[Elecraft] K2 a little deaf? FIXED!

[w2bvh]

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.
>>>>
>>>
>>
>
>