HP 428B, Operating And Service Manual, Page: 25 / 52

Model 428B
Figure 5-7. Detector Phase Display.
5-53. Preliminary Adjustment.
5-54. Preset the controls as follows:
a. Ground the bases of QI and Q2 (TP1 and TP2).
Open the feedback disconnect (see Figure 7-3).
a. Switch the 428B to the 300 mA range.
b. Adjust R84 for minimum ac at the wiper
connection of R84. The reading should be less
than 0.1 volt.
c. Adjust R82 for minimum ac at the wiper
connection of R8·2. The reading should be
approximately 0.02 volt.
d. Thoroughly clean probe head jaws (see Paragraph
5-8).
e. Degauss probe head (see Paragraph 3-29).
5-55. Drive Balance Adjustment.
5-56. With the 428B set to the 1 mA range, and with no
input to the current probe, set R98 for a minimum ac
reading at Test Point 3 (Pin 7 of V2). The minimum
reading should be less than 0.5 volt.
5-57. Residual 40 kHz Cancellation.
5-58. The residual output of the probe head is
cancelled by means of the circuit discussed under
Paragraph 4-45. This procedure assumes that the
preliminary adjustments listed in Paragraph 5-54 have
been completed.
a. Reconnect the feedback disconnect which
was opened in Paragraph 5-54 Step a.
b. Remove the short between the bases of Q1
and Q2 and ground (installed in Paragraph
5-54 Step a).
c. Degauss the probe.
d. Zero the 428B meter with front panel ZERO
control.
e. Adjust R84 for a minimum ac reading at Test
Point 3 (Pin 7 of V2).
f. Continue to alternate Steps d and e until the
adjustment cannot be further refined. The
minimum reading in Step e should be
approximately 0.02 volts or less.
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