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K2 a little deaf?

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w2bvh

K2 a little deaf?

I dragged my HP8640B out of the "lab" (basement) to the second floor
shack to do some antenna testing about a week ago. Before dragging this
50 lb instrument back down two flights of stairs I decided to do a quick
MDS measurement on my K2 and got some results I'm a little concerned about.

Yesterday I did an informal test. K2 set for cw, 1.5 KHz bandwidth,
14.05 MHz, AGC on, preamp off. Attached siggen, tuned the K2 to 550 Hz
tone and cranked down the siggen amplitude until the signal and noise
were about equal, "by ear". Siggen output was -130 dBm. This was enough
to cause me to look further.

Today I took some numbers. Same setup, but with a Fluke dvm instead of
ears and the siggen phase locked . Started at -90 dBm drive and tuned
the K2 to peak voltage reading. Then made the following measurements:

_Sigen (dBm)      Meter(V)_

-90              .38
-100    .15
-110    .050
-120    .017
-130    .011
(off) .010

Then did a rough voltage-ratio-to-dB "calibration" as follows: Set
siggen to -100 dBm & measured the same .15V ob the K2, then ran the
siggen output to 103 dBm and measured .204V. So 3dB signal difference is
a voltage ratio of 1.36.

Finally measured MDS driving the K2 until the measured signal voltage
was (.011 * 1.36 = .015 V). I got this value at a drive level of -123
dBm. Tried a second set of measurements with the preamp on using the
same technique and got the same MDS of -123 dBm. Tried a third set of
measurements with preamp off and AGC off and got a slightly worse MDS
of -121.

Now I know there are flaws with this measurement, especially in
measuring a signal near the noise. But I know the provenance of the
siggen, use it all the time and get good measurements from it. I trust
it. If it was a matter of 2 or 3 dB I wouldn't be writing to the list,
but this is 6-11 dB worse than the 2 numbers on Sherwoods site (-129 and
-135).

FYI, another informal "measurement" I made was on 6M with the XV50. A
signal was easily audible over the noise with the siggen set at -140 dBm
drive!

On air results are equivocal: yesterday afternoon I couldn't hear my
friend Joe N2CX 145 mi away who was qrp portable on 20M (something I've
done multiple time in the past), but I had 3 consecutive easy q's to
eastern EU last night on 30M...

On the K2, gain could be missing or noise could be high anywhere: front
end, I.F, audio stages.

What do you think. Could / should I send it away for work. It'd have to
go somewhere with better instruments, since the K2 isn't exactly
"broken". Or should I don't worry be happy?

73, --Lenny W2BVH

ps -FYI this is K2 1520, I built in 1999 & been in use here ever since.
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John Oppenheimer

Re: K2 a little deaf?

Hi Lenny,

In the past I've used the "Measuring Receiver Sensitivity" found in the
XG3 manual:
http://www.elecraft.com/manual/E740154%20XG3%20Owner%27s%20Manual.pdf
to be rather close to published K2 numbers.

John KN5L
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Don Wilhelm

Re: K2 a little deaf?

In reply to this post by w2bvh

Lenny,

Are your IF filters aligned so a 550Hz signal is within the passband.
With the K2, simply setting STP for 550 Hz will not do it, you must do
CAL FIL to make 550Hz appear well within the receive passband.

A properly tuned K2 will typically have a good response to a -140dBm
signal from the HP8640B.

It is easier to take the K2 to the signal generator than to lug that
heavy signal generator to the hamshack!

73,
Don W3FPR

On 7/1/2018 4:35 PM, w2bvh wrote:

> I dragged my HP8640B out of the "lab" (basement) to the second floor
> shack to do some antenna testing about a week ago. Before dragging this
> 50 lb instrument back down two flights of stairs I decided to do a quick
> MDS measurement on my K2 and got some results I'm a little concerned about.
>
> Yesterday I did an informal test. K2 set for cw, 1.5 KHz bandwidth,
> 14.05 MHz, AGC on, preamp off. Attached siggen, tuned the K2 to 550 Hz
> tone and cranked down the siggen amplitude until the signal and noise
> were about equal, "by ear". Siggen output was -130 dBm. This was enough
> to cause me to look further.
>

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w2bvh

Re: K2 a little deaf?

Thanks Don,

As you say, I also would expect the signal from the 8640B to be clearly
audible at -140 dBm on the K2.

I took your advice checked the filter bandwidths. I used the old noise
source I built when I originally built the K2 ( Tom Hammond design?)
along with the filter measurement instructions from your web site, and
the Spectrogram 5.17 program. No changes were needed. The filters are
centered on 550 Hz. So that is not causing the MDS problem. It was well
worth doing to eliminate that as a possible cause.

