+++++++++++++++++ See also K2 Poor S-meter Action +++++++++++++++++ Date: Fri, 22 Mar 2002 15:38:30 -0500 To: , From: Bill Coleman Subject: [Elecraft] K2 #2548 - Low S-meter sensitivity Problem: K2 has low S-meter sensitivity. Loud signals on other rigs that are S9+40 only read about S4-5 on the K2 S-meter. S-meter has no indication unless strong signal is present. Symptoms: * Calibrates fine with no antenna. CAL S LO = 187, Max RF gain. CAL S HI = 23, Min RF gain. * Rotating RF gain shows indication from S0 through S9+40, according to calibration. * Connecting antenna shows little increase in background noise (40m at night!) unless the preamp is on. * Loud stations (Region 1 SWBC) only indicate about S4-5. * REALLY strong signals (eg connect an MFJ 259 across 50 ohm load == about 20 mW on input) will peg S-meter at S9+40. * Very little AGC action can be heard. Turning AGC off doesn't increase signal strength that much. Diagnostics: * Troubleshooting on Appendix E, Page 6: - Calibrated as above CAL S LO and S HI - RF Gain is at maximum - Control board X1 crystal is oscillating (verified with oscilliscope) * Troubleshooting on Appendix E, Page 12: AGC - No-signal max-gain AGC is 3.86 volts. - No-signal min-gain AGC is 4.86 volts. - AGC at Saturation (MFJ-259 signal ~ 20 mW) is 6.53 volts. - IF Amp AGC input at Saturation is 2.6 volts. -- Thoughts: * What could cause Pin 5 of U12 (RF board) to only show 2.6 volts? Should be closer to 5 volts according to the manual. * Is the no-signal max-gain AGC of 3.86 volts too high? How can I lower it? * Is there a receiver gain problem earlier in the system? Where would I look for it? Transmitter gain is adequate -- I can get 15 watts out on all bands, except 14.5 on 12m, and 11.5 on 10m. How do I verify there is adequate receiver gain at each stage? * I could come up with a better signal generator than the MFJ. I have a 75 ohm step attenuator I could put in-line. I'd appreciate any ideas, things to look for or other probable causes of this problem. Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net Quote: "Not within a thousand years will man ever fly!" -- Wilbur Wright, 1901 ++++++++++++++++++ Date: Fri, 22 Mar 2002 12:51:08 -0800 From: Vic Rosenthal Organization: Transparent Software To: Bill Coleman Cc: elecraft at mailman.qth.net, support at elecraft.com Subject: Re: [Elecraft] K2 #2548 - Low S-meter sensitivity Bill Coleman wrote: > > * Connecting antenna shows little increase in background noise (40m at > night!) unless the preamp is on. Sounds like low gain in the front end. Does it happen on all bands? If it's just one or two bands, there could be a problem in a bandpass filter. 73, Vic, K2VCO Fresno CA ++++++++++++++++ Date: Fri, 22 Mar 2002 11:04:59 -1000 From: John Buck To: Bill Coleman Cc: elecraft at mailman.qth.net Subject: Re: [Elecraft] K2 #2548 - Low S-meter sensitivity What problem? If you can hear a slight increase in noise when you connect the antenna then you have enough gain. According to my very quick calculations based on the numbers you gave and assuming that S9 is correctly about 50 uV, then your meter should saturate on the 20 mW signal. Your data indicates to me that your rig is working just about right. The real problem is that people have become accustomed to receivers with excess noise and too much front end gain. Hearing a lot of noise is just a sign that the gain is higher then needed in the front end of the receiver. The S meter in my K2 is calibrated per the book and verified with a good signal generator/attenuator. And it works about as you describe assuming you can hear antenna noise without the preamp on 40. People have gotten used to very large s meter swings for small signals. This is not an advantage. Aloha, John KH7T +++++++++++++++++ From: "Kenneth A. Christiansen" To: Date: Tue, 26 Mar 2002 12:00:16 -0600 Subject: [Elecraft] My fix for Low S-meter sensitivity >K2 #2548 - Low S-meter sensitivity Bill Coleman >K2 has low S-meter sensitivity. Loud signals on other rigs that are S9+40 >only read about S4-5 on the K2 S-meter. S-meter has no indication unless >strong signal is present. I had the same problem with my K2 SN 1031 and used it for 9 months before I got around to fixing it. The receiver had good noise figure and worked well but I had to add a lot of audio gain and the S-meter was stingy. I just felt it was not working the way it should. I built up the RF detector for my digital volt meter and used my IC737 to a dummy load as a signal generator. Everything was just like the book said till I got to the output of T7. I had loss of voltage instead of a gain from the primary to the secondary. I discovered I had turned T7 180 degrees when I installed it so it acted as a step down transformer instead of a step up transformer. This made a big difference in the receive performance of my K2 as far as AGC, S-meter and AF gain. I do have to say though I don't think I ever lost a QSO when the transformer was wrong as the noise figure and weak signal performance were still good with the AF gain wide open. Hope this helps. 73 Ken W0CZ ++++++++++++++++++++ Subject: Re: [Elecraft] Drum Roll Please Date: Mon, 1 Apr 2002 11:44:50 -0500 To: "John McClain" , "K2" From: Bill Coleman On 3/30/02 4:01 PM, John McClain at digi2 at earthlink.net wrote: >Everything went together fine >except I think I did a pretty bad job aligning the filters and, like a >lot of others, the S-Meter is very stingy. My S-meter problem was solved by replacing the D1 on the control board with one with a .630 Vf (the one that came out was .595 Vf). I also had a bad Q12 on the control board. Replaced it with a 2N2222 (I don't have any PN2222As). Works great now. 40m SWBC stations are 20-30 over S9, right where they should be. Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net Quote: "Not within a thousand years will man ever fly!" -- Wilbur Wright, 1901 +++++++++++++++++