FW: my K3 has some issues-NOT SOLVED- Low Power Amp MOSFETs
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Alas, my K3 power out problem WAS NOT SOLVED by replacing the LPA MOSFETs.
Upon first turn on, the xmit power could be set high on all bands as expected. But then, after a few hours of the radio just sitting there in receive, going to transmit it would not reach a power output past ~40 watts (varied by band and varies a few watts as it sits in transmit) and will fail the transmit gain calibration. Picking up on a possible related thread, I took off the bottom rear case panel and tested the two SWR bridge diodes. They both appear to be good according to my DVM. Is that a good enough test? My 12V supply voltage was a bit low according to other owners (13.4 v dropping to 12.9 v on XMIT)) so I adjusted the Astron SS-30M to 14.5 volts. Now, setting the xmit power higher works when going to TUNE, and the radio passed all the calibrate xmtr gain tests. So again I thought maybe that was it. But again, nope. After being on for over 8 hours in receive, and heating up internally, it fails to reach the set 70W output. But after being turned off for about 2 hours, it resumed producing the set 70W. See the attached chart. Looks like embedded pictures or charts are not accepted here, so this is the significant data I wanted to illustrate: 14 MHz USB Pwr set to 70 Watts At first turn on at 9 AM, the DISP readings were: V in rx 14.4 V V in tx 14.0 V I in rx 1.05 A I in ptt only 2.04 A I in full xmt 15.15 A PA temp 22C FP temp 16C At 4:05 PM when it failed, it had been sitting for 3 hours in receive-only: PA temp 35C FP temp 36C (97F!) Something temperature, voltage, time related? Any other ideas out there? 73, Chet, N8RA -----Original Message----- From: Chet S <[hidden email] <mailto:[hidden email]> > Sent: Tuesday, November 17, 2020 9:22 AM To: '[hidden email]' <[hidden email] <mailto:[hidden email]> >; '[hidden email]' <[hidden email] <mailto:[hidden email]> > Subject: RE: [Elecraft] my K3 has some issues-SOLVED- Low Power Amp MOSFETs Replacing the final output MOSFETs in my K3's low power amp appears to have fixed its transmit power issues. Thank you everyone on the K3 forums for your shared advice and experiences. The "3 screw" tests were pointing to a bad output transistor in the LPA. The idle current increase on the K3 meter when putting the radio into XMIT (not TUNE) was only 310 ma, where at least 600ma was expected, again pointing to a FET being open. To be sure of getting the right vintage of FETs, I ordered a couple of them (and thermal insulator) from Elecraft. My order was shipped that same day and arrived via priority mail 2 days later. Great service. Following the procedure described by others, I replaced both output MOSFETs. Since one had been inoperative, the other had been carrying the whole load to produce output so may have been overworked, overstressed and hurting. Here are my additional comments on the repair process: 1) The Elecraft document "KPA3CONMDKT MOD Kit Manual" has a wonderful description of how to remove and replace the 100W amplifier to allow access to the LPA's adjustment potentiometers. 2) Turning the power to zero is a good safeguard when doing the bias adjustment. I also made sure the radio was in USB or LSB so no power output would be commanded in any case. Now with the FETs replaced and the bottom cover back on, the moment of truth.. Three LPA potentiometers turned down. Turn first pot, +200ma. Turn second pot, another 200ma. Turn third pot, yet another 200ma. Success! Put the 100W amp module back in and connected to the Elecraft K3 utility. All 3 transmitter gain tests passed, and now the power setting is stable on all bands. On 40M where the reported SWR into a dummy load before the repair was 1.4 instead of 1.0 was now down to 1.2. Still do not know why but I can certainly live with that. So. why did this even happen in the first place? I can only speculate. Older generation MOSFETs were difficult to keep in their safe operating region without snubbers. Did some transient zap it? Power semiconductor chip bonding is subject to failure after a high number of thermal cycles, and this has been a contest radio. In January during the VHF contest, my 6M antennas became badly detuned with wet snow and ice accumulation. This led to a high swr propagating thru the transmit chain to the K3 which suffered a total loss of power output on 6M with a very high swr indicated. After a rest, the K3 recovered, but did that event set this failure in motion? GOK. And a closing suggestion- if your K3 is working OK now, save a copy of its configuration: this may turn out to be quite handy in the future. TU all and 73, Chet, N8RA -----Original Message----- From: <mailto:[hidden email]> [hidden email] < <mailto:[hidden email]> [hidden email]> On Behalf Of Chet S Sent: Tuesday, November 10, 2020 4:02 PM To: <mailto:[hidden email]> [hidden email] Subject: [Elecraft] my K3 has some issues Good afternoon K3 folks, I am new to this list and honored to be accepted. Recently my K3 has experienced a couple of issues. I have been unable to get a response from Elecraft e-mail or telephone service regarding the situation. It's been almost two weeks now so thought I'd ask here if anyone has some suggestions for me to try myself. Here is the main concern: When getting ready for CQWWDX SSB a few weeks ago, I went through setting the transmit power on each band to drive an amplifier properly. The power display seemed a bit sluggish and had a delay in indicating the SWR. Worse yet, when I got to 40M, I could not crank the power up past about 30 watts; it would momentarily display a higher number but then fall back to 30w. All other bands cold be set higher. First I reloaded all the current firmware in the main and subreceiver. No change to 40M. So next I went to the K3 Utility ver 1.20.1.15 to do a transmitter gain calibration. Starting with the 5W calibration, it stepped thru the first few bands OK but then stopped (I think on 40M) and displayed "ERR TXG" and the message "calibration settled at 3.6W, TX gain failed because transmit power did not reach the expected value". In anticipation of having to send the radio to Elecraft where it might be totally reset, I manually started recording all the configuration settings. When I looked at my filter settings, I was shocked to see that filter 1 in the subreceiver was set to 1.35. Huh? It should have been 2.80 !! Was my config info corrupted? I reset that to 2.80 and then for the heck of it tried again to set the power on 40M. That now worked "OK" and the swr and pwr display during "tune" showed instantly on all bands. Something had changed. Next, I decided to rerun the TX calibration gain routine. This time if successfully completed the 5w and 50w routines. However, with a good dummy load, 40M displays an swr of 1.4, while the other bands are 1.0 or 1.1, so something is still a little funny on 40M. What might that be? I have now been considering doing a total reset of the radio using the EE INIT function and then the self calibrations. Is this wise? Any experiences or suggestions appreciated. 73, Chet, N8RA ______________________________________________________________ Elecraft mailing list Home: <http://mailman.qth.net/mailman/listinfo/elecraft> http://mailman.qth.net/mailman/listinfo/elecraft Help: <http://mailman.qth.net/mmfaq.htm> http://mailman.qth.net/mmfaq.htm Post: <mailto:[hidden email]> mailto:[hidden email] This list hosted by: <http://www.qsl.net> http://www.qsl.net Please help support this email list: <http://www.qsl.net/donate.html> http://www.qsl.net/donate.html Message delivered to <mailto:[hidden email]> [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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