HP 740B Operating And Service Manual page 62

Section V
REFERENCE LOOP TROUBLESHOOTING TREE
NOTES
1. All voltage measurements made with respect to
W.
Function Switch must be set to STD.
2. Reference Loop schematic and component loca-
tion diagrams are shown In FIgures 7-13 and
7-14.
3. A13, Oven Assembly, Is not field reparable.
2. + 12 V ",0.11 V present at
A2TP7.
PASSES
t
1. Select STD, 1 V Range.
+
5. 9 V
present at AITPI (AITP7 on
Instruments below serial No.
1110-003711).
FAILS
3. + 12 V present at A2TP7.
740B-0-30019
Figure 5-17. Reference Loop Troubleshooting
Tree
5-26
Model 740B
4. ApprOl<lmately - 23 Vdc
present at A2 pin 18 when
Instrument
Is
!Irst turned
on--decreases to apprOl<I.
mately -13 V after warmup.
5. Troubleshoot series Regu-
lator (A2Q14 and QI5).
II. + 5. 9 V pre8ent at Al pin 21.
7. Troubldhoot Serle8
Regu-
lator (A2Q14 and QI5).
01794-1
8. + 1 V pre8ent at S4E(l' -II)
(whlte/red/blue wire on
Xl Divider).
9. Remove A13 aDd m_re
real8tance from AU pin 11
to A13 pin 12. Re8lMance
apprOl<lmately 115
O.
10. TrodIle1ihoot Referecce
Adjualmenia (p/o Al)o.
11. A13 Control DIfferential
Ampllfler bad. Replace
A13.
12. Select Ii VM; Input Shorted·;
1000 V Range. Successively
depre8s Xl thru xl04
SENSITIVITY buttons whUe
varying correspoDding
VOLTAGE SET from 0 to 9
aDd back to O. Meier deflecls
from 0 to -0.9 and back to 0
on each check.
13. Remove AI. Resistance
from S4E
(F -6)
(whitel red!
blue Wire) to
W
Is 1 kn.
14. Replace A13. Voltage at A2
pin III appl'Oltlmalely - 4 V
when Instrument 18 flrat
turned on.
15. A13 Heater bad. Replace
A13.
Ill. Reference Loop working
properly.
17. Troubleshoot corresponding
VOLTAGE SET and SENSI-
TIVITY Switching.
18. Trow>leshoot SSRI.
19. Troubleshoot S4.
20. Troubleshoot Error Ampli-
fier (A2Q12 and Q13) and
Heater Control (Q3).
21. AU Temperature Senaing
Bridge bad. Replace AU.