Short circuited LM3886 amp, what burned?

[Morriz]

Good evening,
I just build my first amp, a LM3886 stereo kit from DIY Chip Amplifier Kits, PCB's, Components and Information. and Im embarrased to say that I ****** up big time :/

Somehow I managed to switch the V+ (positive voltage supply) and PG+ (ground) on one of the amplifier boards.
This lead to the diodes in that rectifier blowing up before I could cut the power.

I've been studying the schematics to try and figure out what more could have been damaged by this mistake. Obviously the positive power supply got short circuited, hence the magic smoke, but other than that Im at a loss.

So if any of you guys can help me figure out what components I would need to replace after this incident I would be forever grateful!

The manual for the amplifier including schematics can be found here:
http://www.chipamp.com/docs/lm3886-manual.pdf

 

[Iain McNeill]

I would guess the PSU would survive. You can diode test the bridge to make sure. Then power up and see if you get volts.

The chip is almost certainly toast. Reverse power is pretty fatal to most IC's.

How long was it on and where did the smoke come out of?

 

[Morriz]

Ops I forgot to write that

The smoke (and flashes) came from one of the diode bridges, the side of the PSU that was shorted. Nopthing else showes any physical sign of being damaged. It was on for max 1 second.

Do you think both amp-chips are toast, or just the one on the board that caused the short?

Thank you for your answer!

 

[AndrewT]

A bulb tester in the mains feed may have saved your amplifier & PSU from any damage. The bulb usually lights up to tell you the circuit is trying to draw too much power.

 

[richie00boy]

I don't think the chip saw reverse voltage, if it did the PSU caps would have popped.

I would replace the blown diodes and power up, but make sure you use the light bulb as Andrew suggests. This should save your PSU if the amp chip has gone faulty. A 40W incandescent bulb should be OK.

 

[pjp]

The datasheet says "Output over-voltage protection against transients from
inductive loads" which I would take to mean there are diodes across the power supply (+-output--).

Its probably too much to expect, but maybe you're incredibly lucky and the 3886 survived.

 

[godfrey]

Do you think both amp-chips are toast, or just the one on the board that caused the short?

Woth dual-mono design, I'd expect the other channel to be fine. You could totally disconnect the fried channel and see if the other one still works.

A bulb tester in the mains feed may have saved your amplifier & PSU from any damage. The bulb usually lights up to tell you the circuit is trying to draw too much power.

This is a very good idea when you're first testing something. It's wired up as shown below.

 

[Morriz]

Thanks for the answers everyone! I was just about to ask for how the light-bulb test was supposed to be connected, but you beat me to it

Based on your comments my plan is now the following:
replace the four diodes in the affected bridge, de-solder and test the other four diodes just to be sure they are OK.
Then I should be able to test the power supply by itself to make sure it works.

I bought a new LM3886 chip for the affected board, but since Im still note entirely sure that the other one survived I will use the light bulb setup and test each amplifier board separate (without any load, just measuring DC offset at the output as suggested in the manual).

I'll get back with the result hopefully later tonight!


Another thing that Ive been thinking about is adding fast fuses somewhere on the secondary side of the transformer (I already have a slow-blow fuse on the primary, it didn't blow in the "incident" tho). Is this a good idea, and where would it be best to put them in that case?

Directly after the transformer, or between the power supply board and the amplifier boards?

Would such a configuration be safe or could bad things happen if lets say one of the fuses blows leaving the amplifier boars with one voltage potential instead of two?

 

[richie00boy]

There's no need to desolder and risk damaging the diodes, test them in situ.

Put your fuses after the main PSU caps before the amp. Yes some amps can fall foul of having just one rail fuse blow, putting the full remaining rail voltage into the speaker. But I think the LM chips are safe from this.