K2 Difficulty Fine Adjusting AGC

[Brian - N5BCN]

(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

[Don Wilhelm-4]

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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[Brian - N5BCN]

Hi Don,

Thanks for the suggestions.

In reference to Problem 1, all of the testing was done with the antenna disconnected and preamp off.  Even so, still having problems differentiating much of a sound difference will adjusting AGC Threshold Voltage.  Is this perhaps because of the gain problem I'm having that you suggested?

I'll perform the MDS measurements and see where that takes me.

I can't really detect much of a problem on receive.  How would this gain problem (between the antenna and product detector) manifest itself in normal operation?

Thanks!

73

Brian - N5BCN

[Don Wilhelm-4]

Brian,

A receiver gain problem will usually result in S-meter problems similar
to what you describe - to get a 50 uV signal to show S-9, the CAL S HI
value will be far removed from that obtained when you do the adjustment
with the RF Gain full counterclockwise.
Normally, the S-9 point with a 50 uV input will only be a few numbers
different than the initial adjustment (i.e. 16 instead of 19 as an
example- but those numbers are not unusual).

It may be that you have noticed the lack of receive gain, but if you
have to run the AF Gain up to the 11 o'clock position or more for a
normal listening level with moderately strong signals, that is one
indicator that something is amiss.

If the MDS measurement indicates a problem, build the signal generator
shown in the K2 manual Appendix E and using it to do receiver signal
tracing (also as detailed in the manual) may be able to identify the
problem stage or section.

If MDS measurement show adequate receive sensitivity (-133 to -138 dBm),
then you can look for the problem in the AGC/S-meter section of the K2
(this is on the Control Board).

It is sufficient to say that your K2 is not acting normally, and the
problem should be found and corrected.

73,
Don W3FPR


Brian - N5BCN wrote:

> Hi Don,
>
> Thanks for the suggestions.
>
> In reference to Problem 1, all of the testing was done with the antenna
> disconnected and preamp off.  Even so, still having problems differentiating
> much of a sound difference will adjusting AGC Threshold Voltage.  Is this
> perhaps because of the gain problem I'm having that you suggested?
>
> I'll perform the MDS measurements and see where that takes me.
>
> I can't really detect much of a problem on receive.  How would this gain
> problem (between the antenna and product detector) manifest itself in normal
> operation?
>
> Thanks!
>
> 73
>  
>

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