this, since both must have a lot of experience with these pcbs and LM3886 chips...
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Old 10th November 2006, 07:40 AM   #13
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Quote:
Are there any fake LM3886 around? I bought these from some source in Asia (don't remember which) and it is labelled PM14BV above the LM3886TF inscription. Or did I just catch a faulty one?
Can you make a clear picture of the front of the chip. I have experience with fake transistors from Asia ....

http://www.sayang.demon.nl/projects/sanken/sanken.html

I have some real (directly from National) LM3886TF as well to compare yours with.

http://www.sayang.demon.nl/projects/.../kimberly.html

Quote:
If I do not find the clue to this, I guess I will have to try the second ampboard and see whether it will smoke too. If I would insert fuses, what values would do?
Use small 24V 2w bulbs in series with + and - rails, in case of high current these will light up and limit the current. LM3886 with no load will not light the bulbs. I used this when testing with Kimberly her amplifier.
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Old 10th November 2006, 08:30 AM   #14
Lucas_G is offline Lucas_G  Netherlands
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Quote:
Originally posted by sayang001


Can you make a clear picture of the front of the chip. I have experience with fake transistors from Asia ....

http://www.sayang.demon.nl/projects/sanken/sanken.html

I have some real (directly from National) LM3886TF as well to compare yours with.

http://www.sayang.demon.nl/projects/.../kimberly.html



Use small 24V 2w bulbs in series with + and - rails, in case of high current these will light up and limit the current. LM3886 with no load will not light the bulbs. I used this when testing with Kimberly her amplifier.
Hi Sayang,

I cannot come that close wityh my camera to the chipamp.
I enclose one photo from above and one made with my scanner of the backside of the chip where one can see the damage.

Regards,

Lucas
Attached Images
File Type: jpg chipampscan01.jpg (95.6 KB, 357 views)
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Old 10th November 2006, 08:33 AM   #15
Lucas_G is offline Lucas_G  Netherlands
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Picture of backside of LM3886
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File Type: jpg chipampscan002.jpg (92.5 KB, 332 views)
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Old 10th November 2006, 08:48 AM   #16
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Quote:
I cannot come that close wityh my camera to the chipamp.
Desolder the chip from the pcb and put it on the scanner with it's front, it is broken anyway
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Old 10th November 2006, 12:59 PM   #17
MikeB is offline MikeB  Germany
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Quote:
Originally posted by Lucas_G

1. Might it still be oscillations that killed the chip so fast? I did not install the feedback capacitor, only the feedback resistor (22,1K) and its loop is a bit long-legged...

OMG, i am not familiar with the board, but it looks like you skipped the dc-blocking cap instead of shorting it. This makes the amp unity gain and oscillate like hell.



Why do you simply skip parts ?

Mike
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Old 10th November 2006, 01:56 PM   #18
Lucas_G is offline Lucas_G  Netherlands
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Quote:
Originally posted by sayang001


Desolder the chip from the pcb and put it on the scanner with it's front, it is broken anyway

You are right. I will do this this evening...
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Old 10th November 2006, 01:58 PM   #19
Lucas_G is offline Lucas_G  Netherlands
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Quote:
Originally posted by MikeB


OMG, i am not familiar with the board, but it looks like you skipped the dc-blocking cap instead of shorting it. This makes the amp unity gain and oscillate like hell.



Why do you simply skip parts ?

Mike

Hi Mike,

The reason can be found here:

I opted for the standard configuration with just R3 (680 Ohm) and no Ci (47 uF) feedback capacitor as described on page 8 of this pdf: http://www.chipamp.com/docs/lm3886-manual.pdf

I checked the pcb. The 22,1 kOhm resistor is connected!

Regards,

Lucas
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Old 10th November 2006, 02:17 PM   #20
MikeB is offline MikeB  Germany
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Quote:
Originally posted by Lucas_G



Hi Mike,

The reason can be found here:

I opted for the standard configuration with just R3 (680 Ohm) and no Ci (47 uF) feedback capacitor as described on page 8 of this pdf: http://www.chipamp.com/docs/lm3886-manual.pdf

I checked the pcb. The 22,1 kOhm resistor is connected!

Regards,

Lucas
Sorry Lucas, my fault. I should not post not knowing the pcb...

But, you might check the soldering next to r3, the "SG" point. It looks like the pad on the bottom is broken. In this case you would get the same result, oscillation.

It just looks suspect on the photo...

Mike
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