Tuning indicators
(“magic
eye
"
valves)
are also
tested
in
sections,
first
normal
tests on
the amplifier
section,
then
a test on the shadow
section.
For
the
latter,
moving
the
Anode
and
Screen
volts
selector from
the
position
given
opposite
“Shadow”
m the valve
chart, to
that opposite
“
Test
should cause
a distinct
change in
shadow
angle.
In the case
of gas
-filled
valves, particularly
mercury vapour types,
it is
important
to
wait for one minute
on
the
“
Cathode
Leakage
11
position before switching
to
“
Rect
"
or to
!
‘
Mutual
Conductance
.
1
1
This
is
to
ensure
that
the cathode is at operating
temperature
before
anode
voltage
is
applied.
Mercury vapour
ana
gas -filled triodes
(thyratrons) are tested
with the voltage
settings
given but
on
Rectifier
”
position. The
grid
volts
should be reduced
slowly
from
15
and
at
some
lower '^alue
the valve
should start
to
conduct,
as shown by
movement of the
meter
pointer to the
“
Good
”
sector.
The value
obtained
varies
considerably
with
the type of
valve, and
its
rated
“
control
ratio,”
i.e., anode
voltage
/grid voltage
at
start
of
conduction.
NOTE
:
In the following
six
sections
the setting
of switches
not
mentioned
is immaterial.
2/2
RECTIFIERS
•
Set
ABC selectors
to
positions
specified in Valve Chart,
Anode/Screen
switch to
100/100
then turn
main
selector to
“
Rect
”
and read
on coloured meter
scale. Repeat
for
second
anode
if
any.
TESTING
COLD
CATHODE
RECTIFIERS
When
testing
cold
cathode
rectifiers
e.g.
OZ-4 the
anode
screen volt switch
should
be
set
to
250/200,
which will
cause
the
OZ-4 to
strike. The
anode
screen
volt switch
should
then
be
returned to the
100/100
position when
the
correct
emission will
be indicated
on the
meter.
Failure
of the OZ.4
to strike
or
the stopping
of emission
before
the
100/100
switch
position
is
reached
indicates a faulty
valve.
Care
should
be
taken
when testing other rectifiers
that the
anode screen volt
switch
be
set
to 100/100
as failure
to
do
so will damage
smaller
types
of rectifiers.
2/3
DIODES
Same as
for rectifiers
but main
selector is at
“
Diodes.”
2/4
FILAMENT
CONTINUITY
Set
main
selector
on
“
Fil.
Cont.” and
if
pointer
moves
to
about half
scale
then the
filament is intact.
2/5
ELEMENT
JjHORTS
Set
main
selector
to
“
Element
Shorts
”
and the
top cap, if any,
io green
socket.
Set
ABC selectors initially
to
000.
Turn
A selector
slowly
from
0 to
17 and back.
If
there is
a
definite frieter reading
on
any
switch
position
there
is an
internal
short circuit in
the valve.
Repeat with B and
C
selectors.
Also
see
Section
3/4.
2/6
CATHODE
LEAKAGE
Set
ABC selectors
and
filament
volts
as
specified,
then
set main selector to
“
Cathode
Leakage.” The
meter
pointer
will
kick
to the right,
then
fall back
to some steady
reading.
It
is not
possible
to
lay
down
a hard
and
fast rule
as
to the
minimum
acceptable
resistance as so
much
depends
on the
type of
valve and
the
circuit
in which it is
used. As
a general guide,
however,
at
least 2
megohms
should
normally
be obtained and a reading
of
1
megohm or
less
will
usually
indicate
a faulty valve.
Whether
it need be rejected on
this
account
depends
entirely
on
the circuit in
which
it
is used.
2/7
MUTUAL
CONDUCTANCE
Set
filament
volts,
anode
/screen volts,
grid volts
and ABC selectors
as
specified
and
turn A
and B
knobs
fully
anti
-clockwise then, and then only,
set
main
selector
to
“
Mutual
6
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