Power Supplies - HP 3478A Technical Manual

7-E-1. INTRODUCTION
7-E-2. This Service Group has information used to
troubleshoot the 3478A's Power Supplies and Reference
Circuitry.
7-E-3. POWER SUPPLY TROUBLESHOOTING
7-E-4. The Floating Common Section of the 3478A has
three power supplies which are: + 5V, -15V, and + 15V.
The Chassis Common Section has one + 5V supply.
The following paragraphs have some troubleshooting
information for the Chassis Common and Floating
Common supplies (refer to Schematic 4)
7-E-5. Chassis Common Power Supply
7-E-6. A low supply can be caused if zener diode CR766
and capacitor C762 are shorted, if there is an excessive
load on the supply (shorted component in the Chassis
Common Logic Circuitry), or if regulator U760 is defec-
tive. A high supply can be caused by U760.
7-E-7. Floating Common Power Supplies
7-E-8. + SV Supply. A low supply can be caused by an
excess load on the supply, a shorted CR712, or a defec-
tive U701. The supply can be checked by lifting jumper
JM701. If the supply is still low, troubleshoot the supp-
ly. Otherwise, troubleshoot the Floating Common Cir-
cuitry.
7-E-9. A high supply is most likely caused by a defec-
tive U701.
7-E-10. + 15V and -1SV Supplies.
supp- ly is a mirror image of the + 15V supply, use the
following procedure for both supplies.
difference is that the -15V supply has an additional filter
capacitor C706.)
-E-11. A low supply can be caused by an excessive load
on the supply, a shorted filter capacitor or protec-
SERVICE GROUP E
POWER SUPPLIES AND REFERENCE TROUBLESHOOTING
Service Group E Contents
Title
Introduction ............................................................ 7-E-1
Power Supply Troubleshooting.............................. 7-E-3
Chassis Common Power Supply ........................ 7-E-5
Floating Common Power Supplies ..................... 7-E-7
Reference Circuitry Troubleshooting ................... 7-E-13
Since the -15V
(The only
tion diode, an open temperature sensitive resistor
(RT706 or RT707), or a defective regulator. The supply
can be checked by lifting jumper JM703 for the + 15V
supply or jumper JM702 for the -15V supply. If the sup-
ply is now good, troubleshoot the Floating Common
Circuitry. If still low, troubleshoot the supply.
7-E-12. A high supply is most likely caused by the
regulators (U702 or U703)
7-E-13.REFERENCE CIRCUITRY
TROUBLESHOOTING
7-E-14. The 3478A's Reference Circuitry is used to
develop three reference voltages: + O1V, -IOV, and a
buffered + O1V. Since the -O1V and buffered + 10V
depends on the + 10OV reference voltage, make sure
the + IOV is good before troubleshooting the other
reference supplies. Perform the following checks (refer
to Schematic 3).
a. If the + 10V reference is inoperative, do the
following:
1.Measure the voltage at zener diode U461.
The voltage should be quiet and at approximately + 7V.
2.If the voltage is wrong, replace U461.
3.If the voltage is good, short across capacitor
C431. Measure pin 6 of U405.
4.If the voltage at pin 6 is approximately + 7V
and quiet, replace U403.
5.If the voltage is still incorrect or noisy, replace
U405.
b. If the + 10V is good and the -10V is incorrect or
noisy, replace U405. If still incorrect or noisy, replace
U403.
7-E-1
TM 11-6625-3071-14
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