HP 3438A Operating And Service Manual page 57

M o d e l 3438A Section V I l l
SECTION Vlll
SERVICE
8-1. INTRODUCTION.
8-2. I-his scction contains the Multirneter theory
o l
operation and troubleshooting information. A l s o
included are block diagrams, simplified schcrnatics and
complete M u l t i ~ n c t e r schematics.
8-3. I'hc f o l l o w i n g sequence w i l l be used i n 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 t o
dlgital converter and i t provides sensc current t o the
" u n k n o w n rcsistancc" l o r each o f the 7 o h m \ 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 t o the input ampli-
fier i n the ac or dc ~nilliamps function.
8-12. Analog to Digital Converter. l he analog t o
dlgital c o n ~ c l - t e r use the
clrcol .\lol)c r t i ~ ~ - , q r . o r i o t ~ r ( ~ ( . l i t i i y ~ c ( ~
t o translate alialog i n p u t signals i n t o digital t i m i n g
pulses.
8-13. Controller. 1-he controller processes rangc and
l ' u ~ ~ c t i o ~ l inl'ol-mation and arovidcs d i e ~ t a l c o n t r o l t o
PclOS t.1 I' switches i n thc i n p u t and post amp11lier-s and
the analog t o digital converter. 'I he corltroller also
D. COMPLETE SCHEMATICS.
conierts the comparator output ( r u n d o w n tinic) i n t o
appropriate digit and segment d r i ~ e voltages t o operate
tile displa).
THEORY OF OPERATION
8-14. Display. The display provides a n annunciated
digital readout o f the i n p u t signal using light e m i t t i n g
8-4. Block Diagram and Simplified Theory.
diodes.
8-5. t ' i g u r . ~ 8-1 i\ a block diagram
01'
the M u l t i m e t e r .
Each block 1 s discussed to g i ~ c the hasic tileor). of
operation o l t l ~ c M u l t i m c t e r f r o n ~ i n p u t t o displa!..
8-6. lnput Switching. I he input s\4itching block
col~sists 0 1 tlie I - u r i c t i o ~ l s w i t c I ~ e ~ ; ~ ~ i d the 11angeswitchcs.
I liesc h\*itche\ pr-ogl.am thc contl-oller using
a
3 line
I'unct~on code
( t
N i x .
I N
8. I-
K C ' )
and a 4 line rangecode
(bl 1 1 1 ) . M l 1 L , V l 1 I-'. A u t o ) .
8-7. lnput Amplifier. I llc i n p u t a m p l i 1 1 ~ 1 - is a m ~ ~ l t i -
gain opcl-ational amplil'ier.
I t
is used Sol- a l l l'i\,c i n p u t
lunctions. I he g;iln is selected b y M O S 1.C I switchcs
WIIICII ;ire c o ~ l t ~ - o l l e ~ I
by the contl.oller (11 13) o r bh. tile
~ n a n u a l rangc \witcllcs.
8-8. Post Amplifier. A c and dc voltages are amplified
by the post amplifier. T h e gain is x I o r x IOand isselected
by M O S
t'E-I
switches or b y the controller (U 13).
8-15. Power Supply. I ' h e power supply provides d c
voltages o f
+
7. -7, -2 ( V S L I ~ ) , and 6.5 V ( V 1 ) i s l ~ ) t o the
A I M u l t i m c t e r circuitry. A n ; ~ d d l t i o n a l $ 9 V ( V
I)).
+ j
V ( V
c),
and V (11725 Back Ciate Hias) are also
p r o \ iclcd
I"or
the H ['-I13 logic cil-cuitl.y.
8-16. DETAILED THEORY.
8-17. Power Supply.
8-18.
+
7 V Power Supply. I h c
+
7 V Power Supply is
I'u1Iu;rvc rectified by CR403 and C R 4 0 4 a n d 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
f u I I u a \ c rectified by C'11405 and C'l1400~111d r e g u l ~ i t c d by
0 9 and 0 4 0 3 . 1 lie +-7 V s u p p l y pro\.ide\ the rclcrcncc l o r
the - 7 V supply as s l l 0 ~ 1 1 1 ili I-igurc 8-2.
8-9. AC Converter. '1-he A C Convertel. is
an
average
8-20. -2 V (VSUB) Power Supply. I ilc V s t l j Suppl), is
responding detector used i n ac Loltage and ac
current
the substrate o r back gate bias supply f o r I J I I ( I n p u t
measurements. I - h e o u t p u t
01'
the AC' Converter is
a
dc
l i y b r i d ) . U 12 Integrator- H! brid, and 1113 Contr-oller.
voltage equal t o the rms value o f theac i n p u t voltagc. I n
I his supply is d c r i ~ e d b y d i ~ i d i n g b j ' - 7 V supply across
the ac currcnt l'unction. the i n p u t voltage t o thcconverter R413 and 11415. 1 he VhI:H Supply 1 s not regulated and is
is the ac \'oltagc d r o p across the currcnt shunts, times the thcrel'orc, load scnsitivc. I.his I'actor. is a n aid i n
gain of the i n p u t and post amplil'iers. t r o u b l c s l ~ o o t i n g .