[Elecraft] K1 Receiver Sensitivity Issue.

[Les Garwood]

Hi Don,

Success!!!
I mostly followed your advice--I say mostly because I got diverted when I
found a clue.  I would still be searching without your insights and
expertise, however.

I was getting about 150 mv out of pin 4 of u7.  I traced it to pins 1and 2
of J6, which was still hot on the scope through the secondary winding of T1
ON THE FILTER BOARD but was dead upon arrival at pin 6 on the return leg.
This sent me on a merry chase of testing all the relay solenoids for
continuity, and their contact leads.  They all checked good so then I leapt
at the conclusion that the controller IC was at fault.  I tried resetting
it to factory default but no apparent change resulted.
So, I went back to the scope and finally found the exact dead end: C15 was
wrongly mounted.  Somehow, I misread the silkscreen hole indicator, I'm
embarrassed to say.  Anyway, I resoldered it in the right holes
and, instantly, robust CW screamed at me.  I'm thrilled and humbled.

Thanks again, Don

73

Les
KE7SLX

On Monday, August 18, 2014, Don Wilhelm <w3fpr at embarqmail.com> wrote:

> Les,
>
> If you can get at least 200 mV peak to peak out of the Pre-Mixer bandpass,
> the VFO level is OK.
> The VFO and band module xtal oscillator are mixed in U7 and the output of
> that mixer (after the bandpass filter) is applied to U1 pin 6.
>
> If you find you need to check for loading of the U1 output, lift one lead
> of C22 to isolate it.
>
> 73,
> Don W3FPR
>
> On 8/18/2014 6:08 PM, Les Garwood wrote:
>
>> Hi Don,
>>
>> Extremely helpful and encouraging; thanks.  Actually,  I was able to get
>> the trimmers set pretty well, i think, using a strong signal source.  But
>> I
>> will make the measurements you recommended.
>> Right now, I suspect either low VFO output or something is pulling down
>> the
>> gain of U1.
>>
>> Les
>> On Aug 18, 2014 3:00 PM, "Don Wilhelm" <w3fpr at embarqmail.com> wrote:
>>
>>  Les,
>>>
>>> Yes, the NE612 should show plenty of gain.  When it does not, the usual
>>> reason is little or no oscillator signal to pin 6.
>>>
>>> The Pre-mixer Bandpass filter must be aligned before you will get much of
>>> a signal at U1 pin 6.
>>>
>>> Since you have a 'scope (hopefully with a 10X probe), I will give you an
>>> alternate method of tuning the Pre-Mixer Bandpass filter.
>>>
>>> Since you have the 4 band board, you must follow the proper band order -
>>> adjust 15 (or 17) meters before adjusting 20 meters, and likewise adjust
>>> 30
>>> meters before 40 meters.
>>> Make a check before starting - align all the slots in the trimmer
>>> capacitors parallel with the long edge of the band board.  Then check the
>>> activity of the crystal for each band - put your probe on band board P1
>>> pin
>>> 4 and not the amplitude of the signal as you switch through the bands -
>>> you
>>> should have greater than 200 mV peak to peak.  The position of the
>>> trimmer
>>> slots when tuned should be no more than 30 degrees from the center
>>> position
>>> if your toroids are correct.
>>>
>>> Then switch to 15 (or 17) meters, and with your probe on P1 pin 6, adjust
>>> the Pre-Mixer trimmer caps for 15 meters for the maximum amplitude.
>>> Switch
>>> to 30 meters and do the same for that bands trimmers.  Do the same for 20
>>> meters and 40 meters.  You should be able to get 200 mV peak to peak
>>> there
>>> on all bands.
>>> Remember that your 'scope probe will load the Pre-Mix filter a bit, so
>>> the
>>> trimmers to the right of the filter will not be at their optimum setting,
>>> but it will be enough for receive - you will set the optimum position
>>> when
>>> you adjust for transmit.
>>>
>>> You should check to see that the Pre-Mix signal is getting to U1 pin 6 -
>>> if it is not, suspect the coax link on the bottom of the board between J9
>>> and J10.
>>>
>>> Once you have a good signal at U1 pin 6, you should see that the output
>>> at
>>> pin 4 is much greater than the input at pin 1.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>>
>