Please help - Chipamp lm3886 amp blowing fuses

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
I

iggy111

Member
Joined 2007
Just finished soldering my first lm3886 kit from chipamp.com, 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:
 
T

Tekko

Banned
Joined 2004
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.
 
I

iggy111

Member
Joined 2007
@bobodioulasso:
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...

@Tekko:
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.

@Boscoe:
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?
 
I

iggy111

Member
Joined 2007
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:

An externally hosted image should be here but it was not working when we last tested it.


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

AndrewT

R.I.P.
Joined 2004
iggy111 said:
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:

An externally hosted image should be here but it was not working when we last tested it.


Let the young ones learn from this (he said knowingly) :)
Click to expand...
Once again, the mains bulb tester would have prevented this mishap getting through !!!!!!!
Another unhelpful post of mine, because Members won't read.
 
I

iggy111

Member
Joined 2007
And it's working :)))
So I removed the 4 diodes and hooked up everything as in the pic, and now it works. Still have to solder the inputs and outputs again before I test it with some cheap speakers...

@AndrewT - you're right, but I was too anxious to get it going that the bulb tester seemed like another project :) Will DEFINITELY build it before any other project! I swear!
 
I

iggy111

Member
Joined 2007
Ok, so just to finish this topic and move on - my amp now works fine, the problem was in using center tap (CT) trafo and not doing the necessary changes in the psu. The fuses kept blowing but luckily nothing else was damaged.

After the changes (desoldering the 4 diodes and connecting the trafo differently) everything works fine. Tested with speakers and all :)
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom