output transistors are dead?

[mdm102]

Hi,

Could you help me to understand, I tried to bias amplifier module, but overturned VR at some moment, it is single turn VR, and seems that I burned output transistors. I measured 42-50 DC voltage on the speaker terminals. Is this mean that output mosfets 2xIRFP240 and 2xIRFP9240 are dead?

Thank you!

MD

[Mooly]

Do they read short circuit Drain to Source ? and what's the amp ?

[sakis]

yeap .... post a circuit ...that behaviour rings a bell ...that could be fun to see !!!

[mdm102]

Thank you for reply, guys! Drain to Source opened in both directions. The model of the module is HPA-nxV300se by Anthony

[AndrewT]

what does "opened" mean.

Open to the free flow of test current?
or
Open circuit when tested?

[nigelwright7557]

If the bias became too great you would turn on both sets of transistors at the same time and short out the power supply.

Thye usualy go short Drain to Source although i have sometimes seen MOSFETs go int all sorts of weird states. I even had one with a partial short from gate to drain.

To be safe I would replace all the MOSFET's not forgetting to wind the bias back to zero before you power up again.

[mdm102]

Originally Posted by AndrewT

what does "opened" mean.

Open to the free flow of test current?
or
Open circuit when tested?

I meant current can flow in both directions, seems diode between Drain and Source allow to flow current in both directions. http://www.vishay.com/docs/91210/91210.pdf

[mdm102]

Originally Posted by nigelwright7557

If the bias became too great you would turn on both sets of transistors at the same time and short out the power supply.

Thye usualy go short Drain to Source although i have sometimes seen MOSFETs go int all sorts of weird states. I even had one with a partial short from gate to drain.

To be safe I would replace all the MOSFET's not forgetting to wind the bias back to zero before you power up again.

Yes, I measured all 4 power transistors, they allow to flow current from Drain to Source in both directions, seems they all are dead. I don't know should I change also small biasing bipolar transistor MJE340, could you advice how to check it? Thank you!

[Mooly]

If they are short they will read 0.00 on DVM on diode check.

The MJE340 needs to be tested out of circuit to fully check. If you are unsure how to interpret readings then it might be wiser to replace it... that said a FET amp shouldn't do much damage to drivers etc when the FET's fail.

When you power up use a 100 watt bulb in series with the mains supply until you are sure the amp is OK.

[mdm102]

Originally Posted by nigelwright7557

To be safe I would replace all the MOSFET's not forgetting to wind the bias back to zero before you power up again.

Anthony said in his manual to set VR to 450-500ohm (VR is 0-500ohm single turn), strange

I just desoldered all mosfets and checked these with DMM, 1 of 9240s is dead for sure, the second 9240 is alive, 240s are alive also.