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.
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