Re: [Elecraft_K3] K3 output power erratic

Bob McGraw - K4TAX

Since you have a good dummy load and a clean test setup, I'd suggest you run the Calibrate Transmitter Gain as found with the Elecraft Utility.    


Bob, K4TAX

On 1/10/2018 8:35 AM, Charles Garner skyviewcharles@... [Elecraft_K3] wrote:


About a year ago I sent my K3 to Elecraft due to the output power behaving erratically on AM operation. Now it started acting up once again. This time on bands higher than 14 MHz the output power can take several seconds to ramp up to 100 Watts output. I'm referring to already having the output power control set for 100 Watts and then pressing the XMT button or setting to FM mode and keying the mic; in other words simply commanding the K3 to put out a 100 Watt carrier. Sometimes on 29 MHz it's particularly bad and will sit at about 50 Watts for 15 seconds and then slowly creep up to 100. Sometimes it initially continues past 100 and stops around 115 Watts and then settles back to 100.
Thor original repair replaced the driver and final FETs within the LPA and also changed to new gold faceplate connector pins.
So I opened it up and found that Q3 on the LPA was blown "open". Q3 is the bias control switch, an SMT bipolar transistor. I honed my SMT skills and I replaced Q3. I also replaced the LPA's driver and final FETs; in other words the 3 primary FETs in the LPA. I tried FW versions 5.60 and 5.50. I reseated the faceplate connections. I ran the transmit gain alignment and it fails usually on the 29 MHz cycle. The error message states "power cannot settle". One hint is I checked the transmitter gains and all look reasonable except 29 MHz which shows a gain of 115 for the HPA. Another hint on 29 MHz is the HI CURRENT warning shows occasionally.
Since the HPA does output 100 W I conclude it's not the issue. I also conclude that the LPA is OK especially since I just changed all of its FETs. So what else is there to cause a slow ramp up in power? Keep in mind this problem is only seen at frequency bands higher than 14 MHz albeit perhaps it's just slightly doing it on 14 MHz. The rig operates apparently perfectly from 10 MHz on down to 1.8 MHz.

Has anyone seen this erratic power behavior and if so what was the root cause? Thanks in advance.

Problem in low pass filter sections? Spurious output? Bad connection from output board to antenna connector on back panel?

All the testing is being done into a known good 50 Ohm dummy load. The test setup is clean.

73 N6AF Frustrated Chuck

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