Brian, Since you are unable to measure any RF voltage, I am going to start at the beginning - proper assembly of the RF Probe.
The RF probe voltage is not read "across" the 4.7 meg resistor, it is read between the end of the 4.7 meg resistor and ground. That is an important difference. In other words, the 4.7 megohm resistor is in series with the detector. Since the nominal input resistance of a DMM is 11 megohms (established in the "old days" of VTVMs), that resistance in series with the 11 megohms will cause the DMM to indicate the RMS value of the RF voltage being measured. There are other techniques that will provide DMM readings that indicate the zero to peak RF voltage, but the RF probe parts provided with the K2 will indicate the RMS voltage values when used with most DMMs. The RF Probe parts and board that were provided with your K2 kit should give you the correct connections and RF voltage readings. The TIP should connect to whatever wire you choose to use for the probe tip - I suggest one of the heavy diode leads so it is physically sturdy enough to be pressed onto the point to be measured. The ground wire (with the alligator clip) should connect to the ground symbol point near the tip with a 4 to 6 inch wire and the alligator clip on the far end. The coax should be connected as indicated on the board silkscreen - the shield to ground and the center conductor connects to the end of the 4.7 ohm resistor (follow the PC trace on the board). At the far end of the coax, the red banana plug connects to the center conductor of the coax and the shield connects to the black - you should use a 4 inch length of wire between the shield and the black banana plug. The schematic on page 9 of Appendix E shows "which is connected to what". The banana plugs connect to your DMM, and the DMM will read the RMS value of a sinewave that is probed by the tip (the alligator clip must be connected to one of the ground loops that you should have installed on your K2 RF board). I cannot discern from you postings whether you have things set up correctly to measure RF voltages correctly or not. An oscilloscope can be used in place of the RF Probe, but it must be used with a 10X probe (the voltage readings must be corrected for the 10 times attenuation of the probe), and the "expected" values listed in the manual must be massaged because the 'scope will show you peak to peak voltages which are 2.8 times the RMS values listed in the manual. 73, Don W3FPR On 9/6/2010 10:28 PM, Brian Denley wrote: > Don: > I am officially stumped. I cannot find the fault in my transmit circuit but > that is basically do to my inability to accurately measure ANY RF voltage. > The RF probe doesn't seem to function. I have replaced the resistor and > resoldered all the connections twice now but I continue to read betwen 50 > and 70 mV with any handheld DMM and basically nothing with an HP3478A, I am > using the DC volt setting. And all that 50 to 70 mV is across the 4.7Mohm > resistor on the probe board. This reading masks almost anything I am trying > to measure and without that capability, I am at a loss trying to trouble > shoot the transceiver. Connecting the ground lead to the probe still leaves > me with the same reading. > > Is there any other instrument that I can buy or borrow that will accurately > measure small RF voltages? Can I buy a working RF probe from Elecraft? > Are there only certain DMMs that it will work with? > > Again, the receiver section and the panel and control functions work just > fine but I only get about 0.5 mW on the readout with keydown. > Brian Denley > http://home.comcast.net/~b.denley/index.html > > > Brian, > >> Yes, I am sure about the 4.7 Megohms, but your 4.6 meg resistor will do >> just as well. >> It is DC voltage through the resistor, and it scales the peak RF voltage >> down to approximate RMS. >> Look at the schematic for the RF Probe on page 9 of Appendix E in the K2 >> manual. >> With the RF Probe, you will be able to measure RF voltages down into the >> millivolt region. >> Go through the steps listed in Appendix E of the manual under Transmit >> Signal Tracing. >> When you get to the points to measure on page 14 (assuming you do not >> find a problem with the VFO or BFO), the RF voltages will be HIGHER than >> the expected value because you do not have enough transmit output. What >> you are looking for is the first point where the RF voltage is LOWER >> than the expected value. That point is just beyond the failing stage, >> and you will know which stage to examine in detail. >> Only after your K2 is operating properly for RF output will you see the >> expected values - go with the higher or lower criteria. >> Don > ______________________________________________________________ > 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 > ______________________________________________________________ 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
