Girlfriend radio: Hallicrafters S-38A ! Any for sale?
cfandt at NETSYNC.NET
Mon Nov 15 21:12:12 EST 1999
At 05:09 PM 11/15/99 -0500, Richard Post wrote:
> The hot-chassis S-38 cannot be made safe by simply using a polarized line
> cord. Several of the sets (up through and including the S-38C, I believe)
> have one side of the line cord directly switched to the chassis. Let's
> suppose that the neutral side of the line is switched to the chassis. That
> makes it safe when it is switched on. The chassis is at line neutral. When
> that same set is switched OFF, the chassis is at line potential and
> dangerous! The hot side of the line is now connected to the chassis by way
> of the tube filaments. Turn the plug around and the chassis is electrically
> hot when
> the power switch is turned on and neutral when turned off.
> A capacitor is connected between the metal cabinet and the chassis. The
> dried-out rubber insulators, the paper back, and the paper on the bottom are
> about the only protection you have against contacting a very live metal
> chassis in the on or the off switch position. Keep that insulation intact.
> Matt's recommendation of an isolation transformer is on target. If you do
> not have an isolation transformer, then at the very least, plug the radio
> into a GFCI (Ground Fault Circuit Interrupter) protected outlet.
A GFCI usually does not work as it trips from the small amount of leakage
current normally flowing thru the typical capacitor connected from either side
of the line to chassis even if its not leaky from failure. I presume a smaller
value cap than usually found, say around 500 pf, may offer less leakage current
and be less prone to trip a GFCI and still offer the signal grounding
characteristics for which the cap is there.
Of course, an isolation transformer is the only way to ensure safety. I have a
1KV unit at my bench with which a set under test is connected plus sometimes
the AC-operated test gear used on that set. I've scrounged several small
isolation transformers from obsolete industrial controls and use them to run my
hot chassis radios (including the S38C) if I have them down in the basement
Small isolation transformers ( ~0.1 KVA) are not too expensive. Also, two
filament transformers like found at Radio Shack can be wired
secondary-to-secondary (they should be identical units) and the 120V winding on
the isolated side can be used to run an S38. Additionally, a single transformer
with two 120V primary windings (either 120V mains when wired in parallel or
240V mains when in series) can be used. Leave the secondary leads insulated
form contacting anything by taping securely or use heat shrink tubing to
accomplish same. Hook the mains to one primary and the load to the other
primary. Fuse your primaries no matter which method chosen.
Christian Fandt, Electronic/Electrical Historian
Jamestown, NY USA cfandt at netsync.net
Member of Antique Wireless Association
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