Amplifier kit troubles

[Starwalker298]

I recently purchased two Silicon Chip Classic-D amplifier kits (250W into 4Ω, 150W into 8Ω) and ran them in stereo after construction. Before building a box to mount it all in curiosity got the better of me and I decided to test it out before I mounted it properly. Long story short, I didn't have my speakers (4Ω impedance) very well connected and one fell out, which caused a fault in the other channel, frying the MOSFETs, shorting the supply rails together. I saw a blue flash at the time of failure, but I didn't see if it was from the speaker or the fuses blowing.

I changed the MOSFETs and it failed again. I didn't notice any busted or swollen caps, no obvious burn marks and I wouldn't have a bloody clue how to test the driver IC (which is a surface mount part that I really don't want to take off if possible). Obviously something else is wrong, but my expertise is limited, and I don't want to just keep replacing parts for others to fail. I'd rather just buy another kit and start from scratch if that's going to happen.

Long story short I am very confused, particularly with the left channel failing when the right speaker fell out (the only common thing is the power supply). Why might this happen? The right channel still works fine.

And can anyone give a little insight to what else might fail in such an event?

Here's a link to my dropbox which shows the schematic. It's pretty high res so if you want to have a look you should be able to see everything clearly.

https://www.dropbox.com/s/c280r572xxnnwyx/20140815_205143.jpg

A few things I've already determined:

1. Supply rails are shorted together with a resistance of 0.6Ω
2. The circled diodes survived. I took them out and they were healthy. Replaced them with fresh ones after the effort of pulling them out of a damn tiny space with a chunky soldering iron.
3. MOSFETs are probably the casualties, not the cause of the fault.


Thanks in advance

 

[Wp100]

Hi,

Assume your PSU is just a transformer, diodes / bridge and caps, has it fried some of the diodes in that as well ?

Often when things like that happen you can go round in circles replacing one part only to find another blown or partly damaged active component that soon blows when power is restored.

Unless you have the equipment and experience to check out all the active parts I would say either replace all the active parts in one go , diodes, op amps , transistors ,diodes and partic zenners; though as you say it might be simpler just to buy a complete new kit...

We have all been there and done that so you are not the first and not the last to do it !

 

[Starwalker298]

I probably should have clarified that it is the rails on the board that are shorted together, not at the PSU, which is constructed as you describe.

All I've got is a multimeter so I think I'd be limited in testing the driver IC. It might be worth replacing the active components, but I don't know how well the solder pads will hold up with all the heat.

Haha. That's true. It's the not the first time I've destroyed an amp either, and I doubt it will be the last.

Thanks!

 

[audioguru]

If the plus and minus 50V rails are shorted together then the Mosfets must be shorted because they should not conduct without gate voltage to turn them on.
The Mosfets will probably be destroyed if the driver IC is faulty and turns on the upper and the lower Mosfets at the same time.

With the Mosfets removed and no input signals, measure the voltages at the outputs of the driver IC that connects to the gates of the Mosfets and compare the voltages with the good channel.