Electronics-World-1959-05

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positive! Now we come to what ought
to be a dead end, a 300 -µµtd. capacitor
(Crc). The capacitor has the same
voltage at both ends, so I guess he's the
culprit."
"Dead on the ball," approved Tommy,
"and that's the best way to approach
a fault like that too. Put a decent
capacitor in there, one of those
750 -volt types. I'd better give the rest
of the set a check over I guess. Hey!"
he erupted a few minutes later, Who
checked these tubes."
"I did," admitted John, "what's
wrong ?"
"See those shields ?" Tommy pointed
to the i.f. strip. "You must tuck those
little grounding strips inside the shield
(Fig. 3) when you replace it, especially
on this particular model. It has a
tendency to take off into oscillation if
those tubes aren't properly shielded."
The Second G -E Chassis
Jack had been quietly making a voltage
check of the second chassis and
now came up with a list that practically
matched the figures on the schematic.
"That doesn't get us far, does
it ?" he observed disappointedly. "I
guess this one needs a different approach
-and I was looking forward
to playing Sherlock Holmes myself on
this one."
"Don't worry," said Tommy, "there
are other ways of bringing the fault to
justice without tracking him everywhere
he goes. Let's look for some
more evidence." He picked up the low -
capacitance probe and turned up the
intensity control on the scope above
the bench. "Let's have a look for the
sync pips coming through from the
clipper circuit." He attached the probe
to pin 8 of the phase detector in Fig. 1.
"Note we have plenty of spike from
the sync department which disappears
when we tune off- channel. Always a
good idea to remove the signal temporarily
to make sure it is the sync
pulse you are looking at, rather than
some induced signal from the high -
voltage section. And you can see the
saw -tooth there from the horizontal -
discharge tube. Better check along
that line as John did with the other
set. We'd look rather foolish if it was
the same thing and we went batting off
on some other tack." This was done,
but it turned up nothing.
"Now look here," said Tommy, "I
think the best way here would be to
sift the evidence through, exhibit -byexhibit
in the routine. component -
check method. Check each part in the
grid circuit of the reactance tube and
then those between the reactance tube
and the oscillator grid. I'm going to
nip out for an early lunch." When
Tommy returned, the set was working
merrily on the bench, its horizontal
hold as stable as a rock.
"It was this 470 -µpfd. capacitor, C,.,,"
reported Jack. "It has no capacitance
at all according to your checker."
"What does that capacitor do in the
circuit anyway ?" asked John. "I don't
know much about this reactance -tube
business."
"Well," explained Tommy, "you can
regard the reactance tube as being
in parallel with the oscillator tuned
circuit, like this (see Fig. 5). Now the
capacitor that went open was coupling
the reactance tube to the circuit so,
with it open, the reactance tube might
as well not be there for all the good it
does. Under such conditions, when the
oscillator drifts off- frequency, the reactance
tube is not able to change the
phase of the circulating currents in the
oscillator tuning network to compensate
for the drift ... what was that
you said, John ?"
"I just said, 'I pass,' " mumbled
John.
SOLDER GUN WIRE STRIPPER
By FRITZ C. HOFFMAN
WHEN INSULATION must be removed
from wire, an irritating situation
usually results, especially when the wire
involved is in a cramped working space.
A slender knife might work, but even
the slight nicks that inevitably result
weaken the wire and thus tend to invite
breakage. The little device shown in
parts (A) and (B) of the accompanying
illustration, made from a miniature, copper
alligator clip of the "Minigator"
type, solves the problem neatly.
To make up the wire stripper, cut a
"V" into the very tip of the clip, and
then bend this tip over 90 degrees as
shown in (A). The entire unit is then
clipped onto the tip of the soldering
gun, as shown in (B). Now all you
have to do is press the trigger of the
gun and rotate the adapted tip around
the wire to be stripped. With the help
of the applied heat, insulation comes off
cleanly and quickly.
Where a considerable amount of
stripping is to be done, as in wiring up
kits, a stripping clip can be made up for
use on a regular or pencil type iron.
Shown in part (C), this adapter can be
fashioned front copper and left per-
manently in place on the iron. While
this version may not work quite as well
as the gun adapter in close quarters, it
is more practical for extensive work and
is always at band when needed. -30-
ELECTRONICS WORLD