K3 repairs-3 screw FET check
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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
Hi Chet; NOTE to ALL; on the 3-Finger FET test!!! Do NOT go into transmit!! You can cook the FETs.
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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.
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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;
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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;
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Chet;
Bias is 200ma per device. Support has a procedure. I also sent a direct email. Keith
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