[Hammarlund] SP-600 service tip

[Craig Roberts]

It's 2 AM and I've just finished repairing my SP-600.  No, I haven't 
been struggling with this for weeks, days or even hours -- just about 45 
minutes.

Y'see, I was on my way to bed and stopped into my shack to turn off my 
old Hammarlund which had been left running since a short SWLing stint 
earlier in the evening.  It was tuned to the 31 meter band, which seemed 
unusually devoid of  noise. A twirl of the dial revealed the entire band 
to be dead, as were the other upper bands.  I expected to hear at least 
a couple of signals here and there -- but nothing!

The lower bands were fine.  A strong evangelical station at about 7450 
KHz boomed in on the lower band setting, but not on the same frequency 
on the band above.  Obviously my second IF (3955 KHz) had ceased to 
function. 

I couldn't hit the sack without at least looking into the problem. You 
know how it is...  I was taught long ago to look for the simple 
solutions first.  It's not advice I always heed (how many difficult 
disassemblies have YOU performed to replace perfectly good components?) 
but -- perhaps because of the lateness of the hour -- I troubleshot 
simply this time.  There's this flimsy little switch, you see, that is 
activated when you turn to the upper (double conversion) bands. It hangs 
on the back of the turret assembly. It's the simplest thing I could 
think of that might malfunction so dramatically. Tubes, in my 
experience, rarely do (though I've had a couple of OA2s crack open).  
So, I wrestled the receiver from its case (and to think I actually 
WANTED that case) and staggered over to my workbench with it. I turned 
the beast over and watched the switch as I rotated the bandswitch.  Sure 
enough, it didn't move from the lower band position.  I, of course, 
imagined some sort of nightmarish mechanical linkage, forged of 
Unobtainium, had self-destructed.  I warily lifted the lid on the turret 
box (fastened by approximately 144 machine screws). There, inside the 
box opposite the switch, I spied a common little spring dangling from an 
actuator arm which, in turn, is operated by a cam plate on the back of 
the turret assembly.  After staying in place for 50 years, this little 
coil had, at 1 AM, decided to let loose. I simply hooked it over the 
edge of the turret box, replaced the lid, and resumed normal listening. 

So, if your SP-600's second IF suddenly ceases to run, you might try 
checking the operation of that little switch before you haul out the 
oscilloscope and kiss your family goodbye for a couple of days.

Oh, one other thing -- original 1950's and 60's "bulletproof" mica caps 
are starting to wimp out.  They're becoming the BBODs of the early 21st 
century. I've found several in both my Collins and Hammarlund rigs 
recently that have gone sour.  Repair gurus are reporting the same 
thing.  So, add those "postage stamp" condensers to the list of 
components to check when you're troubleshooting mysterious anomalies.  
Incidentally, I read an SP-600 modification article the other day in QST 
that recommended replacing all the crappy old Black Beauties in the 
receiver. The piece was dated August, 1970!!

Anyway, g'night!!

73,

Craig
W3CRR