开云体育

Well, pushing the limits on the irf510 mosfets, they caved on me. So, digging through my junk I found four irfp260's and soldered them in as an effort to get back on the air. So far, it seems to work, but I noticed the current draw is much lower and a bit more heat is created, however, I have pretty get distance on wspr and about the same reports on ft8.

Does anyone know if there are any changes that should be made to the ubitx to get it to work better (with more output) using these mosfets? The obvious answer is to order some more irf510's from jameco and put them in, and quit limping the radio along on these other mosfets...but..ya know, spare parts bin and all.
--
----------
N5WLF, Greggory (or my nickname, Ghericoan)
General Class, Digital Radio Hobbyist

These HEXFETS are rated for 200V and 50A. They will (sorta) work as you have shown.

But the gate input capacitance is 4000+ pF. That increases your standing wave in
the drive circuit and lowers your output considerably. You have 2 choices:
Either replace the finals with IRF510s (after you have figured out what blew them up),
or figure out how to lower the input capacitance to 180 pF. or so. You can also probably
safely use a drain voltage of 48v or so, but there would probably be oscillation which would
have to be accounted for. Incidentally, how do you know they are not oscillating now?
Since the potential drain amperage is so high, it is probably wise to provide a current limiter
if you have not already done so, especially using wspr or another more or less continuous
wave function.

It's better to replace the finals.

john
AD5YE

I have a two amp fuse inline. How do you tell if it is oscillating?

And a really bad swr is what I am guessing killed the irf510's.
--
----------
N5WLF, Greggory (or my nickname, Ghericoan)
General Class, Digital Radio Hobbyist

My finals blew due to a bad load and testing out the PA on higher voltage supply. My own fault here.
--
----------
N5WLF, Greggory (or my nickname, Ghericoan)
General Class, Digital Radio Hobbyist

Greg:

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.

john
AD5YE