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Old 20th April 2017, 12:12 PM   #1
ppan is offline ppan  France
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Question LM4780 issue

Hi!

I used for 3 years two LM4780 kit (stereo 2x LM3886 chip) with Connex' SMPS300RE 2x32V successfully, in an 3 way active system (home made MiniDSP 2x8 HD). This amp driving treble & medium part.

I planed to modify my system to add some power, because of my children' party
So I have built only one LM4780 amp for trebles (over 3kHz) with same SMPS power supply, and two Hypex UcD180ST for medium amp.

I own 5 LM4780 PCB (I bought it on Ebay). During several test out of the box, 3 of them where shutting down: all presenting discontinue voltage in final stage (ex. 13V for 18V linear power).
I have changed one LM4780 chip but no way!

Any idea to test and fix this?

Two potential errors:
1. micro short circuit because of metal dust (I have a little space to work)
2. some test has been made with SMPS240QR 2x45V, which is the chip upper limit (no signal)

PS : all few components around are 50V (electrolytic capacitor) to 100V.
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Old 20th April 2017, 07:15 PM   #2
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I would suggest you to post pics to let others see the circuits you are talking about. As for the wild guess of what can be the problem in order from higher to lower probability:

- ebay assembled amps aren't populated with the original LM4780 (these are mostly fakes these days)
- the high supply voltage killed the chips. Note the highest supply voltage of the LM4780 with and without input signal present.
- combination of the two above factors: fake LM4780s did not survive the elevated supply voltage.
- all other possibilities involving wrong connections, bad assembly, cold joints etc.

Regards,
Oleg
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Old 20th April 2017, 10:46 PM   #3
ppan is offline ppan  France
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Quote:
Originally Posted by OlegSh View Post
I would suggest you to post pics to let others see the circuits you are talking about.
Thanks for your reply.
Here is photo. It's a double sided PCB. I think a good work.

Quote:
Originally Posted by OlegSh View Post
ebay assembled amps aren't populated with the original LM4780 (these are mostly fakes these days)
I bought 17,5 to a Czech guy 3 years ago.
It was perfect since I change my plan and power it too much, probably?
I do not see other component which could be damaged.
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File Type: jpg IMG_20170420_231741_1.jpg (198.8 KB, 192 views)
File Type: jpg IMG_20170420_231822.jpg (172.4 KB, 187 views)
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Old 21st April 2017, 03:29 PM   #4
jaycee is offline jaycee  United Kingdom
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PCB looks OK, but the chips themselves are almost certainly fake if they came from eBay.

Note the the LM4780 is also discontinued by TI, though major suppliers should still have some.
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Old 21st April 2017, 03:55 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
the chips themselves are almost certainly fake if they came from eBay.
probably.
Report your faults through feedback.

If you are sure they are non genuine, then also report them for "Fraud"
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Old 21st April 2017, 04:50 PM   #6
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Can u post a picture of the actual chip?
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Old 21st April 2017, 05:05 PM   #7
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The PCB and the passives look OK to me. Since it has been working before, the problem is most certainly with the IC.

Last edited by OlegSh; 21st April 2017 at 05:09 PM.
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Old 22nd April 2017, 01:02 AM   #8
tomchr is offline tomchr  Canada
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Do you have a heat sink attached to the LM4780 during the test?

Tom
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Old 22nd April 2017, 10:39 AM   #9
ppan is offline ppan  France
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@asuslover: see picture below. On top the damaged one, IC changed (from Ebay) without success. Below the original one, in perfect condition, working.
ICs are National Semiconductor (not very visible here).

@tomchr: yes heat sink attached, isolated from ground because of -V
I noted it was necessary to use nylon screw or metal with thermal sheath: the metal sole isn't far from screw aperture.

@OlegSh: that's my conclusion because of overload power... or fragile fake chip. But almost 3 years function for 3 of them, it's strange.
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Old 22nd April 2017, 02:44 PM   #10
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This is an 100% authentic chip.
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