52
4.3.1 Basic
DC
Instrument
Equipment Requirements:
4’/2
digit
Digital Voltmeter e.g. Datron
1041
Variable
5V, 1 amp DC supply
5mV/division
Oscilloscope e.g.
Telequipment
D83
1Mn5%
resistor
in
parallel with
lOnF
capacitor
10Mn5% resistor in
parallel with lOnF
capacitor
DC Voltage Calibrator,
e.g. Datron
4000/4000A
Autocal
Standard.
Procedure:
Power Supplies
1. Turn instrument on and allow
5
minutes warm-up
period.
2. Connect DVM Hi to TPS and Lo to TP28
on
the
Digital
assembly.
Adjust R2 on
the Rear (Power
Supply) pcb assembly
to
give +5.
100V +25mV.
3. Connect DVM Hi to
TP1
and
Lo to
TP20
on the Ana-
log assembly.
Adjust R7 on the Rear (Power
Supply
)
pcb assembly to
give
-t-15.000V
+15mV.
4. Connect DVM Hi
to
TP2
and
Lo
to
TP20
on the
Ana-
log assembly.
Adjust
R12
on the
Rear
(Power Sup-
ply)
pcb
assembly
to
give
—15.000V
±15mV.
Digital
Assembly
5.
Switch
the instrument off and disconnect the
power
lead.
6. Isolate
the
Digital
Board
by
removing the connectors
along
the centre
panel
(J1-J5).
7.
Connect variable
5V supply and DVM
Hi's
to TPS,
Lo's to
TP28. Reduce supply
to 4.750V
-t-10mV.
8.
Set
R83
fully
clockwise. Connect
oscilloscope
Lo to
TP28
and monitor
M53
pin 40.
Turn R83 anti-clock-
wise
until TP30
undergoes a
high
to
low transition
(or begins
to pulse low).
9.
Remove
variable supply
and reconnect items
discon-
nected in
steps 5 and 6. Disconnect the oscilloscope.
Switch on the instrument.
10. Connect
DVM Hi to battery positive terminal.
Low
to TP28.
Check battery voltage
is
>2.5
volts.
11. Disconnect DVM and
connect oscilloscope
Hi
to
TP25,
Lo
to TP28.
Adjust scope trigger until
the
trace is triggered by the first
pulse of
each
reading
burst.
Adjust R11
so that the
pulses
occur every
5ms
±
0.5ms.
12. Place
instrument into
HOLD. Connect oscilloscope
Hi to
TP7.
Adjust
L2
to
give a
stable
-H.05±0.03V.
NOTE: This signal
contains
about
200mV
peak
to
peak high frequency noise.
13.
Insert calibration key into
keyswitch
on the back
panel and
turn, placing
the instrument into
CAL
mode.
NOTE: The display CAL
legend
will
be
lit.
14. Short together pins
'D'
and 'E' on
Digital assembly.
NOTE: All the
calibration store correction factors
are
now
reset to zero.
15.
Turn the calibration key back to RUN
mode.
Analog
Assembly (DC
Isolator Section)
16.
Centralize R
150
and
R
160.
17. Select 0.1V range DC with FILTER
out.
Apply
a
10Mf2 resistor between
instrument Hi and
Lo.
Connect DVM
Hi
to TL8, Lo to
TP20.
Adjust
FSV R152 with a metal
film resistor
(50ppm/°C)
for a
reading of
<
lOmV, using R159
for 'fine'
adjustments.
Do
not solder
in
R152.
18. Apply a short circuit across the input terminals and
adjust R
1 50
for a
reading of
<
50/iV
at
TP1
3.
19. Connect
DVM
Hi
to
TP33
and adjust R160
for
a
reading
of
<20
/jV.
20. Repeat steps 17 to 19
until readings
are within
specified limits.
NOTE; The
following
step
is only required after the
replacement
of
Q12 or
any component associated
with the
temperature compensation
circuitry.
21
.
(i)
Re-apply
10MS2
resistor across the input
terminals. Note the
reading
on the
front panel
display
(=A).
(ii)
Note
the ambient
temperature
(=X°C).
(iii) Place
the
instrument
in
a
temperature
con-
trolled oven
at
approx 50°C without top
cover
and with
power 'on'.
(iv) Leave
the
instrument for at least 1 hour then
note the reading on the
display
(=B) and the
temperature of the oven (=Y°C).
(v)
Compute
(B-A)/(Y-X)
=
Drift/°C.
(vi)
Remove
instrument
from
oven and allow
to
stabilize, with power
'on'
to ambient
for
one
hour.
(vii)
If
the drift was <10 digits/°C
proceed
to (x).
(viii) For
drifts
in
excess of 10 digits/°C
R151 must
be adjusted.
If
the
drift
was
positive turn
R151 clockwise, if
negative turn
R151
anti-
clockwise.
(ix)
Repeat from
(i).
(x)
Lock
R151
with a clean soldering iron.
(xi)
Repeat steps
17
to 20.
22. Solder
in
R152, with instrument turned
off.
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