HP 5384A Service Manual

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8-182. HP 5385A Channel B
8-183. In Figure 8-51, input channel B of the HP
5385A has a fuse in the input BNC connector. When
the fuse opens, an error message appears on the
display of the counter. A Fuse Blown Detector is used
to send the information to the microcomputer. A de
current is passed through R31, R66, L3 and the fuse to
ground. If the fuse is open, the voltage will rise on pin
13
of U18A which causes a high output on U18A. The
high output instructs the microcomputer to output a
message on the display of the counter.
8-184. In the input attenuation circuit, there is a
constant voltage across zener diode CR30. The
current path from CR30 flows through R26, CR4 and
CR5, CR7 and R27 to ground. A second current path
sourced from pin 8 of U18B flows through CR6 and
also through R27 to ground. With a small input signal
the voltage on the anode is high enough to force
enough current through CR6 to drop the diode
resistance. With low resistance through CR61ow level
input signals pass through to U4 pin 1. Also, with low
resistance
on CR6, less voltage is dropped across it so
the voltage across R27 to ground is higher. The higher
voltage at R27 means less voltage drop across CR4, 5
and 7, so less current passes through them. With less
current through the diodes their resistance is high,
therefore input signals are steered toward CR6.
8-185.
The limiter consists of CR24 and CR25. They
are a pair of back to back diodes that turn on when the
signal is about 1.5V or greater.
8-186. The input amplifier consists of three stages;
U4, U 5
and U16. Together they provide approximately
20 dB of gain from 90 MHz to 1 GHz.
8-187.
The signal passes through C62 and goes to the
1/256 frequency divider (U19, pin 10) and the signal
detector (CR29) (the AGC feedback path). When
there is no input signal present, the frequency divider
oscillates at about 950 MHz. With a signal present, the
output is 1/256th of the input frequency.
8-188. In the signal detector, the detector diode
CR29 is biased by a temperature compensated de
reference voltage. The de voltage reference is ob-
tained from between R97, CR26,R35 and R33. The
signal goes through C63 to the detector diode CR29
and blocked by L 15. The signal is rectified by CR29
and filtered to a de level by C64. The filtered de level
adds to and raises the de bias level on the cathode of
CR29.
The detector is adjusted by R34 for the proper
detection level.
HP 5384A
and HP 5385A
Service
8-189.
The feedback is routed through U17A, U18D
and U18B. A high feedback level causes the output
voltage on pin 8 of U18B to drop. When the voltage
drops the current through CR6 drops, thereby
increasing the resistance of CR6 and attenuating the
signal passing through. With more voltage dropped
across CR6, less voltage is dropped across R27.
Therefore the voltage across CR4,5 and 7 is increased.
The increased voltage causes an increase in current
flow through the diodes. The increased current
decreases the diode's resistance, thereby conducting
more signal to ground through C16. R23 and R24 help
maintain 50 ohms input impedence.
8-190. The feedback is then passed through a switch
U17C
to amplifier U18D (pin 2). Trigger circuit U18D is
used so that noise will not activate the amplifier U18B.
8-191.
The manual level/ AGC switch U17, changes
the input of the AGC Amplifier from the detected
voltage to the manually controlled voltage. U17
switches when the microcomputer sends a low to pins
9, 10, and 11 of U17.
8-192. The AGC amplifier U18B,
completes the
feedback path to the attenuator. U18B amplifies the
reference voltage from U18D or the manual setting,
depending on the setting of U17.
8-193. 3870 Microcomputer
8-194. The 3870 microcomputer U2, runs at an
internal clock rate of approximately 3.5 MHz as set by
L4. It continually cycles through programmed rou-
tines, which are stored in ROM within the IC. The
block of front panel switches are routinely polled via
the Switch Status bus and the status is stored in
internal RAM. Any change in the status is detected
and verified. A new function switch. contact must
remain stable for 32 scans to be accepted.
8-195. The 3870 controls the operation of the MRC
via
the Address/Control bus. Address lines RGO and
RG1 select one of four internal registers of the MRC.
NSO, NS1, and NS2 are nibble select lines (nibble= 4
bits)
which address data within the various registers.
8-196. Command line R/W (Read/Write) controls
the direction of data transfer on the bidirectional
Data/Control Bus.
8-197.
The Data/Strobe bus contains four bidirec-
tional data lines, DO-D3, and the command line STR.
STR indicates an address or command is valid.
8-25

HP 5384A Specifications

General

Brand	HP
Model	5384A
Category	Cash Counter
Language	English