Blown FETs?

[Robert Cheek]

I think I may have blown FETs.  I was on 20 meters, using the HR500 with Palstar HF-AUTO on a dipole fed with open wire line.  (I have used this antenna with the HR500 for years) 

Got a drop out when I tried to send 50 watts to the tuner to retune to 80 Meters.  The screen on the HR500 showed a FAULT.  From there it is making no power out.

I see drive on the HR500 meter, I see VDD, no IDD, and now output.  Relays click LEDs on the rear panel show 50 V is good, PPT is good and RF Sense flashes with CW input. 

Checking the Drains on the FETs I have 55 Volts but I also have 55 Volts on the gates....don't think that is correct!

Anyone have thoughts?

50 V on the gates make me think the FETs have shorted. 

Regards,

Bob - N4RFC

[jcve...@gmail.com]

Hi Bob,

55V in the drains indicates that the MOSFETs are blown. The protection diodes should have prevented any further damage to the amp PCB.

73,

Jim WA2EUJ

[Robert Cheek]

Jim,

I think you meant to say 55 volts on Gates indicates the MOSFETs are blown...Spec sheet says Gate max voltage is +/- 40 volts so I figured the Gate were shorted to the Drain. 

I saw the diodes in the schematic and was thinking they were there for when things go bad.  And things went bad.  Do you know that the Gate bias typically runs on the HR500?

New MOSFETs on order.

Thanks for the quick reply

Bob - N4RFC

[AC1I]

I just thought I would throw this out there:

A couple of years ago several of us suffered from blown finals; the theory at that time was that temperature cycles were causing the screws securing the MOSFETs to the heatsink to loosen. The suggested cure was to utilize lock washers on those screws (I myself used high temperature Loctite).  I can only say that I have not had a failure since then.

Patrick AC1I