JBL VRX932LAP Amp - Shorting IGBTs when applying power.

[beavernuggetz]

Hello everyone,

I am working on 3 of these JBL Class-D Amplifiers for a DJ friend of mine. The initial issue was of course some sort of power surge fried some components on the board.

On the underside of the PCB, the GB20B60PD1 IGBTs were blown to pieces (literally) and on the top there were lots of SMD components which are near the IGBTs which were also shorted.

I gathered the list of dead components and ordered replacements from DigiKey and Mouser. Once all the parts were replaced but without having the TL494 (Switching Controller) & FAN7380MX (High-Side Gate Driver IC) connected to avoid frying the SMD components all over again, supplied power to the amp, and after a few seconds the current limiter light bulb became very bright indicating there was a short somewhere; sure enough, the newly installed IGBTs were fried while the SMD components were unaffected.

Without the IGBTs on the board, applied power again, measured 17 volts between SMD resistor R604 (SMPSREF) and the U1J diode which is the proper voltage for the amp to power up as shown in this YouTubeVideo.

Please find the schematics attached, if you go on to page 6 the IGBTs are shown there labelled as Q605 & Q606 (see attached picture).

What could be the cause of these IGBTs shorting out as soon as power is applied? I'm not an expert by any means, my knowledge is middle-of-the-road but do own good test equipment and my soldering skills are pretty good IMHO.

Any feedback would be greatly appreciated.