UR12 wrote:Interesting. The mosfet usually doesn't have a ground on any of the pins or the back so other than having a faulty insulator I don't see how that can happen. It may be that you have another short somewhere from B+ to ground, like a filter cap before the VVR that is shorted or something. It would ber easy to remove the screw holding the mosfet down and lift the mosfet a little off of it's insulator and see if you are getting the same reading (short to ground) that would at least tell you if it was the Mosfet or something else.
Dana, thanks for chiming in mate, That makes sense. I have dropped off to the tech with a spare mosfet this morning but I'll give him a call to put forward the suggestion. I was a little worried about where the tech placed the VVR initially as the mosfet is so close to one of the power tubes (heat). But he wanted to minimise the RF noise by putting it where he did.
Hey Ron, I did have the cab hooked up at the time it blew, and I did check the jack and speaker wires, speaker cable etc to make sure there wasn't an issue with the cab.
I had a problem once with a mosfet amp I built with a burr working it's way through the pad and making contact with the device. You always have to watch that, especially around the mounting hole. Since then, I have always sanded and polished any surface used for sinking transistors. I hope your tech finds something simple.
It is a 38 watt per channel Class A stereo amp. Weighs in around 70 lbs and has a 700 watt power supply. The pc cards were bought but loaded by hand. For those of you who like to look.
Gargoyle wrote:She's fixed!!!!!
passfan you were on the money, she had arced through the the insulating pad under the mosfet.....
Well good for you. I hope your tech sprung for the repair. In my case the amp worked fine for a few months and it wasn't until I took it down and checked the torque on all my mosfets that i forced the burr through. I think it was more like a scratch really and I never really found it. When I took the whole output card out for testing the short disappeared. Each of my power rails was fused so it blew before it damaged the device. I wonder if a secondary B+ fuse for the VVR would be possible to save a 10.00 device ? Have to refer back to the designer on that one as I don't know enough about the current draw of the design. I know I want to add a VVR to my Express and will in the future. Enjoy your amp.