HP TM 11-6625-2779-14&P - Manual (Page 34)

Section IV Model 3555B
4-35. Now consider the peak detection. (See Figure
7-5) When A3Q14 is turned on and A3Q17 is turned off,
no current flows through the meter from the peak
detector When A3Q14 is turned off and A3Q17 is turned
on, the current path is as shown by the heavy lines in
Figure 4-7 Diodes A3CR12 and A3CR13 are included to
offset the junction drop of A3CR15 and A3CR16
respectively.
4-36.
When the average detection and the peak
detector are combined in the proper proportion, an
equivalent rms response is produced. The advantage of
this type of rms detection is fast response.
4-37.
POWER SUPPLY AND SERIES REGULATOR
(Schematic No. 4)
4-38.
The 3555B can be operated from 115V or 230V
ac, the internal 48V dry cell battery or from a central
office battery (tip negative). When operating from an ac
source power is applied through transformer TI and the
AC/BAT switch S1 to the rectifier CR1. This rectified
voltage is filtered by C2 before being applied to the
series regulator through J2, S3, CR1 and cable W1.
4-39.
The regulator is of the conventional series type
with A3Q19 acting as the sensing element and A3CR20
as the reference. Changes in the output level are
amplified by differential amplifier A3Q18 and A3Q19.
The output of the differential amplifier is amplified by
A3Q20 and applied to A3Q21 which controls the
conduction of the series transistor A3Q22. The output
of this series regulator is held at - 20 volts
±
1 volt. The
maximum ac ripple and noise on the output voltage is
200
µ
V rms.
4-40. It should be noted that when operating the set
from either the battery or from an ac source, capacitor
C2 will always be charged whether the set is turned on
or not. Caution should be exercised when servicing the
power supply.
4-8