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

 

I keep seeing that many people seem to be having problems with the IRF510's in their uBITX radios, but that hasn't been my experience here at all.

I have 3 uBITX's, one from each of the first 3 batches. ?Of all 3 kits, only the one from the third batch has given me any grief and that was due to the WX branded audio chip. ?Fortunately I had some UTC branded ones on hand and since replacing it have had no problems whatever.

I have always operated my uBITX radios into antennas that had less than a 2:1 SWR but I wasn't particularly careful except that I always had an antenna of some sort or a dummy load connected to them when I first powered them up just in case they started transmitting due to a shorted key line or some other problem. ?I did not change the factory settings for PA Bias or the power controls, I just left them where they were set from the factory's final test.

Either I was just lucky, or maybe that due diligence (antenna or dummy load connected before powering up) has kept me from having problems with the PA transistors. ?I am getting 10 or more watts out of each of the radios on 80-20 meters and about 7 watts on 10 meters with all three radios which falls in line with statements on the HF Signals web page about the uBITX. ?

OH yes, the first thing I did on all three kits was use thermal insulators and insulated bushings to mount the IRF510's to their heat sinks. ?This way if something grounded contacts one or both heat sinks it doesn't blow the transistor. ?

Just my 2 cents on the subject.?

Jim Sheldon, W0EB?

------ Original Message ------
From: "Arv Evans" <arvid.evans@...>
Sent: 3/16/2018 11:09:57 AM
Subject: Re: [BITX20] Blew the irf510's up, substituted irfp260's in there place... #ubitx-help

It seems that just blindly replacing blown IRF510 devices with more IRF510's
is an exercise in treating the symptom instead of fixing the problem.? There
are many ideas and myths about why the IRF510 devices sometimes fail but
only limited definitive information on what actually causes these failures.?
We have empirical information indicating that if the SWR is high they will fail,
but limited information on exactly why that is the case and what the mechanism
of failure really is.

These URLs may help, or may not:








Arv? K7HKL
_._


On Thu, Mar 15, 2018 at 10:59 PM, John Backo <jabac@...> wrote:
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
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