low pre-driver
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Hi,
When doing the 40-Meter Transmitter Alignment, I couldn't keep the output power in th 1-2 watt range. Turning the power all the way up only got up to .7 watts. I checked the output across the dumy load and it was practically non-existent (.007Vrms) though key-down was noticeably different than key-up. I walked through the Transmitter signal tracing. Everything was within tolerances until I got to the pre-driver output. I expected about .132Vrms but got .07 Vrms. I've checked all the components in the pre-driver section up through T1 (not sure where pre-driver ends and driver begins) and they all have the right markings. I verified T1 and reheated the connections. On qth.net I only found references to high pre-driver issues. Suggestions? Transmit mixer output: .016 Vrms Buffer output: .216 Vrms Bandpass filter output: .035 Vrms TR switch output: .033 Vrms Predriver output: .072 Vrms (expected .13 ) Robert, K6RJG ______________________________________________________________ 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 |
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Robert,
OK, the Pre-driver output is low - next step is to check the DC voltages for Q5 - but that must be done during transmit. It is difficult (almost impossible) to check the emitter without removing the heat-sink. WARNING - even though the power output is low, do not operate the K2 without the heatsink attached. So during transmit, check the collector voltage - should be close to 12.4 volts And check the base voltage which can be reached at the end or either R40 or R41 - should be 1.3 volts. If all was well for those checks, remove the heatsink and measure the resistance between the emitter of Q5 and ground - if it is more than 27 ohms, check the values of R42 and R43 and check their soldering. If the resistance was correct, check the value of C117 (should be .047 uF) and reflow its soldering. Also check C116 (33 pF), C118 (.01 uF) and C115 (.01) and reflow the soldering for those too. Then put the heat sink back on and see if there is any change. BTW - you can consider the division between the pre-driver and the driver as being between the primary and the secondary winding of T1. 73, Don W3FPR Robert Greenwalt wrote: > Hi, > > When doing the 40-Meter Transmitter Alignment, I couldn't keep the output > power in th 1-2 watt range. Turning the power all the way up only got up to > .7 watts. I checked the output across the dumy load and it was practically > non-existent (.007Vrms) though key-down was noticeably different than > key-up. > > I walked through the Transmitter signal tracing. Everything was within > tolerances until I got to the pre-driver output. I expected about .132Vrms > but got .07 Vrms. > > I've checked all the components in the pre-driver section up through T1 (not > sure where pre-driver ends and driver begins) and they all have the right > markings. I verified T1 and reheated the connections. > > On qth.net I only found references to high pre-driver issues. Suggestions? > > Transmit mixer output: .016 Vrms > Buffer output: .216 Vrms > Bandpass filter output: .035 Vrms > TR switch output: .033 Vrms > > Predriver output: .072 Vrms (expected .13 ) > > Robert, K6RJG > ______________________________________________________________ > 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 > > 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 |
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Thanks for the suggestions Don,
Everything checked out. The collector was 13.7 not transmitting and dropped to 12.0 when transmitting. R40 was 0V not trans, 1.2V trans. Resistance of Q5 emitter was 26.6 ohms. R42 and 43 had the right markings - I reflowed the solder. I checked all those caps and reflowed their solder. I put the heatsink back on and there was no change. The Q5 collector was still off by a factor of 2 (.072 Vrms measured, .13 expected). Any other ideas? Robert K6RJG On Sun, Jul 11, 2010 at 3:04 PM, Don Wilhelm <[hidden email]> wrote: > Robert, > > OK, the Pre-driver output is low - next step is to check the DC voltages > for Q5 - but that must be done during transmit. It is difficult (almost > impossible) to check the emitter without removing the heat-sink. WARNING - > even though the power output is low, do not operate the K2 without the > heatsink attached. > So during transmit, check the collector voltage - should be close to 12.4 > volts > And check the base voltage which can be reached at the end or either R40 or > R41 - should be 1.3 volts. > If all was well for those checks, remove the heatsink and measure the > resistance between the emitter of Q5 and ground - if it is more than 27 > ohms, check the values of R42 and R43 and check their soldering. > If the resistance was correct, check the value of C117 (should be .047 uF) > and reflow its soldering. Also check C116 (33 pF), C118 (.01 uF) and C115 > (.01) and reflow the soldering for those too. Then put the heat sink back > on and see if there is any change. > > BTW - you can consider the division between the pre-driver and the driver > as being between the primary and the secondary winding of T1. > > 73, > Don W3FPR > > Robert Greenwalt wrote: > >> Hi, >> >> When doing the 40-Meter Transmitter Alignment, I couldn't