VVR & SAG

[katopan]

It's really hard to tell in the photo (one from directly above would be better) but does the black wire onboard connect to the middle pin2 of the mosfet (which it should) or the outer right pin3?

Is the mosfet fully insulated from the chassis?

[geoff 1965]

yes katopan the black B+ in goes to pin 2 on the mosfet and i used 2mm thick heatsink pad under the mosfet, i will double check i haven't tightened it too much. the sparks looked to come from the pot but the mosfet sits underneath and i switched off straight away so i'm not sure. just waiting for a correct 10R and new mosfet "just in case" to arrive, but i'll start by removing the potentiometer and checking for shorts/resistance.
hard to see from angle of the pic but here's the layout

VVR.jpg

[geoff 1965]

okay i've replaced the 10R with a correct voltage rated resistor and checked the potentiometer and it's good. now there is no fireworks or burning resistors but the B+ voltage is at max with no adjustment from the potentiometer,can i assume this mosfet is fried?

[katopan]

Can you set the pot to 50% and measure the voltage on the three pins of the mosfet? That will tell you if it's fried or not.

[geoff 1965]

set the potentiometer at 50% katopan and i measure 448VDC on all 3 pins of the mosfet.

[geoff 1965]

001.JPG

up and running now with new mosfet installed,getting 35 to 450 VDC "unloaded" so i'll see what it sounds like on minimum and if it's too tinny i can bring the 35 volts up.
thank's katopan & ZP.

[katopan]

That looks much better. Must've been the mosfet then.

[geoff 1965]

yeah the difference was in the packaging,the first one that failed came in a piece of plastic with celotape around it! the new one was in a sealed bag with the warnings about electrostatic and handling on.
the current passing through the faulty one must have been massive because it fried the 12W resistor in seconds,i'm glad i did'nt have my hand on the chassis at the time!