[Hallicrafters] Tech Tip - SX-117 Chassis Ground

[Duane Fischer, W8DBF]

                  Check For An Open Ground!


I can still copy the LSB station. Anyone else run into this
issue before?
If so, what was your fix?

Tom,

I suspect leakage of signal from the input to the output of
the filtering circuits.. could be the low IF.  Likely
culprits include;
bad bypass caps (paper ones??)
corroded metal parts at shield edges and clamping hardware
any and all terminal strip ground legs.  you may need to get
a BIG solder iron (like the roofing guys used) and solder the
ground legs to the chassis.

To test for these conditions, set up your signal generator,
well shielded, to feed the input of the IF strip or other
section under test.  Set the frequency off the edge of the
bandwidth you are interested in. Crank up the signal level
till you get a reading on a detector at the output ( for
instance the agc voltage or detector dc output).

Then go around with a nice new modern ceramic capacitor with
flea clips on its short leads and parallel it onto the bypass
caps in question, watching the output indicator.  A drop in
output means you have located a poor bypass cap.  Similarly,
with a screwdriver, short  the ground tab and tube
socket grounding rings to the chassis.

One guy reports that his highly regarded TMC  GPR-90 suffered
mightily from oscillations and poor selectivity till he
soldered all the riveted ground tabs to the chassis.  I have
not yet done that to mine..


Roy
-Keep em glowing!
Roy Morgan, K1LKY since 1959
7130 Panorama Drive
Derwood MD 20855
301-330-8828
roy.morgan at nist.gov


Duane Fischer, W8DBF - WPE8CXO
E-Mail: dfischer at usol.com
Hallicrafters web site: www.w9wze.net
HHRP web site: hhrp.w9wze.net