Re: K3s no power on TX

Richard Williams

I would venture to say that no power out (especially in the 1 - 12 watt range indicated the failure of both MOSFETs (about half power indicates failure of one of them).   I have had that happen twice now; and it is not a difficult task to replace the two RD15HVF1s.

I purchased mine from RF Parts; they are $4.95 each, and shipping is $12.15;  SO, I purchased six of them (now have 4 spares).

Easy to remove the KLPA3A  board.  Remove the top cover, and bottom read cover. Then remove the fan(s) and the KPA3A 100W Amp board (three screws - two in back and one on the right side when looking from the back).  This gives you access to the KLPA3A board.  You can now pop out the KLPA3A board, and replace the two MOSFETs.

Once done put the board back in secure the bottom cover; leave the KPA3A amplifier out (you need access to the 2 pots on the KLPA board to set the bias).

Fire up the K3S with an amp meter in the power line; if you do not have one, you can use the meter in the K3S.  To do that, tap the DISP button, then use the VFO B know to change the display to Amps.  Set the radio to SSB mode, power knob all the way down, and press the tune button (puts the radio in transmit mode).

With the two pots on the KLPA3A board full CCW, adjust either one for a 200 MA increase in current. Then adjust the other pot for another 200 MA increase.  You are now done, power down the radio, re-install the power amp, fan board on back of the power amp, and then the top cover.

When I adjusted the pots, I had a meter in series with the power lead from the power supply, and watched the meter on the front of the K3S.  The readings were surprisingly close, so either is more than satisfactory to use.

One other note, when you remove the back bottom cover, make sure there are grey (mine are) thermal pads on the RD15HVF1s; you need to transfer those over to the new ones.

Dick, K8ZTT

Flying is the second greatest thrill known to man.
What is first, you ask? Landing, of course.

On Thursday, May 28, 2020, 07:52:27 AM MDT, Ray Maxfield <wa6vab@...> wrote:

Hi Brian

I did the Finger Heat Test on My K3 when it died.

Reach under the radio where the 3 Power fets are mounted

And Feel the Screws that Mount the Fets.

Key the Transmitter ( maybe AM Mode) you should feel the Screw get Warm

If no Heat, most likely Bad.

You can get The Fets from Elecraft ,ebay   internet.

Ray   WA6VAB   K3.   Rocklin CA.


From: Brian
Sent: Thursday, May 28, 2020 4:15 AM
Subject: Re: [Elecraft-K3] K3s no power on TX


Hi Bob


I have the Sub Receiver removed. The interface card has been removed and replaced a few times in my testing.


I have reseated all the connections within the radio.


Although I first noticed the problem after it was moved from CA to NY, after some serios checks I no longer believe the problem occurred in transport.


I am 90 % sure the RD15HVF1s have failed. As this is now  a spare K3S I have time to play fixing it, I am in no hurry to start unsoldering parts in my diagnostics.


I have a few ways to go, get some conformation from Elecraft that my diagnostics is reasonable correct, run simulations  of the circuit, compare to my working K3S or just change the parts, I am trying the first for a start.


This radio was assembled by a kid who built a K2/100 at age 6 and assembled both my K3S, with little supervision, so I have not delved into these radio much in the past.


73 Brian KF6C.


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