K3 power problem with LPA FETs
Others have asked so I took a little time today to consolidate my disorganized notes and will add it below.toggle quoted messageShow quoted text
Assembled below are the notes from my experience in finding and fixing a power out problem on my K3. Where possible, I tried to give credit to the authors, to all of which I am grateful.
TESTS FOR THE MOSFETS.
The 3-screw finger test from WA6VAB and others:
There are 3 MOSFETs of the LPA that are heat-sinked to the bottom of the radio by being connected to the bottom plate using phillips head screws. Check your manual for their location
Place the radio so it is raised up using the front bail and place your fingers on the 3 screw that are holding the 3 MOSFETs.
Key the PTT so the XTR is on. All 3 should get warm, Not HOT or Cold.
Hot = shorted, Cold = open, Warm = normal.
WE6R factory tech comments:
The 3-finger bottom cover on and going into full TX test is valid, just takes longer and a bit harder to tell what is happening.
You can do the 3-finger check with the bottom cover OFF, but DO NOT Transmit, just TAP xmit to turn the Bias ON. (and have the mode set to USB or LSB-ed). Stop the test after 10-15 seconds. Good devices will get hot fast.
(N8RA- Note that the MOSFETs have sticky pad attached to them and if the bottom cover has not been off for a while, it may take some gentle wiggling to get the bottom cover off.)
N8RA test using the front panel power supply current display:
If there is a KPA3 installed, configure it to NOT INST and cycle power so you do not have its current added. Tap DISP and turn the VFO-B knob to get to the current display. Turn transmitter power below 10w and put the transmitter in “test” mode with the mode button. Note the difference in current from receive to that when tapping the xmit button. The difference should be about 600 ma. If significantly less or more, then one of the MOSFETs in the low power amp (LPA) may be bad or not biased properly. Reset your KPA3 configuration back to NOR (normal).
ORDER THE RIGHT MOSFET
Only one of my MOSFETs was bad, but I replaced both output MOSFETs. With one failed, the other had been carrying the whole load to produce output so may have been over-worked, over-stressed and hurting.
To be sure of getting the right vintage of RD15HVF1 MOSFETs, last year I ordered them (and thermal insulators) from Elecraft. My order was shipped that same day and arrived via priority mail 2 days later. Great service.
RF Parts is also a source. It has been reported that the earlier MOSFET with -101 is the one to get for the K3.
DOING THE REPAIR
Obviously, the bottom cover must come off. The MOSFETs have sticky pads between them and the bottom cover, so it may take some gentle wiggling to free it.
MOSFET means USE CARE to prevent ESD damage. Use a soldering iron with a grounded tip. Wear an ESD strap, if you do not have one, keep your other wrist on a metal part of the K3 case.
Note the bending direction and bent lead distance of the MOSFETs to prep the new parts.
Take extra ESD care when handling these transistors by themselves.
Cut the leads of a bad part near the body and then unsolder each lead from its pcb hole.
Use a solder sucker or wick to open the pcb holes.
Mount the new MOSFET and reflow its 3 pcb holes.
Add the heat sink pads to the MOSFET tabs and put the bottom cover back on.
MOSFET BIAS ADJUST
From Elecraft Customer Support:
You'll need to follow this sequence for setting bias after the fix:
1. Remove the 100 watt amplifier (if installed) by disconnecting the fan panel and their 2 connector wires, and the 2 wires to the circuit breaker. Use care to not pull on the KPA3 too hard and thus let it fly off the connectors and cause damage to it or to the chassis. Use gentle but firm finger pressure, or pry carefully with a plastic tool (such as the K2 green plastic alignment tool), near the two PCB connectors. This will usually help the KPA3 start to move.
(N8RA: The Elecraft document “KPA3CONMDKT MOD Kit Manual” has a wonderful description of how to remove and reinstall the 100W amplifier to allow access to the LPA’s adjustment potentiometers.)
2. Locate small SMD pots R18, R19 and R24 near the center of the LPA board. Turn the pots to minimum. (White pots fully CW, Black pots fully CCW)
3. Turn the power knob down to ZERO.
4. Tap DISP, (display button), use VFO B to display the current drawn by the radio. For more accurate results, insert a DMM in the positive power supply lead that is set to a current range that can handle up to 2-3 amps.
5. Hold TEST (the right hand side of the MODE button). This puts the K3 in TEST mode with no RF drive.(but it will not light the red xmt led)
6. Tap XMIT. You will see a current of approx 1 to 2A (depending on options in radio) but the red TX LED will not come on. Adjust R18 to increase the reading by 0.2A IE 200ma
7. Adjust R19 to increase the current another 0.2A
8. Adjust R24 to increase the current another 0.2A
9. Tap XMIT to exit bias on condition. (Current should drop approx.600ma). NOTE: if you are just checking current and you do have the KPA3 installed, set it to NOT INST and cycle power so you do not have its current added, thus confusing your readings.
10. Set Power to 11.9W. Hold TEST again to cancel TEST mode so RF output is possible. The LCD shows TX NORM. (Reinstall the 100-watt amplifier.)
Summary and tips from Elecraft Super Tech WE6R:
We bias each device at 200ma increase, from fully off, which is Counter Clock Wise, CCW. (The total for all 3 is 600ma.)
NOTE there were a *FEW* of the older LPA boards that have 3 WHITE pots and they are REVERSED action, IE you start fully clockwise equals no current (vs CCW).
Only tap XMIT to turn on Bias (quiescent current), NOT a long press to full transmit.
Check your power supply voltage on the front panel display. Many have reported that older K3’s seem to work better with this in the 14 volt range.
Connect to the Elecraft K3 utility and run the transmitter gain tests.
If all is well, save a copy of the K3’s configuration file.
Thank you everyone on the K3 forums for your shared advice and experiences.
From: Gordon LaPoint <email@example.com>
Sent: Wednesday, March 17, 2021 9:03 AM
To: firstname.lastname@example.org; email@example.com
Subject: Re: [Elecraft] Elecraft contact- K3 power problem
Thanks! Please send me the notes, I will look at the problem closer. I don't know if the problem is the low power amp or the 100 watt amp yet.
I should be able to fix it, I built a K2 from scratch and have repaired other radios.
Gordon - N1MGO
On 3/17/2021 8:55 AM, firstname.lastname@example.org wrote: