[Elecraft] K2 AGC/S-meter calibration - cw vs. ssb

[Don Wilhelm]

Dale,

I can think of a few things that could cause a change from CW to SSB.
For one thing, the wider filter does let more total energy through, and the
ALC and S-meter respond to that total energy.  That effect is not as
noticable with the narrower CW filters because the signal at the center of
the passband contains a greater percentage of the total energy in the IF
passband.  Your 2.5 kHz filter can make it more noticable.

It could be that you have a filter blow-by increase when the SSB filter is
selected.  Compare the noise floor about 300 Hz outside the passband using
Spectrogram to see if that is the case - if you have the KAF2 or KDSP2
installed, I would suggest removing them for that test (removal positively
gets the low pass filter out of the picture).

I have found on several K2s that grounding the IF crystals very near the
bottom on both sides helps this (noise floor) condition, and grounding the
SSB crystals near the bottom also helps - yes, you can ground all but one of
the SSB crystals near the bottom, it is soldering at some tricky angles, and
I 'steal' the ground side from two of the capacitors (C4 and C13) to ground
X1 and X7.

Once the crystals are grounded, you can do measurements with the XG1 to set
S9 at 50 uV for various combinations of AGC Threshold settings, CAL S Lo and
CAL S Hi.  I usually make a chart and select the AGC Threshold setting that
gives the best S-meter 'action' and still just produces no LED
indication )and no AGC action with a dummy load connected.  The procedure
takes a bit of time to accomplish, but does result in a receiver that hears
a bit better because the AGC Threshold is set correctly (and that may not
always be the 3.8 volts stated in the manual - it will be close, but not
always 'just right').

73,
Don W3FPR

----- Original Message ----- 
From: "Dale Boresz" <dmb@lightstream.net>
To: <Elecraft@mailman.qth.net>
Sent: Thursday, April 22, 2004 11:35 AM
Subject: [Elecraft] K2 AGC/S-meter calibration - cw vs. ssb


> I've only had my K2 now for about a year and a half, so one would think
> that I'd have noticed this before...
>
> I set up my CAL S LO, and CAL S HI adjustment while in CW mode, on the
> widest filter bandwidth (~1.4KHz). Using the XG1 I can confirm that 50
> uV input corresponds to S9 on the S-meter, and with no signal input to
> the K2, the left-most bar on the S-meter just barely flickers. So all
> seems fine in CW mode.
>
> However, if I switch to either USB or LSB, with no signal input to the
> K2, the left-most 2 bars of the S-meter are lit, and 50 uV input causes
> two bars beyond S9 to light. (at least it's linear!)
>
> The only modifications to the KSB2 are the 2.5KHz mod. The BFO/filter
> balance on the OPT1 filter looks very good on Spectrogram, and sounds
> good on the air, so the problem is not the result of a grossly
> mis-aligned BFO adjustment in either of the SSB modes.
>
> Has anyone else noticed a similar disparity in the S-meter calibration
> between CW and either of the SSB modes?
>
> 73, Dale / WA8SRA
>
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