Heathkit IM-18 User Manual

Page 4
HEATHEIT
CIRCUIT
DESCRIPTION
The
combination
AC-OHMS-DC
test
lead
of
the
VTVM
is
connected
tothe
Function
switch,
which
needed
for
a
specific
measurement
function,
The
COMMON
test
lead
is
connected
to
the
case
(ground)
of
the
instrument.
With
the
Function
switch
in
the
DC+
or
DC-
po-
sition
and
the
switching
probe
on
DC,
the
test
voitage
is
applied
through
the
1
megohm
resistor
in
the
probe
to
the
Range
switch,
which
is
a
series
of
precision
resistors
arranged
as
a
voltage
divider,
Depending
on
the
position
of
the
Range
switch,
a
portion
of
this
DC
voltage
is
"picked
off"
and
applied
to
the
input
grid
of
the
12AU7
tube,
With
the
Function
switch
in
the
AC
position
and
the
test
probe
on
AC-OHMS,
an
AC
test
voltage
is
applied
to
the
6ALS5
tube
(half-wave
doubler
circuit)
where
it
is
changed
toa
DC
voltage
which
is
proportional
to
the
applied
AC
test
voltage,
On
the
higher
AC
ranges,
a
voltage
divider
arrangement
is
used
atthe
input
of
the
6AL5
tube
io
insure
tml
Ge
AG
woltwes
appliodis
the
BALA
tube
does
not
exceed
the
tube's
rating,
The
DC
yoltage
output
of
the
6AL5
tube
is
applied
to
the
Range
switch
and
then
to
the
input
grid
of
the
12AU7
tube,
in
the
same
way
that
DC
test
voltages
are
applied,
A
capacitor
is
used
at
the
output
of
the
6AL5
to
hold
the
applied
pulsating
DC
voltage
at
its
peak
value,
so
that
the
VTVM
responds
to
peak
voltage
regardless
of
the
test
voltage
waveform,
The
AC
balance
control
is
used
to
"buck-out”
the
small
amount
of
contact
potential
in
the
6AL5
tube,
thus
eliminating
residual
readings
on
the
lower
AC
ranges,
The
ohmmeter
section
of
the
VTVM
uses
a
1.5
volt
battery
connected
in
series
with
part
of
the
voltage
divider
network
(determined
by
the
Range
switch
position)
and
the
resistance
to
be
meas-
ured,
The
ratio
between
the
ohmmeter
voltage
divider
network
resistors
and
the
measured
resistance
determines
what
portion
of
the
ohm-
meter
battery
voltage
is
applied
to
the
input
grid
of
the
12AU7
tube,
Thus,
for
all
measurement
functions,
a
voltage
dependent
upon
the
quality
being
measured
is
applied
to
the
grid
of
one-half
of
the
12AU7
twin
triode,
With
zero
voltage
input
to
the
12AU7
bal-
anced
bridge
circuit,
each
of
its
triode
sections
draws
the
same
amount
of
cathode current
and
therefore
each
cathode
is
at
the
same
voltage
potential,
The
meter
movement
is
connected
between
the
cathodes
of
the
12AU7
tubeandcon-
sequently
will
not
deflect
since
both
cathodes
are
at
the
same
potential,
When
a
positive
voltage
(from
the
Range
switch)
is
applied
to
one-half
of
the
12AU7
tube,
this
half
of
the
tube
draws
more
current
than
the
other
half,
causing
a
difference
in
cathode
po-
tential
between
the
two
tube
sections,
Since
the
meter
is
connected
between
the
two
cathodes,
a
current
flows
through
the
meter
movement,
The
meter
pointer
responds
proportionally
to
this
current,
indicating
the
value
of
voltage
or
re-
sistance
being
measured,
The
DC+
and
DC-
switch
positions
are
used
to
reverse
the
meter
connections
between
the
cathodes
so
that
cur-
rent
always
flows
through
the
meter
in
the
same
direction,
The
use
of
the
bridge
circuit
eliminates
any
change
in
the
voltage
reading
if
the
Bt
voltage
in
the
VTVM
should
vary
since
the
resulting
variation
in
tube
conduction
will
occur
in
both
triodes
and,
therefore,
will
not
affect
the
dif-
ference
in
cathode
potential,
Also,
the
maximum
conduction
characteristic
of
the
12AU7
tube,
as
used
in
the
VTVM
circuit,
is
such
that
the
voltage
applied
to
the
meter
terminals
cannot
be
Jarge
enough
to
damage
the
meter
move-
ment,
This
is
one
of
the
primary
advantages
of
the
VTVM
circuit,
The
meter
movement
can-
not
be
burned
out
by
inadvertently
measuring
a
voltage
that
is
higher
than
the
Range
switch
set-
ting,
However,
if
excessive
voltage
is
applied,
the
pointer
may
be
bent
as
it
hits
against
the
stop,
Caution
must
aleo
be
exercisedto
avoid
annlying
any
test
voltage
to
the
test
probe
whenthe
Fune-
tion
switch
is
set
in
the
OHMS
position,
The
precision
resistors
in
the
ohmmeter
voltage
di-
vider
network
have
very
low
power
ratings
and
can
easily
be
burned
out
in
this
way,
The
power
supply
of
the
VTVM
uses
a
silicon
rectifier
in
a
half-wave
rectifier
circuit,
An
electrolytic
capacitor
is
used
for
tiltering
the
DC
voltage
from
the
power
supply,
The
power
supply
provides
both
B+
voltage
for the
12AU7
tube
and
positive
DC
'buck-out'
voltage
for
the
AC
balance
circuit,

Heathkit IM-18 Specifications

General

Brand	Heathkit
Model	IM-18
Category	Measuring Instruments
Language	English