HP 3478A Technical Manual page 80

3478A 3478A
f. f. If the reading is If the reading is good, do the following: good, do the following:
1. 1. Measure for a Measure for a stable 3V ac at pin
2. 2. If the If the reading is good, reading is good, replace U303.
3. 3. If the reading is If the reading is wrong,
capacitor C304 which is connected to pin 4 of U303. capacitor C304 which is connected to pin 4 of U303.
Then measure for a stable 3V ac at the lifted end of Then measure for a stable 3V ac at the lifted end of
the capacitor. the capacitor.
4. 4. If the If the voltage is good, voltage is good, replace U303.
5. 5. If the If the voltage is voltage is still wrong
voltage at pin 6 of U302 is voltage at pin 6 of U302 is a good stable 3V ac.
6. 6. If the voltage If the voltage is good,
C306 is defective. C306 is defective.
7. 7. If the If the voltage is voltage is unstable or w
pin 6 to pin 2 of U302 (use a very short lead to pin 6 to pin 2 of U302 (use a very short lead to
connect the pins, connect t he pins, to preve
reading at pin 6 is now a stable .12V, the feedback reading at pin 6 is now a stable .12V, the feedback
resistors of U302 resistors of U302 are defe
wrong, replace U302. wrong, replace U302.
g. g. If the re If the reading at JM3 ading at JM303 is other th
unstable, replace unstable, replace U301. U301. If the A
inoperative, inoperative, U102 may U102 may be defective
U102, make sure the A/D Co U102, make sure the A/D Controller (U462) is
to paragraph to paragraph 7-B-16 to check the 7-B-16 to check the A/D Controller.
controller is good, replace U102. controller is good, replace U102.
7-B-8. Inaccurate Reading 7-B-8. Inaccurate Reading on All Rang
7-B-9. 7-B-9. Since t Since the 3 he 3478A's 478A's AC to
calibrated with an input voltage at a frequency of 1KHz, calibrated with an input voltage at a frequency of 1KHz,
inaccuracy can result from poor frequency response. inaccuracy can result from poor frequency response.
Since the high frequency is compensated by C302, make Since the high frequency is compensated by C302, make
sure the capacitor sure the capacitor is good. is good. If the capacitor
replacing U301 and if still inaccurate, try U102. replacing U301 and if still inaccurate, try U102.
7-B-10. 7-B-10. Overload, Overload, Inaccurate, Inaccurate, Floating,
Readings on Some Ranges Readings on Some Ranges
7-B-11. 7-B-11. Since all ranging of t Since all ranging of the AC to DC Co
done in the Input Hybrid U102, the hybrid is the most done in the Input Hybrid U102, the hybrid is the most
likely cau likely cause for se for ac failures ac failures on some
replacing U102, make sure it is not set to an incorrect replacing U102, make sure it is not set to an incorrect
mode by the A/D Cont mode by t he A/D Controller (U462). roller (U462). Go to paragraph 7-
B-16 to B-16 to check the A/ check the A/D Controller. D Controller. If the
good, replace U102. good, replace U102.
7-B-12. 7-B-12. AC AC CURRENT CURRENT TROUBLESHOOTING
7-B-13. 7-B-13. Make su Make sure t re the DC
Functions are operating correctly on all ranges, before Functions are operating correctly on all ranges, before
troubleshooting fo troubleshooting for ac current r ac current failures.
7-B-3 to troubleshoot the AC Volts Failures and Service 7-B-3 to troubleshoot the AC Volts Failures and Service
Group A for the Group A for the DC Current DC Current Failures.
stable 3V ac at pin 4 of U303. 4 of U303.
replace U303.
wrong, lift the lift the end of
replace U303.
still wrong, make sure t , make sure the
a good stable 3V ac.
is good, capacitor C304 capacitor C304 or or
unstable or wrong, connect rong, connect
to prevent oscillations) nt oscillations). .
are defective. ctive. If the If the .12V is
03 is other than .12V ac or an .12V ac or
If the AC Volts Function C Volts Function is still
be defective. . Before replacing Before replacing
ntroller (U462) is good. good. Go
A/D Controller. If the
on All Ranges es
AC to DC Conver DC Converter is
If the capacitor is good, try is good, try
Floating, or or Noisy
he AC to DC Converter is
on some ranges. ranges.
Go to paragraph 7-
If the controller is controller is
TROUBLESHOOTING
he DC Current Current and AC and AC Volts
failures. Go to parag Go to paragraph
Failures. If the If the functions
are good, the only component that can cause a failure is are good, the only component that can cause a failure is
the I the Input Hy nput Hybrid U102.
sure the A/D sure the A/D Controller is good
to check the A/D to check
replace U102. replace U102.
end of
7-B-14. OVERLOAD 7-B-14.
TROUBLESHOOTING TROUBLESHOOTING
7-B-15. 7-B-15.
circuitry is in U102. Replace the hybrid, circuitry is in U102.
he
7-B-16. CHECKING THE 7-B-16.
7-B-17. Do the 7-B-17.
Refer to Schematic 3. Refer to Schematic 3.
a. a. Turn the
If the If the
b. b. Move jumpers JM502
"D" position (JM403 pin 1 and 2), as shown "D" position (JM403 pin 1 and 2), as shown in in Figure
.12V is
7-B-1. 7-B-1.
c. c. Obtain
as follows (shown in Figure 7-B-1) as follows (shown in
is still
Start: Start:
Go
St Stop op: :
If the
Cl Cloc ock k : :
Hold: Hold:
Self-Test: Self-Test:
Gnd: Gnd:
ter is
d. d. Turn the 3478A on
signatures. signatures.
U462 pin U462
U462 U462 pin
U462 pin U462
Noisy
e. e. If any sig
D (Flowchart D) for troubleshooting. D (Flowchart D) for troubleshooting.
nverter is
f. f. If the signatur
defective. defective.
Before Before
Volts
raph
functions
7-B-2 7-B-2
brid U102. Before replacing Before replacing U102, make
Controller is good. . Go to paragraph 7-
the A/D Controller. Controller. If the
OVERLOAD PROTECTION PROTECTION CIRCUITRY
All of t All o f the he ac ac functions functions overload
Replace the hybrid, if defective.
CHECKING THE AID CONTROLLER AID CONTROLLER (U462)
Do the following to following to check the A/D check the A/D Controller.
Turn the 3478A off 3478A off. .
Move jumpers JM502, JM503, an , JM503, and JM403 to th
Obtain a a Signature Signature Analyzer. Analyzer. Set
Figure 7-B-1): :
TP401 (JM403 TP401 (JM403 pin pin 6) 6) ( (
TP TP40 402 2 (J (JM4 M403 03 pi pin n 5) 5) ( (
TP TP40 403 3 ( ( JM JM40 403 3 pi pin n 4) 4) ( (
Out Out
Out Out
Ground Pin Ground Pin (next (next to to C203)
Turn the 3478A on and check for the and check for the following
pin 32: 32: 7ACA 7ACA
pin 33: 33: 20F0 20F0
pin 34: 34: 666H 666H
If any signatures are wr natures are wrong, go to Service Group
If the signatures are good, the es are good, the Input Hybrid is
Figure 7-B-1. Figure 7-B-1. JM403 SA JM403 SA Connection
TM 11-6625-3071-14 TM 11-6625-3071-14
U102, make
Go to paragraph 7-B-16 B-16
If the controller is g controller is good, ood,
CIRCUITRY
overload protection protection
if defective.
(U462)
Controller.
d JM403 to the e
Figure
Set and and connect connect
) )
) )
) )
C203)
following
ong, go to Service Group
Input Hybrid is
Connection