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K3 repairs-3 screw FET check


Chet, N8RA
 

Thank you all for the input!

 

I performed the 3 screw test with the xmit power set to 8.0 W. My supply voltage is 13.2v in xmit. How long should I wait to check the temperature rise?

 

After about 5 minutes, the middle screw was starting to approach uncomfortably hot  - even thru my old calloused finger, while with the other two it was difficult to tell they were warming.  Does this indicate that the middle one (Q4) has a problem or would that have been expected after 5 minutes of continuous output? Q4 and Q5 were definitely different temperatures though and I would have expected them to be the same. I removed the rear bottom cover. The silpads were stuck well to the cover but did eventually come free of it. I did not try to remove them from the FETs to check the part number.

 

Replacement of the questionable or all FETs looks doable, but then how does their bias current potentiometers get set?

 

Could it be that the bias of the hotter Q4 is set too high? Am I overthinking?

 

I see RF parts does still carry the MOSFETs and it has been mentioned that Elecraft may also be a source.  Is there a better replacement than the parts list numbers (RD06HHF1 and RD15HVF1)?

 

Please let me know what you make of this.

 

Tnx and 73,

Chet, N8RA

 

I do not have the KAT3 and will check into RF board LP filter area next.

 

 

 

From: Elecraft-K3@groups.io <Elecraft-K3@groups.io> On Behalf Of Robert W5AJ
Sent: Wednesday, November 11, 2020 1:23 PM
To: Elecraft-K3@groups.io
Subject: FW: [Elecraft-K3] K3 repairs

 

Keith,   I believe in your mind you removed the bottom cover for FET temperature test, which is a no no for this test

 

Leave the K3 as is, place finger on three screws on bottom cover (The screws with locking washer on them are for three FETs)

 

One has to be able to use the XMIT function (long press)  with bottom cover on and FET’s screwed on to bottom cover (NORMAL OPERATION!)

 

I set my TUNE power to 10 watts for the temperature test on three FETs on bottom

Or 100 watts, or 50 watts,  I don’t use 11 or 12 since FT8 at those levels is most likely cause of Driver FET burn out

insure your favorite dummy load is attached.

 

73 W5AJ

 

PS:  Moderator really – really needs to fix list outgoing messages so replies to messages include original message

 

Robert

Midland, Texas

http://w5aj.eqth.net/Infinity.pdf

 

From: Elecraft-K3@groups.io [mailto:Elecraft-K3@groups.io] On Behalf Of Keith_WE6R
Sent: Wednesday, November 11, 2020 11:04 AM
To:
Elecraft-K3@groups.io
Subject: Re: [Elecraft-K3] K3 repairs

 

Hi Chet;
Sub RX filter settings will NOT affect transmit.
But if there was a glitch, certainly check the MAIN filters. The filters with TX settings are important.

NOTE to ALL; on the 3-Finger FET test!!! Do NOT go into transmit!! You can cook the FETs. 
Just TAP XMIT to turn on the BIAS only! (Red LED will be on, with NO RF going out).

If you have a KAT3, check the L10-C10 8.215 trap. It can get burnt and affect some bands. If burnt, just put a jumper in place of L10.
Also inspect the LPF components on the RF board under the KPA3 (pull the KPA3). Look around L20 or check the schematic for the components for your trouble band.
Keith WE6R Elecraft K3 Tech.


Keith_WE6R
 

To All;
To clarify, I do my 3-finger test with the bottom cover OFF and I do NOT Transmit, just TAP xmit to turn the Bias ON.
In 10-15 seconds the test is over as good devices will get hot fast.
The cover ON and going into full TX test is valid, just takes longer and a bit harder to tell what is happening.

Chet, the device getting hot is the GOOD one! It is making RF and doing its job.
Q2, the driver, should also heat, but it is doing less work. If you are getting RF, Q2 is probably fine.
***The two finals Q4&5 are in parallel and should heat EVENLY! I would replace yours, re-bias and re-run the 5w tx gain calibration.
Keith WE6R, Elecraft K3 Tech.


Ray Maxfield
 

To Clearify……….

KPA3A Power Amplifier(Q4&5) consists of two RF power transistors operating as a broadband push-pull

amplifier;  Ray  WA6VAB  K3

 

From: Keith_WE6R
Sent: Thursday, November 12, 2020 8:26 AM
To: Elecraft-K3@groups.io
Subject: Re: [Elecraft-K3] K3 repairs-3 screw FET check

 

To All;
To clarify, I do my 3-finger test with the bottom cover OFF and I do NOT Transmit, just TAP xmit to turn the Bias ON.
In 10-15 seconds the test is over as good devices will get hot fast.
The cover ON and going into full TX test is valid, just takes longer and a bit harder to tell what is happening.

Chet, the device getting hot is the GOOD one! It is making RF and doing its job.
Q2, the driver, should also heat, but it is doing less work. If you are getting RF, Q2 is probably fine.
***The two finals Q4&5 are in parallel and should heat EVENLY! I would replace yours, re-bias and re-run the 5w tx gain calibration.
Keith WE6R, Elecraft K3 Tech.

 


Chet, N8RA
 

Thank you Keith. That confirms what I was concluding.

Is there a design value for the Q4 and Q5 quiescent current?

 

Their MOSFET datasheet has them operating at 0.5 amps, but that seems way too high for the K3’s power out and heatsinking.

 

73,

Chet, N8RA

 

 

From: Elecraft-K3@groups.io <Elecraft-K3@groups.io> On Behalf Of Keith_WE6R
Sent: Thursday, November 12, 2020 11:26 AM
To: Elecraft-K3@groups.io
Subject: Re: [Elecraft-K3] K3 repairs-3 screw FET check

 

To All;
To clarify, I do my 3-finger test with the bottom cover OFF and I do NOT Transmit, just TAP xmit to turn the Bias ON.
In 10-15 seconds the test is over as good devices will get hot fast.
The cover ON and going into full TX test is valid, just takes longer and a bit harder to tell what is happening.

Chet, the device getting hot is the GOOD one! It is making RF and doing its job.
Q2, the driver, should also heat, but it is doing less work. If you are getting RF, Q2 is probably fine.
***The two finals Q4&5 are in parallel and should heat EVENLY! I would replace yours, re-bias and re-run the 5w tx gain calibration.
Keith WE6R, Elecraft K3 Tech.


Keith_WE6R
 

Chet;
Bias is 200ma per device. Support has a procedure. I also sent a direct email.
Keith