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iggy111 3rd December 2012 03:45 PM

Please help - Chipamp lm3886 amp blowing fuses
Just finished soldering my first lm3886 kit from, and I think I fried the amps - the fuse keeps blowing right when I power it up :confused:

I tried the lightbulb test and it works, which means that my power supply is ok - it outputs 2 x 38.8V (transformer is 2x28, center tap) and the lightbulb glows dim.

Then I tried to connect only one of the amp boards, I heard a short but relatively loud hum and then the fuse blew. I tried 1.6A, 2A and 2.5A, slow-blow (T) type. I don't have a soft-start.

Here is one thing I definitely messed up - I soldered the input resistor R3 as for the "standard mode" from the manual, without the input cap. But later on I soldered the input cap without resoldering the input resistor to the other hole. Could that be the problem? Does anyone know what happened when I powered it up like this?

Also, could someone explain how to test if the chip is still ok by desoldering it and taking measurements outside the circuit?

Any input is appreciated :rolleyes:

bobodioulasso 3rd December 2012 04:33 PM

Power supply alone, without bulb. Does the fuse blow?
Which is the transformer VA rating?

Tekko 3rd December 2012 05:03 PM

I'd say your LM3886 ic's are most likely internally shorted, either from a bad batch(fakes) or overheated when soldered.

Or you simply just reverse polarized the rails, blowing the output stage in the LM3886 ic's, its a easy thing to do, or simply just a short on the amp boards.

Boscoe 3rd December 2012 05:26 PM

+/-40V is high and may be the problem.

iggy111 3rd December 2012 07:26 PM

the psu is working fine, with and without bulb. Transformer is EI type, by the core cross-section (2x5cm) I calculated that it is 100VA, salvaged from an old amp, and had a 1.6A fuse originally. I ordered a 160VA 2x28 toroid, but it's not here yet...

How can I check if its shorted? Don't think it overheated, I have a soldering iron with temperature regulation and a lot of experience soldering. But how could I have reverse polarized the rails?? The psu takes care of everything and the markings on the pcb are clearly visible.

I thought of that, but the chip should go up to 42V, no? Question: what does it actually do when you go over that, say 45V? The chip just blows?!
The only reason I went for 2x28 trafo is cause I plan to build the LM338 regulators and regulate at 2x30V, so this is just temporary.

Thanks for the quick replies guys! So the question remains - how can I check if the chips shorted, or if they simply draw too much current so my fuse blows?

Puffin 3rd December 2012 07:41 PM

I use +/-40 v with no problems.

bobodioulasso 3rd December 2012 08:19 PM

A class AB amp must work whith the bulb inserted.
A shorted chip will show no (or very little) voltage across it when the bulb is used.The bulb will bright taking the whole voltage.

Though if everything is correctly wired and the fuse blows, then the chip is dead.

basi 4th December 2012 09:53 AM

Iggy do you have lm3886t or lm3886tf?

iggy111 4th December 2012 10:04 AM

Ahhh, for Christ sake...
In my hurry to get the amp working, I forgot there is a difference between a CENTER TAP and DUAL SECONDARIES trafo.
Yup, I hooked up a CT trafo to both rectifier bridges by connecting its GND to both...

Now I'm going to try to connect it as in this pic, hoping I didn't fry the amps:

Let the young ones learn from this (he said knowingly) :)

iggy111 4th December 2012 10:08 AM

TF, why?
(iliti zašto? :)

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