HP 740B Operating And Service Manual page 59

Model 740B
Table 5-10. MAIN LOOP TROUBLESHOOTING (Cont'd)
Section V
r
r
r
53. The Operational Filter is producing a correct
output to the Differential Amplifier (step 3) but
the Differential Amplifier is not generating a
correct output (step 41). Troubleshootthe Dif-
ferential Amplifier, p/o A4 (Figure 7 -8).
54. Correct indications verify proper Range switching
operation on the 10 mV and 1 mV range.
55. A correct indication (but excessive response
time) shows that range switching for the Main
Loop Feedback Divider is operating.
56. Since the instrument is capable of delivering
maximum output current at low voltages (step
49) the trouble is probably in the Internal Cur-
rent Limit circuit, p/o A7 (Figure 7-11).
57. Since output current is insufficient at both high
and low voltages (steps 38 and 49) the trouble
is probably in the External Current Limit cir-
cuit (Figures 7 -3 and 7 -9).
58. If the instrument passes all of the preceding
checks, the Main Loop is functioning properly.
01794-1
59. Troub leshoot Range switching for the inoperative
range (Figure 7 -3).
60. Excessive response time but a correct final in-
dication (step 55) may be caused by incorrect
mV Range switching for the Low Level Ampli-
fier. Check for a short between A3 pin 21 and
A3 pin 17. If short is present, troubleshoot
Deck SIB (F) of Range/Function Switch, SI (Fig-
ure 7-3).
61. If problem disappears when jumper wire is con-
nected, the Main Loop Feedback Divider switching
is not at fault.
62. If problem disappears when jumper wire is con-
nected' there is probably a bad switch contact
between AI7Vl/V2 and pin 1 (Figure 7 -3).
Check for continuity between these points on the
mV ranges (VM and
~VM
modes).
63. Since trouble is not caused by Range/Function
switching between A17 and A3 (step 61), the
trouble may be m V Range SWitching associated
with the Main Loop Feedback Divider (Figure
7-3).
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