HP 428B, Operating And Service Manual, Page: 24 / 52

Model 428B
5-33. Remove the clip lead and resistor. Replace the
feedback disconnect jumper.
5-34. ALIGNMENT.
5-35. OSCILLATOR BALANCE.
5-36. Construct a probe as shown below:
Figure 5-5. Oscillator Balance Probe.
Connect the ends of the probe to the T5 side of R94
and R95. Connect an ac voltmeter between ground
and the junction of the two probe resistors. To adjust
oscillator balance, pad C53 or C54 for minimum
reading (on instruments with serial prefixes of 601- or
below, adjust the slug of T5). The ac reading at the
junction of the probe resistors should be 50 m V or less.
Remove the probe and meter before proceeding.
5-37. OSCILLATOR FREQUENCY.
5-38. Connect an ac coupled electronic counter to test
point 4. Adjust the slug of T5 to set the frequency
measured to 40 kHz ± 200 Hz. If T5 doesn't have
enough range, pad C52 (in instruments with serial
prefixes of 601- or below do not use T5; use C52 only).
Remove counter.
5-39. OSCILLATOR LEVEL.
5-40. Connect an ac voltmeter to Test Point 6 (Pin 7,
V7). Adjust R92 for 8 ±0.4 V. Remove meter.
5-41. DETECTOR GATE.
5-42. Connect an ac voltmeter to Test Point 4 (Pin 7 of
V5). Adjust L7 for a peak reading.
5-43. TUNED AMPLIFIER.
5-44. Equipment Setup.
5-45. Set an ac voltmeter to the 1 volt range and
connect it to Test Point 3 (Pin 7, V2) of the 428B. Clip
the 428B probe around a wire which is carrying 35 mA
rms ac. This current can be generated by a filament
transformer, as in Figure 5-6, or by an oscillator
operating into its rated load as in Paragraph 5-20. Set
the 428B RANGE switch to 100 mA. This arrangement
will be used for input, interstage, and detector phase
adjustments.
Figure 5-6. Detector Phase Adjustment.
5-46. Input Alignment.
5-47. Short TP1 and TP2 to ground, open the
feedback disconnect shown in Figure 7-3, then adjust
C2 for a maximum ac reading at Test Point 3 (Pin 7 of
V2).
5-48. Interstage Alignment.
5-49. Set up equipment as in Paragraph 5-45, then:
a. Connect a .0082 microfarad capacitor across the
brass pins which are connected to L10. Adjust L9 for
maximum ac at Test Point 3 (Pin 7 of V2).
b. Remove the capacitor. Connect the .0082
microfarad capacitor across the brass pins which are
connected to L9. Adjust L10 for maximum ac at TP3.
c. Remove the capacitor and voltmeter.
5-50. DETECTOR PHASE ADJUSTMENT.
5-51. With the bases of Q1 and Q2 (TP1 and TP2)
grounded, open the feedback disconnect jumper (see
Figure 7 -3), and install a 390 ohm resistor between
the feedback wire and the pin on the circuit board to
which it normally connects. Connect the horizontal
input of an oscilloscope to TP4 (Pin 7, V5). Connect
the vertical to TP3 (Pin 7, V2). Except for the ac
voltmeter, set up equipment as in Paragraph 5-45 (a
proper setup is shown in Figure 5-6). Turn slug of L7
until the pattern looks like one of the correct patterns
shown in Figure 5-7. If there is a diamond in the center
of the pattern, the diamond should contain no traces.
Remove the 390 ohm resistor and the voltmeter.
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