The reply on the Elecraft reflector from KN5L was helpful for me to make
more MDS measurements and help me validate the ones I made originally. I
used the instructions that come with the XG3 manual to set up the K2 and
calculate the MDS. I used the 8640B as the signal source. The results
of those measurements are:

_XFil BW Hz)                    MDS (dBm)
_400                                  -125
700                                  -123
1500                                -121.5

So these results are actually a couple of dB WORSE (at the same
bandwidth) than the ones I originally got (e.g. the ones from my
original email to the reflector a couple of days ago).

Any additional advice you may have would be greatly appreciated. I don't
know if you have fixed this type of problem before, but if you have,
maybe I should be mailing it off to you to fix it?

I did a qrp-qrp qso with an F5 last night on 40M, but it was pulling
teeth to copy him. Another 1 1/2 s-units (or so) of signal would have
been very welcome!

Please let me know either way.

73 & happy 4th,
--Lenny W2BVH

ps - as for bringing the siggen to the K2 or vice versa, the siggen had
to be upstairs in the shack for antenna measurements (can't bring the
antenna connection down to the basement ;-). MDS measurement was an
afterthought -- which I'm glad I did.

__

On 7/1/2018 11:55 PM, Don Wilhelm wrote:

> Lenny,
>
> Are your IF filters aligned so a 550Hz signal is within the passband.
> With the K2, simply setting STP for 550 Hz will not do it, you must do
> CAL FIL to make 550Hz appear well within the receive passband.
>
> A properly tuned K2 will typically have a good response to a -140dBm
> signal from the HP8640B.
>
> It is easier to take the K2 to the signal generator than to lug that
> heavy signal generator to the hamshack!
>
> 73,
> Don W3FPR
>
> On 7/1/2018 4:35 PM, w2bvh wrote:
>> I dragged my HP8640B out of the "lab" (basement) to the second floor
>> shack to do some antenna testing about a week ago. Before dragging
>> this 50 lb instrument back down two flights of stairs I decided to do
>> a quick MDS measurement on my K2 and got some results I'm a little
>> concerned about.
>>
>> Yesterday I did an informal test. K2 set for cw, 1.5 KHz bandwidth,
>> 14.05 MHz, AGC on, preamp off. Attached siggen, tuned the K2 to 550
>> Hz tone and cranked down the siggen amplitude until the signal and
>> noise were about equal, "by ear". Siggen output was -130 dBm. This
>> was enough to cause me to look further.
>>
>

Don Wilhelm

Re: K2 a little deaf?

Lenny,

Check the obvious - is the Receive Antenna (RANT) on?
Is transmit power full output?

If both are correct, you will have to go through the Receive Signal
Tracing in the Troubleshooting Appendix of the manual.
If you have an oscilloscope with a 10X probe, you can use the HP8640B to
drive the K2 receiver input. Set the signal generator so you have an
obviously measurable signal at the BNC jack. The signal level should
decrease only slightly through the T/R switch, but will decrease some
through the BandPass filter. The preamp will bring it back up as will
the Post Mixer Amplifier.

If you can achieve enough signal to give you as much 'scope deflection
as the listed "Expected" values, that will give you some guide for the
signal levels along the receive path.
I can usually make a judgement about the relative levels as I progress
through the receiver without consulting the receive chart, but then I
have had a lot of practice.
Note that with the inclusion of the back to back diodes across the IF
amplifier, the expected values after that point have changed - if you
get to that point, you can temporarily remove one end of the diode pair.

73,
Don W3FPR

On 7/2/2018 2:22 PM, w2bvh wrote:

