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INSTRUCTION
MANUAL
Digital Mullimeiar
Models 172A. 173A
THEORY OF OPERATION
also
charges
the
8.4V
batteries,
In BAT mode,
the
tl9.2
volts
and the
+8.4
volts
from
./
the
batteries
connect
to
the
input
terminals
of
VROOZ, VR003 and ~~001.
The
line
switch
applies
ac voltage
from
the
transformer
to
the
Model
1722 for
its
regulated
5 volt
supply
Thus,
turning
the
I72A
or
l73A
off
also
turns
off
the
1722.
Transformer
TODI has
two
internal
shields,
one secondary
shield
tied
to pin
IO,
and one
primary
shield
tied
to power
ground,
This
shielding
provides
line
isolation.
When the
dual
primary
of
TOOI
is
tied
in parallel
by switch
5001,
the
Model
172A/173A
is connected
t
for
II7
volt
operation.
Switch
SO01 also
puts
these
primaries
in series,
for
230 volt
operation,
as indicated
on the
schematic.
Provision
for
100 volt
line
is
interchanging
connections
5 and 6 and connections
2 and 3 on the
primaries.
This
is a physical
wiring
t
change
that
must
be made on the
printed
circuit
board,
and
is
a factory
option.
There
are
two connectors
for
the
Model
1728 battery
pack,
P402 and P4Ol.
The se If-check
terminals
are
on the
bottom
of
the
instrument.
There
is
an "ac"
self-check,
"dc"
self-
check
and an "ohms"
self-check.
Connect
from
the
appropriate
self-check
terminal
to
the
input
high
terminal
to exercise
the
instrument.
The self-check
is not
accurate,
the
voltages
and currents
are
only
approximate.
A battery
test
point
("A")
on the
bottom
of
the
instrument
allows
checking
of
the
+19.2
volt
battery
in
the
Model
1728.
Normally
the
low battery
light
will
indicate
when
the
batteries
have
become
low and need
recharging.
However,
the
test
point
could
be used
to
troubleshoot
for
a defective
cell.
5-7.
MODEL 1728 RECHARGEABLE BATTERY PACK, SCHEMATIC
26758C.
The Model
1728 provides
i19.2
volts
and +8.4
volts
from
nickel-cadmium
batteries.
ET401
and BT402 are
19.2
volt,
.45 AH nickel
cadmium
packs
which
are
fused
by
IA,
3AG SIo-Blo
fuses.
ET403
is an 8.4W
pack
composed
of
seven
1.2
volt
"C"
cells
and
is
also
fused
by a IA,
3AG. Slo-Blo
fuse.
The Model
1728 has a built-in
recharging
circuit
which
operates
from
-~29.5V
(unregulated)
supplied
by the
DMM line-power
supply.
a.
BT401
Charoinq.
BT40l
is
charged
via.a
constant
current
from
Q401.
Diodes
CR401
and CR402 and
resistor
R402 place
a diode
drop
across
R40l.
The
resulting
current
is
the
maximum charging
current.
Diode
CR403 prevents
the
battery
from
supplying
current
through
Q401.
The maximum
charging
current
occurs
only
when
the
DMH is set
to OFF.
A
trickle
charge
is maintained
when
the
DMM is
line
operated.
The reduced
current
is
obtained
by inserting
a limiting
resistor
in series
with
the
charging
circuit
(not
shown
on Schematic
26758C).
In full
charge,
the
batteries
are
charged
at
a C/IO
rate
(45 mA
for
BR40l
1. BT402,
f. 200mA for
BT403).
When trickle
charged,
the
charge
rate
varies
according
to
line
voltage
E battery
condition
from
a minimum
of
C/l00
rate
to a maximum
of
C/20
rate.
Thus,
trickle
charge
may never
fully
charge
the
batteries,
but
is
intendal
i
to put
sufficient
charge
into
them for
short
intermittent
use.
6.
BT402 & BT403
Charaino.
BT402 G BT403 are
charged
via
constant
currents
from
~402
3
or Q403,
respectively.
These
circuits
operate
similarly
to
the
above
circuit,
except
that
they
are
powered
by unregulated
?29.5V.
C.
Low Battery
Detector.
This
circuit
monitors
the
voltage
of
all
three
batteries
in
the
Model
1728,
and provides
a drive
signal
to
turn
on the
front
panel
low battery
indi-
cator
if
either
battery's
voltage
drops
below
a predetermined
level.
The approximate
levels
are:
ET403
(+7.2V),
BT40l
(17.lV)
and ET402
(-17.V).
These
levels
were
selected
to ensure
that
a low
battery
indication
will
be given
before
operation
of
the
associated
regulator
circuits
in
the
DMM can be adversely
affected.
The heart
of
the
detector
circuit
is
U401,
which
is
connected
as an open-loop
comparator.
Low voltage
detection
for
each
battery
is
described
as follows:
1
I)
BT403.
The
threshold
of
the
comparator
is
set
at approximately
+7.2
volts
by a
divider
consisting
of
R407 and R4lO connected
between
the
regulated
+I5
volts
and
5-2
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