Low output from 2nd crystal filter and SA602 product detector

Posted by n7zf on Feb 01, 2011; 6:55am
URL: http://elecraft.85.s1.nabble.com/Low-output-from-2nd-crystal-filter-and-SA602-product-detector-tp5979986.html

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