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Troubleshooting - HP 3456A Operating And Service Manual

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Service
put signal magnitude plus the offset value, hence the re­
quirement for that additional dynamic range.
8-95. Slope Amplifier U406.
inverting amplifier with a gain of 100 for the low level
signals near zero volts. This amplifier increases the sen­
sitivity of those signals to more accurately detect zero­
crossing. Diodes CR402 and CR403 clamp the output of
±
U406 to
.6 V to prevent saturation. This speeds up
the slope conversion process. The output of the slope
amplifier is fed to the U409b comparator stage and to
Q408 in the AID autozero circuit.
8-96. Zero Comparator (U409b).
has an open collector output (HCP-High Compare)
that is either
5 V or zero volts. This stage compares
+
the integrator's output to ground. The comparator's
output is off when its positive input terminal (pin 7) is
above ground. The open collector output rises (via pull­
+
up resistor R430) to
5 V at this time. U409's output
changes state immediately
signal at pin 7 goes negative.
The AID autozero should not be
8-97. AID Autozero.
confused with the Autozero that can be enabled/disabl­
ed with the front panel Autozero bullon. Autozero
compensates
for
Input
autozero, which canot be switched off, occurs between
every AID Converter measurement cycle to compensate
+
for any
12 V reference errors (see Paragraph 8-79).
8-98. The output of slope amplifier U406 appears at the
source of the AID autozero FET switch (Q408). The
autozero capcitor C4 1 1 , charges to the voltage level ap­
pearing at the output of U406 when Q40S is closed. The
voltage stored on C411 is fed to Q407a and b, a divide
by 30 amplifier. The output of Q407a and b is applied to
the integrator stage during the next measurement.
8-99. Jumpers JMPR 401, 402, 403, 506, and 507 are
provided for troubleshooting aids if a power supply is
loaded down. JMPR's 401 and 402 are located directly
underneath the U500 fine-line package. JMPR's 506
and 507 are located in the 12 V reference section adja­
cent to JMPR 403.
8- 100. The voltage divider, made up of R431 and R423,
+
provides the
2 V reference for the comparators.
Voltage divider R432 and R433 provide a voltage source
for the 5 V logic signature analysis (SA) equipment
when it is used to check the 12 volt logic in the AID
Converter.
8-101. 1 2 VOLT REFERENCE_
8·102. General.
8-103. The 3456's ability to make precise measurements
depends on the long term and short term stability of the
reference supplies. The reference supplies consist of
8-16
The slope amplifier is an
Comparator U409b
( +
10
5 V
zero volts) when the
Amplifier
offsets.
AID
those 500 series numbered components in the small
reference section on the A20 board and two other com­
ponents (variable resistor R614 and Switch S60I) that
are physically located in the calibration section.
8-104. Two types of plug-in reference boards are
available. These boards (A24 and A25) are inter­
changeable (with resistor change) and provide the short
and long-term stability required.
8-105. The fine-line resistors (U500-dotted box
around resistors) are accurate, temperature-tracking,
matched resistors. The two reference boards require a
different resistor between pins 1 7 and 19 of the fine-line
resistor package. Consequently the fine-line package
must be changed if the reference board is changed from
an A24 to an A25 and vice versa. Refer to schematics 5A
and 5B (Reference Board Schematics) and Figure C3 in
Appendix C for details. The fine-line resistor packages
and the reference boards are not field serviceable.
8·106. Circuit Description.
8-107. As shown on the Simplified Schematic S-27,
- 12 V a is used only in those circuits requiring a very
stable and precise - 12 volts. The - 12 V b is used for
the less critical stages of the AID Converter and as a
reference for the AC Converter. The
good short term stability.
8-IOS. The zener diode voltage reference on both the
A24 (U503) and A25 (U500) boards have internal
heaters to provide as near constant a temperature as
possible for output voltage stability. U503 on the A24
board has a zener output voltage (pin 3 of US03)
specification range of 5.82-6.06 volts. U500 on the A25
board has a zener output voltage (pin 2 of U500)
specification range of 6.S-7.1 volts.
8-109. Buffer Q501 lowers the output impedance of op
amp U501 so that transients generated by the AID
CMOS circuits will not cause regulation problems. The
base of Q501 should be approximately - 12.7 V (output
of U501) which is reduced to - 1 2 V by the base-emitter
junction of Q501.
S-IIO. The effective feed-back resistance established by
fine�line U500, J26, J27, )28, S60I, and R614 provide
the range needed for the course and fine adjustment of
the - 12 V a reference voltage. The jumpers associated
with J26, )27, and )28 are positioned for either - 12 V
or ground for the initial rough calibration. These
jumpers should not be changed unless a reference
board, fine-line package, or QSOI is changed. A 1 6 posi­
tion rotary switch (S601) and a trimpot (R614) are used
for the final front panel calibration. S601 (screwdriver
adjustable) is shown in its "0" position but will normal·
ly be positioned between 6 and 8 on the dial during
calibration. This initial calibration position should be
adequate for a 5-10 year period. Refer to Figure 25 for a
pin configuration and truth table for S60 1 .
Model 3456A
12 V source has
+
J

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