[Elecraft] [K2] Low TX Output on Higher Bands
As I proceed with checking the new-to-me (S/N 5133, bare-bones, no options) K2 out, I'm finding that the maximum transmit power output drops markedly as the frequency goes up. If I set the power to maximum, I can get the following approximate powers (on CW): 80m: 15 W 40m: 15 W 30m: 15 W 20m: 7 W 17m: 4 W 15m: 4 W 12m: 1.5 W 10m: 1.5 W I've done a few rudimentary checks which I won't list in detail here for now. All transmit tests were done into a dummy load with a good SWR and power levels shown on the K2 LCD and an external meter agreed as well as might be expected. Do these symptoms sound familiar to any of you, or will I have to start a full troubleshooting process? It might be the same as observed by W6ICM ( http://elecraft.365791.n2.nabble.com/K2-low-output-tt461601.html). Are there any other theories? 73, Steve VE3SMA __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] [K2] Low TX Output on Higher Bands
Steve, I have found that behavior in K2s that have some problem with RFC3. Make certain the leads are well stripped and tinned (you should see some tinned wire above the board) and RFC3 is wound on the right core - I have found a few where the builder used the black powdered iron core instead of the ferrite core. The winding of T2 can reduce the power on higher bands, but not as severe as you have, but check anyway to be sure T2 is wound as indicated in the manual and spaced 1/16 to 1/8 inch above the board. You will likely have to do some modified transmit troubleshooting using the test points indicated in the manual, but at a higher power setting than 5 watts (I would recommend 10 watts setting). At each test point, compare the RF voltage for 20 meters with that for 15 meters. When you find the place where the RF voltage for 15 meters is substantially less than that for 20 meters, you have identified the output of the failing stage. 73, Don W3FPR On 11/8/2014 10:42 AM, Steve Kavanagh wrote: As I proceed with checking the new-to-me (S/N 5133, bare-bones, no options) K2 out, I'm finding that the maximum transmit power output drops markedly as the frequency goes up. If I set the power to maximum, I can get the following approximate powers (on CW): 80m: 15 W 40m: 15 W 30m: 15 W 20m: 7 W 17m: 4 W 15m: 4 W 12m: 1.5 W 10m: 1.5 W __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] [K2] Low TX Output on Higher Bands
Steve, When comparing the RF voltages between bands, I meant to say to use 30 meters instead of 20 meters as stated. The idea is to compare one band that is producing full output with another band that has the low output. 73, Don W3FPR On 11/8/2014 2:24 PM, Don Wilhelm wrote: Steve, I have found that behavior in K2s that have some problem with RFC3. Make certain the leads are well stripped and tinned (you should see some tinned wire above the board) and RFC3 is wound on the right core - I have found a few where the builder used the black powdered iron core instead of the ferrite core. The winding of T2 can reduce the power on higher bands, but not as severe as you have, but check anyway to be sure T2 is wound as indicated in the manual and spaced 1/16 to 1/8 inch above the board. You will likely have to do some modified transmit troubleshooting using the test points indicated in the manual, but at a higher power setting than 5 watts (I would recommend 10 watts setting). At each test point, compare the RF voltage for 20 meters with that for 15 meters. When you find the place where the RF voltage for 15 meters is substantially less than that for 20 meters, you have identified the output of the failing stage. 73, Don W3FPR On 11/8/2014 10:42 AM, Steve Kavanagh wrote: As I proceed with checking the new-to-me (S/N 5133, bare-bones, no options) K2 out, I'm finding that the maximum transmit power output drops markedly as the frequency goes up. If I set the power to maximum, I can get the following approximate powers (on CW): 80m: 15 W 40m: 15 W 30m: 15 W 20m: 7 W 17m: 4 W 15m: 4 W 12m: 1.5 W 10m: 1.5 W __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] [K2] Low TX Output on Higher Bands
