HP 740B Operating And Service Manual page 57

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Model 740B
Table 5-10. MAIN LOOP TROUBLESHOOTING
Section V
L
1. A correct voltage at the Output terminals shows
that the Main Loop is functioning.
2. The Loop Gain Check is given in Paragraph 5-76.
3. > +0.9 V atA4 pin 8 verifies that the Low Volt-
age Section of the Main Loop is functioning.
4. A correct output shows that the Main Loop is
relatively noise -free.
5. Connecting a jumper wire from A17V2 to
W
creates a 1 V difference between the Main Loop
input voltage (+ 1 V from the Reference Loop)
and the feedback voltage (0 V). If the Modulator
output exceeds O. 5 V P - p, the Modulator is
probably not at fault.
6. If the OVERLOAD Light is on, the Low Level
Amplifier (A3) is probably functioning.
7. > -7 mV present at A4 pin 21 indicates thatthe
Operational Filter is receiving a drive voltage.
8. If output zeroes, the ZERO Control Circuit is
working properly.
9. If noise decreases when jumper wire is con-
nected to A4 pin 8, the circuits between the
output of the Operational Filter and the Output
terminals are probably not causing noise.
10. A waveshape > 20 V
P -
P
at A17V4 verifies
proper operation of the Demodulator (A17V3/V4).
11. Troubleshoot the Modulator (A17V1/V2) according
to the procedure in Paragraph 5-80.
12. If the DC Fuse is not blown, the Power Switch
transistors (Q1 and Q2) are probably not at
fault.
13. Since the OVERLOAD Light is not on, the Main
Loop is probably stable. Since no voltage (or
an incorrect voltage) is present at the Output
terminals, the Output Rotary Switch, K1, is
likely at fault (Figure 7 -3).
14. The Operational Filter (p/o A4) is receiving a
sufficient input (> -7 mY) but not producing a
sufficient output. Troubleshoot Operational
Filter (Figure 7 -8).
15. If the OVERLOAD Light is on, the Low Level
Amplifier (A3) is producing an output and probably
is not at fault.
16. A 10 V output verifies proper operation of the
feedback switching and filtering (10 V range) in
the High Voltage Section of the Main Loop.
01794-1
17. Check the voltage of A10BT1 (1. 3 V minimum).
IfA10BT1 isgood, check the other ZERO con-
trol Circuit components (Figure 7 -3).
18. Since noise decreased when the Operational
Filter output was connected to
W
(step 9), the
noise source is between the Main Loop input and
the Operatioml Filter output
(A4
pin 8). Noise
at A4 pin 8 should be about 1% the amplitude of
the noise at A4 pin 21 if the Operational Filter
is functioning. (Observe noise with oscilloscope).
If the Operational Filter is functioning, the trouble
is probably in the Low Level Amplifier, A3
(Figure 7-7), or A17. Paragraph 5-80 gives
the procedure for troubleshooting A17.
19. If noise increases by a factor of ten on each in-
creasing range, the 1 V Filter is probably not
at fault.
20. The Modulator and Demodulator are functioning
(steps 5 and 10) but the Low Voltage Section of
the Main Loop has insufficient gain (step 2).The
trouble is probably low gain in the Low Level
Amplifier, A3 (Figure 7-7).
21. A waveshape > 25 V p- pat A3TP10 shows that
the Low Level Amplifier is producing a good
output when driven to saturation.
22. A waveshape > 15 V p -p and> 5
Jlsec
duration
shows that the Power Switch (Q1, Q2) and Power
Switch Driver (p/o A7) are functioning.
23. If fuse does not blow again, the instrument may
be working properly. Blown fuse may have been
caused by an excessive overload at the Output
terminals.
24. The Low Level Amplifier is producing a good
output (step 15) but the Operational Filter is not
receiving a correct input (step 7). The Demod-
ulator (A17V3/V4) is probably atfault. Trouble-
shoot the Demodulator according to the pro-
cedure in Paragraph 5-80.
25. Connecting the jumper wire from A17V2 to
W
creates a 1 V difference between the Main Loop
input voltage (1 V) and the feeci>ack voltage (0 V).
If the Modulator output exceeds O. 5 V p - p, the
Modulator is probably working correctly.
26. If the instrument provides correct output volt-
ages on the 100 V and 1000 V ranges, range
switching for these ranges is functioning pro-
perly.
27. The Main Loop provided a correct output on the
1 V range (step 1) so the trouble is probably in
one of the Feedback Dividers, (A9R4 - 12 and
SlRll - 14), Range switching, or the 10 V Filter,
A5C5 (Figure 7-3).
5-21