HP 740B Operating And Service Manual page 58

Section V
Table5-10. MAIN LOOP TROUBLESHOOTING (Cont'd)
Model 740B
28. Since noise increases by a factor of ten on each
increasing range, filtering for each range is
probably not at fault. The noise is probably
originating in the Pulse Width Converter, plo
A5 (Figure 7-9) or the Differential Amplifier,
plo A7 (Figure 7-11).
29. Filtering for the 1 V range is probably causing
the trouble. Check for bad A5C4 (Figure 7-9).
30. The Demodulator receives a correct input from
A3 (step 21) but produces a low output (step 10).
Troubleshoot the Demodulator (A17v3/v 4) ac-
cording to the procedure in Paragraph 5-80.
31. The Low LevelAmplifier (A3) does not produce
a satisfactory output when driven to saturation
(step 21). Troubleshoot the Low Level Ampli-
fier (Figure 7-7).
32. The Power Switch circuits are functioning (step
22) but no voltage (or an incorrect voltage) is
present at the Output terminals (step 1). The
trouble is probably in Range Switching or Fil-
tering (Figure 7-3).
33.
If
waveshape at A5 pin 19 is greater than 20
/lsec duration, the Pulse Width Converter (p/o
A5) is furnishing T3 with a proper input.
34. Connect negative lead of ohmmeter to
W
or
W;
positive lead to collectors of Q1 and Q2 (blue
wires). If resistance measures
>
75 il, Q1 and
Q2 are probably working.
35. Check voltage at Output terminals.
36. The Modulator is apparently working (step 25)
but the Low Level Amplifier is not turning on
the Overload Light (step 15). Troubleshoot Low
Level Amplifier (Figure 7-7).
37. If + 1 V is present at A17V1, the Modulator is
receiving a correct input voltage from the Re-
ference Loop.
38. A correct output voltage shows that the Main
Loop can deliver maximum rated output power
(20 W), verifying proper operation
of
the In-
ternal and External Current Limit circuits.
39. If the 1000 V range is the only range that mal-
functions, the range switching probably is not at
fault.
40. The Pulse Width Converter is producing a cor-
rect output (step 33)but the Power Switch is not
producing proper pulses (step 22). The trouble
is probably in T3 or the Power Switch Driver
(p/o A7). Refer to Figure 7-11.
41.
<
+8 V at A5 pin 20 verifies that the Differential
Amplifier (p/o A4) is furnishing a correct Out-
put to the Pulse Width Converter.
5-22
42. Since fuse blows (step 23) and Q1 and Q2 are not
shorted (step 34), the Power Switch Driver,
plo A7, is probably at fault (Figure 7-3). Pos-
sible trouble: shorted A7Q1.
43. Replace bad Q1 or Q2. Figure 7-1, Detail D
shows the assembly sequence for Q1, Q2 and
attaching hardware. A layer of silicon grease
should be applied to both sides of the mica in-
sulator (MP45) before reassembly.
44. Verify operation in VM and AVM modes. If
operating, there is probably no other malfunc-
tion.
If
a problem occurs in VM or AVM, per-
form procedure in Front Panel Troubleshooting
Tree (Figure 5-14).
45. Since the instrument does not operate, a problem
still exists in the High Voltage Section. Proceed
to step 22.
46. The Modulator is receiving a correct input volt-
age from the Reference Loop (step 37) but does
not produce a good output (step 25). Trouble-
shoot the Modulator according to the procedure
in Paragraph 5-80.
47. The absence of +1 V at A17V1 may be caused
by trouble in the Input Filter: L2, C6, C7 (Fig-
ure 7-3). Check for open L2 or shorted C6 or
C7. If Filter is not at fault, the Reference Loop
is not generating + 1 V. Follow procedure in
Reference Loop Troubleshooting Tree (Figure
5-17).
48. A correct indication shows that the 100 mV
Range switching is operative.
49.
If
the instrument provides a correct output (1 V
across 20
il),
the External Current Limit cir-
cuit is probably not at fault.
50. Marginal gain in the Operational Filter (p/o A4)
will probably show up only on the 1000 V Range.
Using an oscilloscope, compare the noise amp-
litude atA4 pin 8 with the amplitude at A4pin 21.
Waveform at pin 8 should be approximately 1%
the amplitude at pin 21 if Operational Filter
has sufficient gain. If Operational Filter ap-
pears to be functioning, troubleshoot 1000 V
Range switching (Figure 7-3) and Differential
Amplifier (Figure 7 -8).
51. If the trouble also occurs on the 100 V range.
filtering or switching for the bad range(s) is
probably at fault. Troubleshoot C3 and Range
switching (Figure 7-3) and 100 V Filter. A5C6
(Figure 7 -9).
52.
The Pulse Width Converter (p/o A5) is receiving
a correct input (step 41) but is not generating a
correct output (step 33). Troubleshoot the Pulse
Width Converter and 20 kHz Clock (Figure 7 -9).
01794-1