Don: Many thanks for the tips! RFC3 looks to be right. T2 was sitting right on the board but I was able to pry the side next to T3 up so that it is certainly clear of all the leads around it and there was no change. So I guess I will have to wait a day or two until I get home and have a go with an RF probe. I did do a few DC measurements and noticed that the emitter voltage of the driver, Q6, is about 0.25 V rather than the 0.4 V specified in the manual (at 0.1 W power setting...the manual doesn't clearly specify the power setting for the DC checks). I wondered if the beta of Q6 might be a bit weak...but it doesn't quite feel like what would drop the power by 10 dB. The bias is about right...around 1.05V. Thanks & 73, Steve VE3SMA - Original Message - From: Don Wilhelm To: Steve Kavanagh ; "Elecraft@mailman.qth.net" Cc: Sent: Saturday, November 8, 2014 5:35 PM Subject: Re: [Elecraft] [K2] Low TX Output on Higher Bands Steve, When comparing the RF voltages between bands, I meant to say to use 30 meters instead of 20 meters as stated. The idea is to compare one band that is producing full output with another band that has the low output. 73, Don W3FPR On 11/8/2014 2:24 PM, Don Wilhelm wrote: > Steve, > > I have found that behavior in K2s that have some problem with RFC3. > Make certain the leads are well stripped and tinned (you should see > some tinned wire above the board) and RFC3 is wound on the right core > - I have found a few where the builder used the black powdered iron > core instead of the ferrite core. > > The winding of T2 can reduce the power on higher bands, but not as > severe as you have, but check anyway to be sure T2 is wound as > indicated in the manual and spaced 1/16 to 1/8 inch above the board. > > You will likely have to do some modified transmit troubleshooting > using the test points indicated in the manual, but at a higher power > setting than 5 watts (I would recommend 10 watts setting). At each > test point, compare the RF voltage for 20 meters with that for 15 > meters. When you find the place where the RF voltage for 15 meters is > substantially less than that for 20 meters, you have identified the > output of the failing stage. > > 73, > Don W3FPR > > On 11/8/2014 10:42 AM, Steve Kavanagh wrote: >> As I proceed with checking the new-to-me (S/N 5133, bare-bones, no >> options) K2 out, I'm finding that the maximum transmit power output >> drops markedly as the frequency goes up. If I set the power to >> maximum, I can get the following approximate powers (on CW): >> >> 80m: 15 W >> 40m: 15 W >> 30m: 15 W >> 20m: 7 W >> 17m: 4 W >> 15m: 4 W >> 12m: 1.5 W >> 10m: 1.5 W >> >> > __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] [K2] Low TX Output on Higher Bands
Steve, Re-check the voltage on Q6. The emitter should be 0.6 volts *lower* than the base - which should put it at the manual's voltage of 0.4 volts. The power setting should not matter for that measurement. Check the emitter resistor on U6 - it should be 1.5 ohms. If this is a K2 below SN3000 without the A to B upgrades, R50 may be a 1/4 watt resistor, and it was replaced with a 1/2 watt resistor. If R50 has been overheated, it is conceivable that its resistance is lower than the 1.5 ohms. Another thing to check is the TX buffer stage U9. First measure the DC voltage at pin 3 - if Q20 is working properly, it will be at 12 volts (or power supply voltage) during receive, but drop to 1/2 that value during transmit (or TUNE). If it does not change, replace Q20 (2N7000 which is static sensitive). Also check the resistance of R93 (820 ohms) and R94 (82 ohms) as well as the value of C150 (330pF). Note that R94 and C150 are in series which makes the gain of U9 greater as the frequency is increased. 73, Don W3FPR On 11/9/2014 10:56 AM, Steve Kavanagh wrote: Don: Many thanks for the tips! RFC3 looks to be right. T2 was sitting right on the board but I was able to pry the side next to T3 up so that it is certainly clear of all the leads around it and there was no change. So I guess I will have to wait a day or two until I get home and have a go with an RF probe. I did do a few DC measurements and noticed that the emitter voltage of the driver, Q6, is about 0.25 V rather than the 0.4 V specified in the manual (at 0.1 W power setting...the manual doesn't clearly specify the power setting for the DC checks). I wondered if the beta of Q6 might be a bit weak...but it doesn't quite feel like what would drop the power by 10 dB. The bias is about right...around 1.05V. __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] [K2] Low TX Output on Higher Bands
Well, I've been doing lots of measurements and inspections and I'm still not sure what is the matter. As suggested I rechecked the DC voltages on Q6 and got 1.03 V on the base and 0.25 V on the emitter. The difference is 0.79 V - high, but plausible. The emitter resistor R50 measures 1.4 ohms...OK. R93 measures 809 ohmsOK. R94 measures 83 ohms...OK. C150 is marked 331J...OK. Q20 seems to be working fine. W6 to ground measures 1kohm...OK. The ungrounded end of C212 is grounded for DC regardless of which band is selected...OK. RF voltage is seen on Q212 only on 24 and 28 MHz, so the correct filter is selected on those bands. 