Model 3438A Section VIll
SECTION Vlll
SERVICE
8-1. INTRODUCTION.
8-2. I-his scction contains the Multirneter theory
ol
operation and troubleshooting information. Also
included are block diagrams, simplified
schcrnatics and
complete
Multi~ncter schematics.
8-3.
I'hc following sequence will be used in this
section.
A. THEORY OF OPERATION.
1. Block Diagram and Simplified
Theory.
2.
Detailed Theory.
B. TROUBLESHOOTING.
C. SIMPLIFIED SCHEMATICS.
8-10. Ohms Current Source.
'l'he ohlns currcnt
source provides
ohms
rcl'erence volt:igc for thc analog to
dlgital converter and it provides sensc current to the
"unknown
rcsistancc" lor each of the
7
ohm\ ranges.
8-11.
Current Shunts.
The current shunts are used for
ac and dc current measurements. The voltage drop across
the shunt resistors is the input voltage to the input ampli-
fier in
the ac or dc ~nilliamps function.
8-12. Analog to Digital Converter.
l
he analog to
dlgital con~cl-ter use the
clrcol .\lol)c rti~~-,qr.oriot~ r(~(.litiiy~c(~
to translate alialog input signals into digital timing
pulses.
8-13. Controller.
1-he
controller
processes rangc and
l'u~~ctio~l inl'ol-mation and arovidcs die~tal control to
PclOS
t.1
I'
switches in thc input and post amp11lier-s and
the analog to digital converter.
'I
he corltroller also
D. COMPLETE SCHEMATICS.
conierts the comparator output (run down tinic) into
appropriate digit and segment
dri~e voltages to operate
tile displa).
THEORY OF OPERATION
8-14. Display.
The display provides an annunciated
digital readout of the input signal using light
emitting
8-4.
Block Diagram and Simplified Theory.
diodes.
8-5. t'igur.~ 8-1
i\
a block diagram
01'
the Multimeter.
Each block 1s discussed to gi~c the hasic tileor).
of
operation ol tl~c Multimcter fron~ input to displa!..
8-6. lnput Switching.
I
he input s\4itching block
col~sists
01
tlie I-urictio~l switcI~e~;~~id the 11angeswitchcs.
I
liesc h\*itche\ pr-ogl.am thc contl-oller using
a
3
line
I'unct~on code
(t
Nix.
IN
8.
I-
KC')
and a 4 line rangecode
(bl
111).
Ml1L, Vl1
I-'.
Auto).
8-7. lnput Amplifier.
I
llc input ampli11~1- is a m~~lti-
gain opcl-ational amplil'ier.
It
is used
Sol-
all l'i\,c input
lunctions.
I
he g;iln is selected by MOS 1.C
I
switchcs
WIIICII
;ire co~lt~-olle~I by the contl.oller
(11
13) or bh. tile
~nanual rangc \witcllcs.
8-8. Post Amplifier.
Ac and
dc
voltages are amplified
by
the post amplifier. The gain is
x
I
or
x
IOand isselected
by
MOS
t'E-I
switches or by the controller
(U
13).
8-15. Power Supply.
I'he power supply provides dc
voltages of
+
7. -7, -2 (V SLI~), and 6.5 V (V 1)isl~) to the
AI Multimcter circuitry. An ;~ddltional
$9
V (V
I)).
+j
V (V
c),
and V (11725 Back Ciate Hias) are also
pro\ iclcd
I"or
the
H
['-I13 logic cil-cuitl.y.
8-16. DETAILED THEORY.
8-17. Power Supply.
8-18.
+
7
V
Power Supply.
Ihc
+
7
V
Power Supply is
I'u1Iu;rvc rectified by
CR403
and CR404and regulated by
1.110.
1
he
$7
V supply is adjustable by 11417.
8-19. -7
V
Power Supply.
l.hc -7
V
I'oacl- Supply is
fuIIua\c rectified by C'11405 and C'l1400~111d regul~itcd by
09 and 0403.
1
lie +-7 Vsupply pro\.ide\ the rclcrcncc lor
the
-7
V
supply as sll0~111
ili
I-igurc 8-2.
8-9. AC Converter.
'1-he AC Convertel. is
an
average
8-20.
-2
V (VSUB)
Power Supply.
I
ilc Vstlj Suppl), is
responding detector used in ac
Loltage and ac
current
the substrate or back gate bias supply for
IJ
I I
(Input
measurements.
I-he output
01'
the
AC'
Converter is
a
dc
liybrid).
U
12 Integrator-
H!
brid, and 1113 Contr-oller.
voltage equal to the rms value of theac input voltagc. In
I
his supply is dcri~ed by di~iding bj'-7 V supply across
the
ac currcnt l'unction. the input voltage to thcconverter
R413 and 11415.
1
he VhI:H Supply 1s not regulated and is
is the ac
\'oltagc drop across the currcnt shunts, times the thcrel'orc, load scnsitivc. I.his I'actor. is
an
aid in
gain
of
the input and post amplil'iers.
troublcsl~ooting.
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