> Thanks Don,
>
> As you say, I also would expect the signal from the 8640B to be
> clearly audible at -140 dBm on the K2.
>
> I took your advice checked the filter bandwidths. I used the old noise
> source I built when I originally built the K2 ( Tom Hammond design?)
> along with the filter measurement instructions from your web site, and
> the Spectrogram 5.17 program. No changes were needed. The filters are
> centered on 550 Hz. So that is not causing the MDS problem. It was
> well worth doing to eliminate that as a possible cause.
>
> The reply on the Elecraft reflector from KN5L was helpful for me to
> make more MDS measurements and help me validate the ones I made
> originally. I used the instructions that come with the XG3 manual to
> set up the K2 and calculate the MDS. I used the 8640B as the signal
> source. The results of those measurements are:
>
> _XFil BW Hz)                    MDS (dBm)
> _400                                  -125
> 700                                  -123
> 1500                                -121.5
>
> So these results are actually a couple of dB WORSE (at the same
> bandwidth) than the ones I originally got (e.g. the ones from my
> original email to the reflector a couple of days ago).
>
> Any additional advice you may have would be greatly appreciated. I
> don't know if you have fixed this type of problem before, but if you
> have, maybe I should be mailing it off to you to fix it?
>
> I did a qrp-qrp qso with an F5 last night on 40M, but it was pulling
> teeth to copy him. Another 1 1/2 s-units (or so) of signal would have
> been very welcome!
>
> Please let me know either way.
>
> 73 & happy 4th,
> --Lenny W2BVH
>
> ps - as for bringing the siggen to the K2 or vice versa, the siggen
> had to be upstairs in the shack for antenna measurements (can't bring
> the antenna connection down to the basement ;-). MDS measurement was
> an afterthought -- which I'm glad I did.
>
>
>
>
> On 7/1/2018 11:55 PM, Don Wilhelm wrote:
>> Lenny,
>>
>> Are your IF filters aligned so a 550Hz signal is within the passband.
>> With the K2, simply setting STP for 550 Hz will not do it, you must
>> do CAL FIL to make 550Hz appear well within the receive passband.
>>
>> A properly tuned K2 will typically have a good response to a -140dBm
>> signal from the HP8640B.
>>
>> It is easier to take the K2 to the signal generator than to lug that
>> heavy signal generator to the hamshack!
>>
>> 73,
>> Don W3FPR
>>
>> On 7/1/2018 4:35 PM, w2bvh wrote:
>>> I dragged my HP8640B out of the "lab" (basement) to the second floor
>>> shack to do some antenna testing about a week ago. Before dragging
>>> this 50 lb instrument back down two flights of stairs I decided to
>>> do a quick MDS measurement on my K2 and got some results I'm a
>>> little concerned about.
>>>
>>> Yesterday I did an informal test. K2 set for cw, 1.5 KHz bandwidth,
>>> 14.05 MHz, AGC on, preamp off. Attached siggen, tuned the K2 to 550
>>> Hz tone and cranked down the siggen amplitude until the signal and
>>> noise were about equal, "by ear". Siggen output was -130 dBm. This
>>> was enough to cause me to look further.
>>>
>>
>

w2bvh

Re: K2 a little deaf? FIXED!

Hi Don & John,

Thanks for the advice. The MDS problem is now apparently fixed! Before
doing the signal tracing, I decided attend to a simple set of repairs on
the K2 which I've put off for awhile. I changed the Ext Spkr jack on the
back and the Headphones jack on the front (both of which were starting
to get unreliable). Also replace the AF Gain pot, which has been noisy
for awhile (and with a loose shaft). After making the changes, just for
giggles, I re-checked MDS using the same method as I used before (the
one described in the XG3 manual). The results are: before: -125, after
-141. Did measurements at other bandwidths and have similar offsets
with the other bandwidth measurements done before the fixes.

The 8640 signal is audible now with a drive level of -140 dBm, which is
good sanity check on the "fix".

I guess I was losing 16 dB of gain in the audio portion of the K2
because of one of the replaced components. I'm glad it worked out this
way.

I figured you'd be interested in hearing the news.... And thanks again
for your advice.

73, --Lenny W2BVH

On 7/2/2018 6:13 PM, Don Wilhelm wrote:

