(1) The auxiliary power supply for the converter is composed of Q507,
Q508, Q509, and the associated components. Diode CR506 and filter network
R542-C523 supply unregulated dc voltage to series pass transistor Q507. In
the 887AB, regulated dc voltage is supplied from a set of batteries, BT2, in
the BAT CHECK and BAT OPR modes.
The emitter voltage of Q509 is set by
Zener diode CR506. The base input of Q509 is taken from a divider string,
R538, R539, and R540, which samples the output voltage of the -18-volt
supply. Any variation in the -18 volt varies the base drive of Q509, which
varies the output from the collector load of Q509. Since the output of Q509
drives Q508, the collector current of Q508 continuously adjusts the
conductance of Q507 to maintain the auxiliary voltage constant at the value
determined by the setting of R539.
(2) All ac measurements are made by first converting the ac input
voltage into a dc voltage.
The converter provides a dc output of 1 volt
when full range voltage is applied to the 887A in each ac range.
In the
1-volt AC range, the AC-DC polarity switch and the range switch connect the
input binding posts directly to the converter input. The converter gain is
of such a value that the DC output voltage is equal to the RMS value of the
converter input voltage for a sine wave. For the 1000-, 100-, and 10-volt
ac ranges, a separate input attenuator for each range reduces the unknown ac
voltage by a factor of 1000, 100, and 10 respectively. The operation of the
converter is then the same as for the 1-volt range.
Thus, an output of
1 volt dc is provided for full range input of a pure sine wave on any ac
range.
(3) The converter gain is adjusted with R503 in the feedback loop of
the operational amplifier. Capacitor C502 in the feedback loop is used to
adjust the high frequency response of the converter. The attenuation of the
1000-, 100-, and 10-volt attenuators are adjusted with R410, R406, and R403
respectively. Capacitors C405, C403, and C401 are used to adjust the high
frequency response attenuators.
The bias of field-effect transistor Q501
should require adjustment with R508 only when Q501 or a component in its
drain-source is replaced.
The amplifier output level at the collector of
Q506 should require adjustment with R522 only if Q505, Q506, or a component
in one of these stages is replaced.
o. The AC-DC polarity (fig 5-15 (fo)) switch is provided for selecting
either the ac or dc mode of operation. When the AC-DC polarity switch is
set to ac, the ac to dc converter is switched into the circuit by sections
S4A, S4B, S4c, and S4d. Also, sections S4H and S4G are used to switch 1.1
volts dc to the Kelvin-Varley divider.
Section S4E is used to provide
proper attenuation in the TVM attenuator.
p. For the dc mode of operation, the AC-DC polarity switch may be set to
the positive or the negative dc position. The polarity switch reverses the
transistorized voltmeter-reference voltage combination with respect to the
input. Note that a 0.01 uf capacitor (C1) is connected from the COMMON post
to the chassis ground post to reduce the effect of ac circulating currents.
If the instrument did not contain a polarity switch, the grounded side of any
unknown voltage that is negative with respect to ground would have to be
connected to the INPUT post. This would ground the INPUT post and effectively
place C1 across the input. With this capacitance connected across the circuit
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