Low sensitivity on K2...

[Robin Kiszka]

I have two K2s, and I find on one of them that the sensitivity is much lower. I see this when doing a test with an XG1 signal generator.

The problem is that the "low sensitivity" rig is one that I bought built, while the one that works fine is a rig I built myself. I have tried to reallign the rig, but without much improvement to say the least. It just seems that AF gain is far more responsive on my own built rig, and this is accross all bands.

Any suggestions?

thanks and 73
Robin
OZ6ABM

[Don Wilhelm-4]

Robin,

What filter were you using when doing the sensitivity measurements?  Did
you use the same filter width for each?
If they both have the KSB2 installed, try doing a comparison with the
OP1 filter selected.

Is one K2 above SN 3000 and the other below 3000?  If so,  yes there may
be differences.  The very early K2s had a winding ratio of 13:4 on T7
while the newer ones use a 20:5 ratio - that will account for about a 3
dB difference.    The early K2s IF crystals did not make a consistent
filter as the newer crystals.  The newer crystals are installed in all
K2s above SN 2560.  There may be differences in the AGC settings that
will change the apparent response - turn AGC off when making similar checks.

There can be significant differences introduced by the filter
alignment.  I suggest using Spectrogram  (or Spectrum Lab or other audio
spectrum analyzer) to look at the filter shape and the output level - dB
above the noise floor makes a good comparison, while raw signal
amplitude does not compare well because the AF Gain will not be set the
same.

The K2 Dial Calibration article on my website www.w3fpr.com has
information about setting the K2 filters if you are not familiar with
all the steps involved.

If the above does not resolve it for you, you may have to build up the
crystal oscillator shown in the Troubleshooting section of the manual
and do Receive Signal Tracing on both K2s to find where the difference
is coming from.

73,
Don W3FPR


On 2/26/2012 11:51 AM, Robin Kiszka wrote:
> I have two K2s, and I find on one of them that the sensitivity is much lower.
> I see this when doing a test with an XG1 signal generator.
>
> The problem is that the "low sensitivity" rig is one that I bought built,
> while the one that works fine is a rig I built myself. I have tried to
> reallign the rig, but without much improvement to say the least. It just
> seems that AF gain is far more responsive on my own built rig, and this is
> accross all bands.
>
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[Robin Kiszka]

Thanks for the replies. I can try to answer as follows:

The "good" rig is in the 3800 series,  and has:
KDSP2, KAT2, KBT2, KSB2, KNB1, K160RX, KIO2

The "weak" rig is 5900 series and has:
KAF2, KPA100, KSB2, KNB1, K160RX, KIO2

I have used spectogram, but not for the 5900 series rig.

I did a quick VAC measurement across the speaker and found that the good rig had a poweful 2 VAC, while the weak rig had as low as 0.5 VAC. I did some reading and tried to check the settings of R1 on the local board against the VDC on pin 5 of U2 on the weak rig. It looked OK close to the 3.9 mark, but that was close to the edge of its run, just a slight tweak got the DCV up to about 1.3 VAC on the speaker - though audio was beginning to distort. On the good rig the audio is always good even though the same measurement gives 2 VAC.

All these I tried using Preamp off and what I could regard as similar filter settings, though the two radios have different filtering techniques, DSP vs. non-DSP. But that said, I was listening on 10m and could heard DX from state side on the good rig, but hardly audible on the weak rig, about S3 compared to S1. Maybe the DSP unit is preamplifying the signal and that's the issue?

I did try switching the NB module in the rigs, it made no difference. I should have tried with the SSB unit as well, but didn't think about it at the time.

Maybe this has shed some more light on the setup here.

thanks again and 73
Robin
OZ6ABM