Posted by
Don Wilhelm-4 on
Feb 01, 2011; 1:37pm
URL: http://elecraft.85.s1.nabble.com/Low-output-from-2nd-crystal-filter-and-SA602-product-detector-tp5979986p5980976.html
Bob,
The levels measured at U11 and U12 under such strong signal conditions
are a bit confusing.
Do you have AGC turned off? If not, the gain of U12 is being reduced
dramatically.
Since you are working on an early level of the manual, check the turns
ratio used for T7 - the better ratio is 5:20 turns instead of the
earlier 4:13. If the serial number is below 2560, I suggest that you
also change the IF crystals to the newer ones. If the crystals are
marked "-20" they are the old ones - new ones have an "-S" suffix. You
will have a better filter shape with the newer crystals.
I would suggest that you instead test the receiver by seeing how small a
signal that K2 can receive. Set your HP8640 to -130 dBm and look at the
Spectrogram display. How many dB above the receiver noise floor is the
signal pip? If it is well above the receiver noise floor, that receiver
will hear with the best of them.
73,
Don W3FPR
On 2/1/2011 1:55 AM, n7zf wrote:
> I am finishing (wishful thinking) a revision A K2 for a friend who bought the
> kit many years ago and did not build it before having a stroke. I have
> installed all mods published on the Elecraft site, plus the Rework
> Eliminators. There are no options right now (but they are built). The
> filter crystals are the 14 matched ones for a K2 with SSB.
>
> I am doing the voltage checks in the receiver. Everything seemed fine until
> I reach the 2nd crystal filter output and the product detector output. I
> measure about .150Vrms (expect .270Vrms) at U11 pin1. The output from the
> SA602 mixer at U11 pin5 is only .060Vrms (expect .580). I guess if the
> input signal to the mixer is low, then the output will be low.
>
> Both of these voltage measurements are very subjective, because they change
> very dramatically if I adjust the signal generator (an HP 8640B) or the K2
> VFO a minor amount. But even doing that, I cannot get the mixer output
> above .250Vrms.
>
> What I don't understand is how the rms voltage output of the 2nd crystal
> filter (expected .27V) can be expected to be higher than the input voltage
> (.15V)? Is there some amplification going on here?
>
> Am I off in the weeds here? How do I troubleshoot the 2nd crystal filter?
>
> Also, the T7 step-up ratio I measured to be 3.65 but the expected ratio is
> 3.88. (I don't have the D40 and D41 limiting diodes installed.) Is 3.88
> good enough? I have tried pushing the windings around but 3.65 is the best
> I can get.
>
> My RF voltage measurements are done using a Fluke RF probe and an HP
> multimeter.
>
> Bob.N7ZF
>
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