output transistors are dead?

mdm102

Member
2010-01-26 9:52 am
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
 
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

Member
2010-01-26 9:52 am
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

Administrator
Paid Member
2007-09-15 8:14 am
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

Member
2010-01-26 9:52 am
Unfortunately I don't have spare mosfets, I've ordered some matched pairs today at tech-diy.com.

I've burned mosfet, as I said, during biasing. I don't know why, but turning VR made no sense. I adjusted VR to 476ohm (450-500ohm Anthony recommended) before begining of biasing, then I started to turn VR counter-clockwise to 0ohm. VR has 0-500ohm nominal. My DMM was set to VDC and measured across 10ohm 1W resistor. During turning VR I always had 156-158mV. This was very strange for me, because I carefully read instructions and many times. Maybe I did wrong connections? I don't know. You can see my installation here: ImageShack Album - 3 images

here you can see a part of Anthony's instructions regarding pre-flight test and biasing: http://img202.imageshack.us/img202/9833/300se.gif

I've checked every resistor that Anthony mentioned in instruction in Pre-Flight test chapter to check to be sure that module work fine, and all check-measurements for both modules were absolutely correct.

I can't bias the second module too. Same problem with biasing, but I didn't burned it, now I very carefully turn VR, I tried to go down 250ohm, also zero effect, I afraid to burn the second module if I'll go lower 250ohm.

I will be very very glad if you help me to get it work. Thank you all! :)
 

mdm102

Member
2010-01-26 9:52 am
Sakis, this is commercial amplifier module, so no circuit scheme with it. Module goes with installed SMD components, I just installed thru-hole components on it and tested it after this, and it works ok, but I cann't bias it. I don't know why. Seems I did all things right to the instructions, but no effect. I'll try today to test biasing transistor, maybe it does not do its job.

Globulator, I agree with you, but too late :D