Re: Blew the irf510's up, substituted irfp260's in there place... #ubitx-help

John Backo


If you are aware of what happened, just get new deices and install them.
You probably will need to redo the alignment as IRF510s are notoriously
variable. You CAN let the adjustments alone, but you will never be sure
of their real value. If you have ever changed them from the factory settings,
you must re-do the adjustments.

The simplest way is to set RV1, RV2, and RV3 to zero, using a DVM and RF
probe. Then, install the new devices. Then check what the RCVR current draw is
with the DVM. It should be somewhere around 100 ma. But it doesn't matter
much what it is as long as RCV is working. Check the antenna SWR to make
sure it is ok (below 2.0:1) Connect the PA drain voltage -- there should be little or
no change in current draw. Make sure the antenna is connected. While in transmit, SLOWLY
increase RV2 until there is an additional current draw of about 100 ma. SLOWLY
increase RV3 until there is an additional current draw of 100 ma. The RX draw
should be about the same; the idle TX draw is an additional 200 ma. or so.

Then, in TX, speak into the mike with a long "aaahh" while SLOWLY increasing RV1.
The current drain should increase another ampere or so. That should be close
to your final adjustment. You can tweek RV1 a bit until actual distortion occurs;
if that happens back off RV1 a hair until things are clear. (having a QSO to check
your audio helps a lot).

And there you are. You can check the voltage (carefully!) on the gates in TX.
They both should be around 3v, and should match fairly well if the devices you
installed were from the same place and time of manufacture. After that, let
well enough alone.

Incidentally, the same procedure applies for almost any MOSFETS installed as
PA finals.

Hope this helps.


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