On 3/15/2012 12:26 PM, [email protected] wrote: > Hello fellow hams, > > One more K2 (#7293) was born this week in Europe - thanks to the Elecraft > team for designing this kit and making it available for the ham crowd. > > The radio is now assembled, but I have small issues with the testing. During > the initial testing on 40m reception the radio was almost deaf (even 7.3MHz > AM stations were faint), and I found out that both 8R and 8T control lines > were activated at the same time (thus enabling all attenuators). It turned > out that Q3 (2N7000) on the Control Board was zapped and was pulling the base > of Q1 low even without applied G-S voltage, thus activating erroneously the > 8T line. I didn't had any spare 2N7000 parts around, so I had to invent a > BJT-replacement for this low-power MOSFET, based on BC550C and 33K base > resistor (you know, it's hard to buy spares in the night). The replacement > worked OK, but I was wondering whether my circuit modification has changed > the T/R control circuit voltages. I did a comparison between Q4's drain > voltage (the healthy MOSFET) when ON and Q3's (now BC550C) collector voltage, > and the results were 5mV on Q4's drain and 46mV on Q3's collector. To me even > with the higher (45mV vs 5mV) saturation C-E voltage of BC550C, Q1 and Q2 > switches will still be able to saturate properly when enabled. > > So my 1st question is why originally 2N7000 were used to control T/R switches > (especially taking into account that these MOSFETs are very sensitive to > ESD)? I'll also appreciate comments whether my T/R small modification looks > acceptable. > > My 2nd question is what's the acceptable voltage on 8R& 8T control lines? > I'm measuring around 7.5V on those control lines, while the K2 manual > (rev.H1, p.114) says that the voltage should be 8V. While PIN attenuators and > MOSFET switches should be rather insensitive to the 0.5V difference between > the theoretical and practical values, there's a possibility for changes in > the PA bias (although the current mirror and current steering circuit seems > to keep the bias rather stable, at least on the schematic). > > Thanks in advance for your comments& responses. > > Regards, > Nikolay (LZ1NRD) > > ------------------------------------- > Mail.bg: Безплатен e-mail адрес. Най-добрите характеристики на българския > пазар - 20 GB пощенска кутия, 1 GB прикрепен файл, безплатен POP3, мобилна > версия, SMS известяване и други. > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html
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