8T applied to D1 is fine. I measured the Q7/Q8 resistances per the manual. With the positive lead on the base I measured around 520 ohms to emitter and collector. With the negative lead on the base I get about 750 ohms to emitter and about 5k to the collector. The 750 ohm number seems a bit low and might indicate something. Here's my RF voltage measurement results for various requested power settings. These are actually the DC voltage displayed on a DMM using the RF probe circuit described in the K2 manual (though not using the Elecraft board). At W6 (in mV) MHz 1W 5W 10W 3.5 3 10 16 7 4 14 26 10 3 11 22 14 7 25 31 18 5 9 9 21 2 4 4 24 7 8 8 28 11 11 11 The lack of change at higher power levels on the higher bands seems to agree with the fact that the K2 output power stops increasing as the power knob is advanced. At Q5 collector (in V) MHz 1W5W10W 3.5 .088 .175 .25 7 .024 .070 .11 10 .080 .22 .34 14 .166 .36 .42 18 .35 .51 .51 21 .46 .72 .72 24 .60 .64 .64 28 .73 .76 .75 We see much higher output and less variation with power setting on the higher bands. This seems consistent with a following stage having gain roll off at the higher frequencies. At Q6 collector (in V) MHz 1W 5W 10W 3.5 .89 1.5 2.2 7 .90 1.7 2.2 10 1.0 2.1 3.1 14 1.1 2.4 2.8 18 1.6 2.5 2.5 21 2.1 3.1 3.1 24 2.5 2.7 2.7 28 2.9 3.1 3.1 Same behaviour here. At Q7/Q8 base (in V, no 1 W setting, not all frequencies measured at 10 W setting) MHz 5W 10W 3.5 .39/.41 -/- 7 .40/.40 .57/.62 10 .37/.36 -/- 14 .31/.35 .37/.42 18 .35/.33 -/- 21 .40/.40 .39/.37 24 .29/.33 -/- 28 .26/.29 .26/.28 Now we see lower voltages at the higher frequencies, despite what was seen at the collector of Q6. This is probably consistent with the lower base impedance as frequency increases (though feedback will modify this). At Q7/Q8 collector (in V, only some frequencies and no 1 W setting) MHz 5W 10W 75.1/5.3 7.0/7.1 145.4/5.3 6.2/6.1 214.6/4.8 4.6/4.9 283.6/3.8 3.2/3.4 The output voltage drops with frequency and increases with power setting only at the lower bands, which is consistent with what is seen on the power meter. My best guess is that one of the finals (Q7 or Q8) is bad, but it isn't completely clear since the manual only gives RF voltages for 40m and I don't know how it is supposed to vary over frequency. Any clues here for those more familiar with troubleshooting the K2? 73, Steve VE3SMA - Original Message - From: Don Wilhelm Sent: Sunday, November 9, 2014 4:26 PM Subject: Re: [Elecraft] [K2] Low TX Output on Higher Bands Steve, Re-check the voltage on Q6. The emitter should be 0.6 volts *lower* than the base - which should put it at the manual's voltage of 0.4 volts. The power setting should not matter for that measurement. Check the emitter resistor on U6 - it should be 1.5 ohms. If this is a K2 below SN3000 without the A to B upgrades, R50 may be a 1/4 watt resistor, and it was replaced with a 1/2 watt resistor. If R50 has been overheated, it is conceivable that its resistance is lower than the 1.5 ohms. Another thing to check is the TX buffer stage U9. First measure the DC voltage at pin 3 - if Q20 is working properly, it will be at 12 volts (or power supply voltage) during receive, but drop to 1/2 that value during transmit (or TUNE). If it does not change, replace Q20 (2N7000 which is static sensitive). Also check the resistance of R93 (820 ohms) and R94 (82 ohms) as well as the value of C150 (330pF). Note that R94 and C150 are in series which makes the gain of U9 greater as the frequency is increased. 73, Don W3FPR __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] [K2] Low TX Output on Higher Bands
In the remote chance that someone else is tracking down a similar problem, I eventually tracked this issue down to a bad solder joint at R94 (in the transmit buffer circuitry). Thanks to Don, W3FPR, for his (now legendary) assistance! 73,Steve VE3SMA From: Steve Kavanagh To: "Elecraft@mailman.qth.net" Sent: Saturday, November 8, 2014 10:42 AM Subject: [K2] Low TX Output on Higher Bands As I proceed with checking the new-to-me (S/N 5133, bare-bones, no options) K2 out, I'm finding that the maximum transmit power output drops markedly as the frequency goes up. If I set the power to maximum, I can get the following approximate powers (on CW): 80m: 15 W 40m: 15 W 30m: 15 W 20m: 7 W 17m: 4 W 15m: 4 W 12m: 1.5 W 10m: 1.5 W I've done a few rudimentary checks which I won't list in detail here for now. All transmit tests were done into a dummy load with a good SWR and power levels shown on the K2 LCD and an external meter agreed as well as might be expected. Do these symptoms sound familiar to any of you, or will I have to start a full troubleshooting process? It might be the same as observed by W6ICM ( http://elecraft.365791.n2.nabble.com/K2-low-output-tt461601.html). Are there any other theories? 73, Steve VE3SMA __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
