[Elecraft] [K2] K2 Difficulty Fine Adjusting AGC

[Don Wilhelm]

Brian,

The AGC Threshold test that you are doing should be done with no antenna 
and with the preamp off.  The point is that you do not want the AGC to 
be active on receiver internal noise.  As far as noise coupled from an 
antenna, well, that is just your local noise level, and could be 
anywhere between nothing and over S-9, that will depend on your location 
and your antenna - so the only constant is with no antenna connected.

Below 3.75 volts the S-meter adjustments become more and more difficult 
to get 'right'.

Your problem 2 suggests that your K2 receiver has a gain problem 
somewhere between the antenna and the product detector.
I suggest you first use the XG1 to make an MDS measurement (see the XG1 
manual for instructions).  If the MDS is not up to par, you should be 
looking for the cause in the receiver front end or IF stages.

Correct any receiver gain problem before attempting to set the S-meter 
using the XG1 - during the troubleshooting phase, adjust the S-meter CAL 
S-LO and CAL S-HI using the procedure in the manual only.

73,
Don W3FPR



Brian - N5BCN wrote:
> (K2 #6800, KAT2, KSB2, KIO2, KNB2)
>
> Hello all,
>
> I know many electrons have been spilled on the reflector regarding this
> topic, however, I'm still not able to resolve my situation.
>
> Problem 1:
>
> When attempting to fine tune the AGC threshold, I can differentiate a volume
> difference between AGC On and AGC Off as expected.  The problem is there
> isn't much difference in this sound delta when adjusting the AGC Threshold
> Voltage (U2, Pin 5) going from 3.8 V down to below 3.5 V.  The voltage needs
> to be below 3.5 V for my ears to detect a “significant” change.  My
> understanding is that AGC Threshold Voltage should not be set below 3.6 V.
>
> I'll be the first to admit my ears are not that sensitive, so I hooked up
> the K2 to Spectrogram to see if that would help me detect a significant
> change.  At 3.8 V, AGC On versus AGC Off produced about a 3 dB delta. 
> Adjusting down to 3.5 V, the delta increased to maybe 4 dB, but it's hard to
> tell.  Between 3.8 V and 3.5 V it's really difficult to detect much
> difference at all.
>
> The Regulator Voltage on U4 Out is 8.05 V.  I read on the reflector that
> this should equate to a AGC Threshold Voltage of roughly 3.78 V.  At 3.78 V,
> there is virtually no difference between AGC On/Off.
>
> This may be all a moot point, because I simply set the threshold voltage at
> 3.7 V and seem to get decent S-meter deflection.  However, I absolutely can
> not get the S-9 calibration with a XG1 signal generator (see Problem 2).
>
> Problem 2:
>
> Before attempting to calibrate for S-9, CAL S LO is at 185 and CAL S HI is
> at 017 (Preamp off) and I get a working s-meter.  However, when setting CAL
> S HI with a XG1, the CAL S HI value is 002 (Preamp off).  The meter is then
> always pegged at S9, even when there isn't an antenna plugged in.  I've done
> this calibration several times, adjusting CAL S LO, then CAL S HI, with the
> same results.  I'm starting to think there is something wrong with my XG1,
> even though I can hear the signal clearly at 7040 kHz.
>
> Any ideas would be much appreciated!  The K2 is working great other than
> this issue!
>
> 73
>
> Brian - N5BCN
>
> -----
> K2 #6800 KSB2 KAT2 KIO2 KNB2
>   
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