K3- Low Power Amplifier--RD15HVF1 FET failure?


M Gerber
 

My K3 with built-in 100W amp power out into nice dummy load and LP100 wattmeter reads reasonably accurately as PWR is advanced to 6W. Then, with PWR set 6-12W, RF out maxes at ~4-8W depending on band. With PWR set at 12W, which is the transition to turn on the 100W amp, RF out jumps accurately to 12W out and climbs normally from there as PWR setting is advanced. The RF out of my K3 previously had been remarkably accurate from 0-100W on virtually all bands (less RF out on 6 and 10M) prior to this problem.

I am pretty sure that one of the (2) RD15HVF1 FET's in the Low Power Amplifier has died.

I did a search through the group archive and did not see anything related. 

 The FET's are cheap and my inclination is to replace both and rerun the Tx Gain Calibration.

Has anyone else had a similar problem? 

Thanks and 73

Mark WH7W


Doug, VE3MV
 

GM Mark:  

If, as  you suggest, one of the FET's in the LPA has died, I don’t think you would still be able to get 100W out.  Perhaps a intermittent connection on the pins / connecter between the LPA and the main board. 

What happens if you just rerun the TX Gain Calibration?

73 & Good Luck,

Doug  VE3MV

 

On 2018-Apr-22, at 2:01 AM, M Gerber <msgerber@...> wrote:

My K3 with built-in 100W amp power out into nice dummy load and LP100 wattmeter reads reasonably accurately as PWR is advanced to 6W. Then, with PWR set 6-12W, RF out maxes at ~4-8W depending on band. With PWR set at 12W, which is the transition to turn on the 100W amp, RF out jumps accurately to 12W out and climbs normally from there as PWR setting is advanced. The RF out of my K3 previously had been remarkably accurate from 0-100W on virtually all bands (less RF out on 6 and 10M) prior to this problem.

I am pretty sure that one of the (2) RD15HVF1 FET's in the Low Power Amplifier has died.

I did a search through the group archive and did not see anything related. 

 The FET's are cheap and my inclination is to replace both and rerun the Tx Gain Calibration.

Has anyone else had a similar problem? 

Thanks and 73

Mark WH7W


mikerodgerske5gbc
 

I think if 1 were bad, power out would be zero. With power set to zero they will get hot to the touch fast. If ones bad it'll be cool. 
You should be able to find info on this perhaps via babble or google. 
I've been there done that but don't have the 100w amp. 

73
Mike R

7!
Absolutely Awesome 

On Apr 22, 2018, at 6:20 AM, Doug, VE3MV <dougve3mv@...> wrote:

GM Mark:  

If, as  you suggest, one of the FET's in the LPA has died, I don’t think you would still be able to get 100W out.  Perhaps a intermittent connection on the pins / connecter between the LPA and the main board. 

What happens if you just rerun the TX Gain Calibration?

73 & Good Luck,

Doug  VE3MV

 

On 2018-Apr-22, at 2:01 AM, M Gerber <msgerber@...> wrote:

My K3 with built-in 100W amp power out into nice dummy load and LP100 wattmeter reads reasonably accurately as PWR is advanced to 6W. Then, with PWR set 6-12W, RF out maxes at ~4-8W depending on band. With PWR set at 12W, which is the transition to turn on the 100W amp, RF out jumps accurately to 12W out and climbs normally from there as PWR setting is advanced. The RF out of my K3 previously had been remarkably accurate from 0-100W on virtually all bands (less RF out on 6 and 10M) prior to this problem.

I am pretty sure that one of the (2) RD15HVF1 FET's in the Low Power Amplifier has died.

I did a search through the group archive and did not see anything related. 

 The FET's are cheap and my inclination is to replace both and rerun the Tx Gain Calibration.

Has anyone else had a similar problem? 

Thanks and 73

Mark WH7W


M Gerber
 

I signed into the Group to close the loop on this and also replied to Martin's HS0ZED query re: K3 LPA repair. Hopefully my comments will help someone else.

First, kudos to Milton AD5XD for his lightning fast direct email to me last weekend. Great information from Milton. And kudos to Elecraft email Support. I received a response first thing Monday from them.


One of the FET's Q5 (RD15HVF1) did in fact die. It was due to a construction error on my part. Yes, "What was I thinking?", but I can retrospectively understand why I did what I did.

When I secured the rear bottom cover, I placed the internal tooth washers on the adhesive heatsink pads so the washer was sandwiched between the (3) FET heatsink pads and the cover. This obviously prevented the heatsink pads from making good thermal contact with the bottom cover. The K3 Assembly Instructions are quite clear and warn you not to forget the internal-tooth lock washers-----but they go under the 4-40 1/4" machine screws.

My excuse: The internal tooth washers would mar the finish on the bottom cover of my brand new beautiful K3!

As I made the repair today, the eureka moment came when I was thinking: "This washer location makes no sense--you need excellent contact for heat transfer and this washer is hurting not helping. I dug out my buried K3 Assembly Manual and reread the "destructions".

I suspect a significant majority---like 99+%--- of you did not make this same mistake, but if someone reads this and did the same thing, I may have saved you a few dollars and 2 hours of time. A quick flip of your K3 is all it takes to check!

Lastly, these forums are priceless. As we say in Hawai'i --Mahalo to everyone who takes a few minutes to reply to the steady stream of questions!

73 & Aloha

Mark WH7W