Heathkit IM-16 - Factory Repair Service; Shipping Instructions

DC VOLTAGE
MEASUREMENTS
The
Solid-State
Voltmeter
has
many advantages
over
the
nonelectronic
voltmeter. The
greatest
advantage
is its
ability
to
measure
voltages
with-
out
significantly
loading the
circuitbeingtested"
This
characteristic
enables the
voltage to
be
measured more
accurately. This
is especially
desirable
in such high impedance
circuits as
resistance
coupled amplifiers and AVC
net-
works, and
on
oscillator
grids.
To
illustrate
this
advantage
of
the Voltmeter,
assume
that
a resistance-coupled
audio
ampli-
fier
with
a
500
K
ohm
plate
load
resistor
is
operating from a
100 volt
plate
source. See
Fig-
ure
5.
Assuming
that the
actual
plate
voltage in this
case is
50
volts, therefore,
the
tube
acts as
a
500
K
ohm resistor.
When
measuring
the
plate
voltage
with a
conventional
1000
ohms-per-volt
meter
on
the
100 volt sca1e, the
meter
repre-
sents a
100
K
ohm
resistor
placed
in
parallel
with the
tube. See Figure
5A.
The
voltage
on
the
plate,
as
shown
on
the
meter,
wouldthenbe
14,3
volts.
This
large amount
of
error
is
caused
by
the
shunt
resistance
of
the
meter"
When
the
Solid-State
Voltmeter
is
used
on any
scale, the
full 11
megohms
input
resistance
is
placed
in
parallel
with the
tube.
See Figure
bB.
A
plate
voltage
of
about
4g volts
is then
shown
on
the
meter;
only
ZVo
lower than
the
normal
operating voltage" Thus,
more
accurate
readings
in this
instance
can be obtained
only with
a
high
resistance
instrument
such
as
the
Solid-State
Voltmeter.
AC VOLTAGE
MEASUREMENTS
To
measure
AC
voltage with the Voltmeter,
set
the FUNCTION
switch and the
probe
switch to
AC.
Set
the VOLTS
range
switch
to
a range
greater
than
the
voltage to
be measured,
if
an
approxi-
mate
voltage
is known.
If the
voltage
is
com-
pletely
unknown,
set
the
VOLTS
range
switch to
1500.
Then
touch the test
probe
to the
point
in
the
circuit where
the
voltage is to
be measured,
If
the
meter
moves
less
than
t/Z
ot
full
scale,
switch
to
the
next
lower
range"
The
maximum
AC
voltage
that
can be
safely measured
with
your
VOM
is 1500
volts, and this
limit must
not
be exceeded.
F
igure 5
CIRCUIT
UNDER
TE
ST
tooofr,/v
METER
Figure
5A
CIRCUIT UNDER
TEST
F igure
58