Unfortunately I am still having issues with this KX1... I changed out Q7 and there was no change in voltage reading at Q7's base. I asked Elecraft about this and they informed me that it can be hard to measure accurately at this spot with a DMM do to RF. It makes me wonder if that I am seeing inaccurate readings due to RF as PO seems fine! If I turn R4 counter-clockwise I get the correct 0.7VDC. Meanwhile I resolved the low power issue and everything seemed fine, both transmit and receive, but I still had the negative voltage on Q7's base with R4 fully clockwise.
Well things have gone from (seemingly) bad to worse. I looked at other components in the path. I replaced Q6, just because I had a hard time getting it out to test it. No change. T1's windings didn't look quite right so I redid the green windings, re-soldered, looks good. Checked my soldering in general, looks OK. *Now* I have low audio and low power out on 20 meters. Everything else seems OK. I replaced K1 because it got hit with the soldering iron during repairs. I completely redid T2 on the KX1 board. Putting a finger on T2 raises the noise level greatly on 20M only and there is *no* peaking of the 20M signal with trimmer 20A. 20B peaks as normal as do all the other trimmers for their bands. LPF1 installation looks good. So, I am now trying to resolve a low signal/audio/power out on 20 meters. Tnx, Kevin, KF0JX Kevin Simmons said the following on 11/01/2006 10:30 PM: > Don, > > Yes, you are correct. Lifting one end of R28, Q7 still had a negative > voltage at its base during transmit. I'll change out Q7. > > I'd like confirmation weather the KX1 circuit diagram on page 18 of the > KBX3080 manual is correct or not. It shows Q7's base connecting to R2, a > 1Kohm resistor. > > Thanks Don, > 73, > Kevin, KF0JX > > > Don Wilhelm said the following on 11/01/2006 09:55 AM: >> Kevin, >> >> There is no change in the base circuit of Q7 - that is true whether the >> KXB30 or the KXB3080 is installed. You should be tracking down the source >> of the negative voltage at Q7 during transmit. >> >> Yes, the voltage at the base of Q7 should be about 0.7 volts during >> transmit >> (long explaination of why follows:) - The source of the 6 volts during >> transmit is at pin 7 of U3, then it goes through R26 and then through >> R28 on >> its way to the base of Q7. >> >> When the 6T signal becomes active (it should during transmit), among >> other >> things it is applied to the base of Q7 to saturate Q7 - effectively >> producing a short between the collector and emitter of Q7 - the >> reason is to >> protect the receiver input from the transmit RF. When a transistor is >> saturated the base to emitter voltage will be close to 0.7 volts - if >> it is >> anything else, either the transistor is bad or something else strange is >> wrong. >> >> There is not a negative voltage produced intentionally in the KX1 - I >> would >> guess you have a bad Q7 which is acting more like a diode and >> rectifying the >> RF on its collector to produce a negative voltage at the base. If you >> want >> to do a test to confirm my suspicion, lift one end of R28 and try a short >> transmit while measuring the base voltage of Q7 - I would guess that >> it is >> producing a negative voltage all by itself, if so, replace it (and >> put R28 >> back in place). >> >> 73, >> Don W3FPR >> >> >>> -----Original Message----- >>> >>> While tracking down a low power output after installing the >>> KXB3080 into the >>> KX1, I was checking the voltages at Q1, Q4, Q5, Q6, and Q7 during >>> transmit into >>> a dummy load. Page E-6 of the KX1 manual states there should be >>> 0.7VDC at the >>> base of Q7 during transmit with a KXB30 (not KXB3080) installed. >>> I would get >>> between -1.5 to -2.7 VDC at the base of Q7 with the KXB3080. I >>> found no mention >>> of this change in the voltage in the supplied manuals. In fact, >>> looking at the >>> KXB3080 manual, page 18, figure 15, the circuit around Q7 is not >>> what I have >>> installed. The diagram shows the base of Q7 connecting to R2, a >>> 1Kohm resistor. >>> I don't find this change in the procedures. The KX1 manual shows Q7 base >>> connecting to R28, a 1.8Kohm resistor. >>> I resolved the low power output issue (yay!) and the >>> voltage at the base of Q7 >>> during transmit is still ~-2.5VDC. The question is "What /should/ >>> the voltage >>> be?" If it should now be something other than 0.7VDC, then there >>> should be a >>> reference to that in the KXB3080 manual. Do others think the >>> circuit diagram on >>> page 18 is inaccurate? >>> >>> -- >>> Kevin >>> KF0JX >>> >>> >> > _______________________________________________ > Elecraft mailing list > Post to: [email protected] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
