ACCESSORIES
R-F
PROBE
P-75
(K):
An
EICO
R-F
probe
(P-75K
-
kit
form, P-75
-
factory
wired)
for
use in measuring
voltages
up
to
50
volts and to 200
Me
is available
to
extend
the
uses
of the
instrument. This
probe
is
simply
plugged into the
D-C
jack
of the
instrument and the r-f
voltages
are read
on the
regular
D-C
sea I
es
.
HIGH
VOLTAGE
PROBE
HVP-1:
An
EICO High
Voltage
Probe HVP-1
(factory
wired
only)
for
measuring d-c voltages
up
to
3CKV
is available to extend the
uses
of
the instrument. The probe may
be
supplied with
a
multiplier resistor
of
240
Megohms
to
give a high voltage range
of
10,000
volts or witha multi-
plier
resistor
of
740
Megohms
to give
a high
voltage range
of
30,000
volts.
CIRCUIT DESCRIPTION
GENERAL:
The
meter
measures
either d-c
or
a-c voltages
by
making use
of
the rectifying and
amplifying characteristics of
vacuum tubes. The input
impedances
are
very
high (d-c
-
25
megohms, a-c
-
3 megohms),
and
the
current
used to
actuate
the
indicating
meter is
not taken from the
circuit being
measured.
A
bridge circuit,
used to stabilize the operating voltages
of
the
tubes, provides
constant
accuracy despite
line
variations.
When
used as an
ohmmeter, the instrument wi II
measure resistances between
zero
and
1000
meg-
ohms.
Decibel
measurements
between
-
20
and
+
55
db can
be made
using
the
DB
scale.
D-C
AMPLIFIER
CIRCUIT:
(See
Figures 1
and
3) A
balanced bridge
circuit
is
used
in the d-c amplifier, comprising
the
twin
triode
V“2, a
common plate
load resistor R-3,
and the balanced
cathode load resistors R-10,
R-ll,
and
R-12. The
meter
M-l
is connected across the two cathodes
of
V-2. In the
normal
condition,
a
reference current
flows
through
V“2B,
which
has
a
ground-
ed
grid.
Current
flow
through
V-2A is adjusted
by
means
of
the ZERO ADJ.
control
R-12
to
equal
the
current
flow
in
V
_
2B.
The meter then reads
zero.
OPERATION AS
A D-C VOLTMETER:
(See Figures 1 and
3)
The circuit for
operation as
a d-c voltmeter
is as
follows: The
unknown voltage
is
applied
across the connectors
J-l and J-3 (ground). The
FUNCTION
switch S-3
connects the
range voltage divider
across J-l and
ground.
A
voltage, depend-
ing
on the RANGE switch setting,
is
then
applied
to
the
grid
of
d-c amplifier
V-2A.
This
grid
voltage
unbalances the
bridge circuit, and the meter is
deflected in direct
proportion to
the
unbalanced current.
OPERATION AS
AN
A-C
VOLTMETER:
(See
Figures
1 and 3) The circuit
for
operationas
ana-c voltmeter is
as
follows: Theunknown
a-c voltage
is
appl-
ied
across the connectors at
J-2 and
J-3
(ground). The FUNCTION switch
applies the
voltage
to
the
diode rectifier V“1
. The
d-c
output voltage
of VI
-
7
-
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