[Hammarlund] SP-600 RF illness cured

[Craig Roberts]

Hiya fellas --

My new antenna manufacturing business has been keeping me very busy at 
nights so it took me awhile to get into my SP-600's ailing RF section. 
As you may recall, sensitivity had suddenly dropped to nearly zero.  
About the only station I could hear on the old Hammarlund was the 50KW 
broadcaster a half mile from my house (which I can also hear on my 
mattress box springs or a wadded up chunk of aluminum foil).

Last night, though, I took a couple of hours to pull the RF deck. (This, 
by the way, is a little tedious but not at all difficult. Just follow 
the many directions posted here and there.or on Chuck Rippel's 
videotape).  The trouble had seemed to center on the 1st RF amp tube, 
V1. Pulling it actually INCREASED sensitivity very slightly. Naturally, 
I started tracing components at its socket. The caps looked okay, the 
chokes were intact and the resistors measured within tolerance  -- until 
I got to R6 which comes off the send side of the send/receive switch 
line.  It measured 1 MEG. It should be 1K.  The reason for its unduly 
high resistance became obvious under the gaze of my Optivisor magnifier. 
The little resistor was charred and split along its longitudinal axis  
-- just like the many BBODs this receiver once sported 

Once this resistor was replaced and the RF module wired back in, the 
radio played beautifully --better than ever, actually.

I am, of course, curious as to how or why this resistor might have burst 
into flames at some point (or why -- after doing so -- it clung to life 
for awhile, at least).  I've done extensive voltage checks and find 
nothing terribly amiss.

Oh well, as the old coots say: if it ain't broke, don't fix it -- and if 
you fixed it, don't fix it more.

Oh, (he said slyly), if you're interested in my antenna business, please 
come visit my website.  I'll send you the URL, if you email me. I don't 
want to be crassly commercial and publish it here, though.  I've been 
crass enough :-)

73,
Craig
W3CRR