HP 740B Operating And Service Manual page 63

Model 740B
Table 5-12. REFERENCE LOOP TROUBLESHOOTING
Section V
1.
2.
3.
4.
5.
6.
7.
If +5.9 V is present at Al TP1, the Reference
Supply is generating a voltage.
If +12 V ±O. 6 V is present at A2TP7, Series
Regulator (pi
0
A2) is functioning.
+12 V at A2TP7 shews that the Series Regulator
(pi
0
A2) is probably not at fault.
Correct indications show that the Oven Heater
and Regulator circuits are working correctly.
If no voltage is present at A2TP7 (step 2) the
Series Regulator (pi
0
A2) is probably at fault.
See Figure 7-13.
+5.9 V at Al pin 21 shows that the Control
Differential Amplifier (pi oA13) is probably not
at fault.
Since +12 V is not present at A2TP7, the Series
Regulator (PI oA2) may be at fault. Remove A2
and check for +34 V at pin 22 of A2 jack. If
voltage is not present, troubleshoot +34 V supply,
pi
0
A6 (Figure 7 -10). If +34 V is present,
troubleshoot Series Regulator, pi
0
A2 (Figure
7-13).
13. VOLTAGE SET and STANDARD VERNIER con-
trols must be set to first position (0) to make
this check. If resistance is 1 kQ, S4 and S4R5
are probably not at fault.
14. A correct indication shows that the Temperature
Sensing Bridge is probably not at fault.
15. If Heater is bad, entire A13 assembly mustbe
replaced. Components in A13 are not separately
replaceable.
16. If the instrument passes all of the previous
checks, the Reference Loop is probably notat
fault.
17. Trouble in one of the VOLTAGE SET Switches
will probably show up only when that switch is
varied. Once the trouble is isolated, check the
SWitching associated with the malfunctioning
switch position (Figure 7-14). If one of the
switches is completely inoperative, trouble-
shoot corresponding SENSITIVITY Switch.
8. +1 Vat S4E (F-6) shows that the VOLTAGE SET
Dividers are receiving a correct input from the
Reference Supply.
9. A correct reading shows that the Oven Heater
is probably not at fault.
10. Check for open A1R40.
11. The Control Differential Amplifier (pi oA13) is
receiving a correct input (step 6). Remove Al
and measure voltage at pin 21 of Al jack. If
voltage is between +5.9 V and +6.4 V, trouble
may be in VOLTAGE SET Dividers. Proceed
to step 13. If voltage does not measure between
+5.9 V and +6.4 V, the Control Differential
Amplifier
(p/oA13)
is not working. A13 must
be replaced. (Components in A13 are not
separately replaceable. )
12. Correct indications show that the VOLTAGE
SET and STANDARD VERNIER Dividers are
functioning properly.
01794-1
18. Check for open, shorted or changed-value S3R1.
19. To further isolate the trouble, select
t.
VM and
measure resistance from S4E (F-6) toW (S2A
pin 10).If resistance now measures 1kQ,trouble
is in STD mode FUNCTION Switching. Probable
trouble area: deck SlI(F) of RangelFunction
Switch, Sl (Figure 7 -3). If trouble remains,
troubleshoot S4.
20. Since the Temperature Sensing Bridge is working,
the trouble is probably in the Error Amplifier
or Heater Control (Figure 7 -13).
21. Remove A2 and measure voltage at pin 16 of A2
jack. If approximately -4 V is present, the
Error Amplifier (pi
0
A2) or Heater Control is
probably at fault. If approximately -4 V is
not present, the Temperature Sensing Bridge
is not working. Entire A13 assembly must be
replaced since individual components inA13 are
not separately replaceable.
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