[Elecraft] K2 #2548 - Low S-meter sensitivity

[Bill Coleman]

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 @ Saturation (MFJ-259 signal ~ 20 mW) is 6.53 volts.
- IF Amp AGC input @ 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@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901