> Lenny,
>
> Check the obvious - is the Receive Antenna (RANT) on?
> Is transmit power full output?
>
> If both are correct, you will have to go through the Receive Signal
> Tracing in the Troubleshooting Appendix of the manual.
> If you have an oscilloscope with a 10X probe, you can use the HP8640B
> to drive the K2 receiver input. Set the signal generator so you have
> an obviously measurable signal at the BNC jack. The signal level
> should decrease only slightly through the T/R switch, but will
> decrease some through the BandPass filter. The preamp will bring it
> back up as will the Post Mixer Amplifier.
>
> If you can achieve enough signal to give you as much 'scope deflection
> as the listed "Expected" values, that will give you some guide for the
> signal levels along the receive path.
> I can usually make a judgement about the relative levels as I progress
> through the receiver without consulting the receive chart, but then I
> have had a lot of practice.
> Note that with the inclusion of the back to back diodes across the IF
> amplifier, the expected values after that point have changed - if you
> get to that point, you can temporarily remove one end of the diode pair.
>
> 73,
> Don W3FPR
>
> On 7/2/2018 2:22 PM, w2bvh wrote:
>> Thanks Don,
>>
>> As you say, I also would expect the signal from the 8640B to be
>> clearly audible at -140 dBm on the K2.
>>
>> I took your advice checked the filter bandwidths. I used the old
>> noise source I built when I originally built the K2 ( Tom Hammond
>> design?) along with the filter measurement instructions from your web
>> site, and the Spectrogram 5.17 program. No changes were needed. The
>> filters are centered on 550 Hz. So that is not causing the MDS
>> problem. It was well worth doing to eliminate that as a possible cause.
>>
>> The reply on the Elecraft reflector from KN5L was helpful for me to
>> make more MDS measurements and help me validate the ones I made
>> originally. I used the instructions that come with the XG3 manual to
>> set up the K2 and calculate the MDS. I used the 8640B as the signal
>> source. The results of those measurements are:
>>
>> _XFil BW Hz)                    MDS (dBm)
>> _400                                  -125
>> 700                                  -123
>> 1500                                -121.5
>>
>> So these results are actually a couple of dB WORSE (at the same
>> bandwidth) than the ones I originally got (e.g. the ones from my
>> original email to the reflector a couple of days ago).
>>
>> Any additional advice you may have would be greatly appreciated. I
>> don't know if you have fixed this type of problem before, but if you
>> have, maybe I should be mailing it off to you to fix it?
>>
>> I did a qrp-qrp qso with an F5 last night on 40M, but it was pulling
>> teeth to copy him. Another 1 1/2 s-units (or so) of signal would have
>> been very welcome!
>>
>> Please let me know either way.
>>
>> 73 & happy 4th,
>> --Lenny W2BVH
>>
>> ps - as for bringing the siggen to the K2 or vice versa, the siggen
>> had to be upstairs in the shack for antenna measurements (can't bring
>> the antenna connection down to the basement ;-). MDS measurement was
>> an afterthought -- which I'm glad I did.
>>
>>
>>
>>
>> On 7/1/2018 11:55 PM, Don Wilhelm wrote:
>>> Lenny,
>>>
>>> Are your IF filters aligned so a 550Hz signal is within the passband.
>>> With the K2, simply setting STP for 550 Hz will not do it, you must
>>> do CAL FIL to make 550Hz appear well within the receive passband.
>>>
>>> A properly tuned K2 will typically have a good response to a -140dBm
>>> signal from the HP8640B.
>>>
>>> It is easier to take the K2 to the signal generator than to lug that
>>> heavy signal generator to the hamshack!
>>>
>>> 73,
>>> Don W3FPR
>>>
>>> On 7/1/2018 4:35 PM, w2bvh wrote:
>>>> I dragged my HP8640B out of the "lab" (basement) to the second
>>>> floor shack to do some antenna testing about a week ago. Before
>>>> dragging this 50 lb instrument back down two flights of stairs I
>>>> decided to do a quick MDS measurement on my K2 and got some results
>>>> I'm a little concerned about.
>>>>
>>>> Yesterday I did an informal test. K2 set for cw, 1.5 KHz bandwidth,
>>>> 14.05 MHz, AGC on, preamp off. Attached siggen, tuned the K2 to 550
>>>> Hz tone and cranked down the siggen amplitude until the signal and
>>>> noise were about equal, "by ear". Siggen output was -130 dBm. This
>>>> was enough to cause me to look further.
>>>>
>>>
>>
>
>

John Oppenheimer

Re: K2 a little deaf?

In reply to this post by w2bvh

On 07/02/2018 01:22 PM, w2bvh wrote:
> _XFil BW Hz) MDS (dBm)
> _400 -125

Measured K2; 14 MHz with 400 Hz BW using XG3 and Fluke 87V (True RMS
with averaging mode). With full AF gain, the two -107 dBm / off voltages
were 322.3 mV and 12.5 mV, => -135.2 dB MDS

K2 sidetone pitch is 600 Hz.

AGC ON, 50 uV input sets one LED segment above S9

Using a new Rigol DG1023Z, set to 13.8 mVPP and a VNWA measured 73.78 dB
attenuator. 12.6 / 368.4 => -136.3 dB MDS. The 1023 accuracy is +-1mV,
or about +-0.65 dB.

John KN5L
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John Oppenheimer

Re: K2 a little deaf? FIXED!

In reply to this post by w2bvh

On 07/03/2018 04:21 PM, w2bvh wrote:
> The MDS problem is now apparently fixed!

Great news Lenny! I got interested and spent the afternoon measuring my
K2. Once having the results, I had to send them! And wanted to test my
new test equipment.

The K2 is an amazing radio. I'm glad that the Elecraft team is keeping
the kit available.

John KN5L K2 #7212
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