keep the output >> power in th 1-2 watt range. Turning the power all the way up only got up >> to >> .7 watts. I checked the output across the dumy load and it was >> practically >> non-existent (.007Vrms) though key-down was noticeably different than >> key-up. >> >> I walked through the Transmitter signal tracing. Everything was within >> tolerances until I got to the pre-driver output. I expected about >> .132Vrms >> but got .07 Vrms. >> >> I've checked all the components in the pre-driver section up through T1 >> (not >> sure where pre-driver ends and driver begins) and they all have the right >> markings. I verified T1 and reheated the connections. >> >> On qth.net I only found references to high pre-driver issues. >> Suggestions? >> >> Transmit mixer output: .016 Vrms >> Buffer output: .216 Vrms >> Bandpass filter output: .035 Vrms >> TR switch output: .033 Vrms >> >> Predriver output: .072 Vrms (expected .13 ) >> >> Robert, K6RJG >> ______________________________________________________________ >> 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 >> >> >> > 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 |
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Robert,
Compare the RF voltage at the base of Q5 - you can reach the base at the end of R40 or R41 - and compare that with the Pre-Driver output at the anode of D1. If they are not substantially the same, check C107, it is the only component between those two points. It is strange that you do not have higher than the "expected" RF voltages at the early stages. Normally when there is low power output, the RF voltages for the working stages up to the point of failure are at maximum because the microprocessor is trying to increase drive in an attempt to bring the power up. Are you using the indication on the K2 display to determine the power output? Or are you using an external meter? If you are using an external meter, what does the K2 display indicated for power out during a tune? What I am wondering is if the problem is in the output detection circuit. 73, Don W3FPR Robert Greenwalt wrote: > Thanks for the suggestions Don, > > Everything checked out. > > The collector was 13.7 not transmitting and dropped to 12.0 when > transmitting. > R40 was 0V not trans, 1.2V trans. > Resistance of Q5 emitter was 26.6 ohms. > R42 and 43 had the right markings - I reflowed the solder. > I checked all those caps and reflowed their solder. > > I put the heatsink back on and there was no change. The Q5 collector was > still off by a factor of 2 (.072 Vrms measured, .13 expected). > > Any other ideas? > > Robert K6RJG > > On Sun, Jul 11, 2010 at 3:04 PM, Don Wilhelm <[hidden email]> wrote: > > >> Robert, >> >> OK, the Pre-driver output is low - next step is to check the DC voltages >> for Q5 - but that must be done during transmit. It is difficult (almost >> impossible) to check the emitter without removing the heat-sink. WARNING - >> even though the power output is low, do not operate the K2 without the >> heatsink attached. >> So during transmit, check the collector voltage - should be close to 12.4 >> volts >> And check the base voltage which can be reached at the end or either R40 or >> R41 - should be 1.3 volts. >> If all was well for those checks, remove the heatsink and measure the >> resistance between the emitter of Q5 and ground - if it is more than 27 >> ohms, check the values of R42 and R43 and check their soldering. >> If the resistance was correct, check the value of C117 (should be .047 uF) >> and reflow its soldering. Also check C116 (33 pF), C118 (.01 uF) and C115 >> (.01) and reflow the soldering for those too. Then put the heat sink back >> on and see if there is any change. >> >> BTW - you can consider the division between the pre-driver and the driver >> as being between the primary and the secondary winding of T1. >> >> 73, >> Don W3FPR >> >> Robert Greenwalt wrote: >> >> >>> Hi, >>> >>> When doing the 40-Meter Transmitter Alignment, I couldn't keep the output >>> power in th 1-2 watt range. Turning the power all the way up only got up >>> to >>> .7 watts. I checked the output across the dumy load and it was >>> practically >>> non-existent (.007Vrms) though key-down was noticeably different than >>> key-up. >>> >>> I walked through the Transmitter signal tracing. Everything was within >>> tolerances until I got to the pre-driver output. I expected about >>> .132Vrms >>> but got .07 Vrms. >>> >>> I've checked all the components in the pre-driver section up through T1 >>> (not >>> sure where pre-driver ends and driver begins) and they all have the right >>> markings. I verified T1 and reheated the connections. >>> >>> On qth.net I only found references to high pre-driver issues. >>> Suggestions? >>> >>> Transmit mixer output: .016 Vrms >>> Buffer output: .216 Vrms >>> Bandpass filter output: .035 Vrms >>> TR switch output: .033 Vrms >>> >>> Predriver output: .072 Vrms (expected .13 ) >>> >>> Robert, K6RJG >>> ______________________________________________________________ >>> 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 >>> >>> >>> >>> > ______________________________________________________________ > 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 > > 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 |
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