Transceiver; Table 1 Control Head Jumper Table; Table 2 Serial Data Bus Logic States - Motorola Spectra Service Manual

• H/L - (#7 key) used to select/enable radio external
alarms.
• Mon - (#8 key) used to monitor for channel traffic.
• Dir - (#9 key) used for talkaround (mobile-to-
mobile) conversations.
• Rcl - (* key) used to review modes in scan and
phone lists.
• Del - (# key) used when editing stored lists.
B. Theory Overview
The pushbutton ON/OFF switch (S1) or rotary
switch (on rotary control heads) applies battery power to
the control head, transceiver, and accessory connector
via the SW B+ line. Jumpers JU1 and JU2 on the control
head board are used to enable the IGNITION feature.
JU1 is normally out and JU2 is normally in. This allows
complete control over Transmit/Receive functions
shown in Table 1.
Table 1. Control Head Jumper Table**
Customer Options JU1
1. RX & TX enabled OUT
via on/off switch on
control head
2. RX & TX enabled OUT
via ignition switch
3. RX enabled via on/off
off switch on control
head; TX enabled
via ignition switch
* Ignition cable is a part of the cabling which plugs into the rear
accessory connector (P2) on the back of the radio. The ignition
cable must be connected to Pin P2-5 for Table 1 to be valid.
** Refer to Detailed Service Manual board overlays for jumper
location.
Switched B+ is applied to a voltage regulator (U45),
which supplies regulated 5 Vdc to the microcomputer
(U1) circuitry, the interface circuits, the vacuum fluores-
cent (VF) driver (U101), and to the comparator (U3).
The microcomputer (U1) communicates with the
radio command board via the serial bus, scans the con-
trol head keyboard for activity, and generates the
display information for the VF driver (U101) and VF dis-
play (U102). U1 uses a 4 MHz ceramic resonator (Y1)
for its clock source.
When switched B+ is initially applied, the RESET
line to pin U1-1 is held low for approximately 120 mil-
liseconds before it goes high. The high resets U1 and
starts it operating properly. A resultant signal at pin U1-
38 provides a 1kHz tickle pulse to the watchdog timer
circuit in the serial input/output IC (U4), and provides a
strobe signal to the VF display driver (U101). If the tick-
le pulse disappears for more than 120 milliseconds, the
reset sequence is initiated and pin U4-9 goes high.
8
JU2 Comments
OUT Connect ignition cable*
to "Battery Plus (+)"
terminal.
IN Connect ignition cable*
to "Ignition Switch."
IN OUT Connect ignition cable*
to "Ignition Switch."
Table 2 illustrates the serial data bus logic states.
Table 2. Serial Data Bus Logic States
No
Data Line Bus Activity
(Normal)
Busy Low
Busy in High
Busy out High
Bus + High
Bus — Low
TX DATA High
RX DATA High
RESET Low
RESET High
* Data will be sent (bus activity) immediately after a reset.
** Sending only.
The VF display is an eight-character, fourteen-seg-
ment, alphanumeric device. The anodes and the grids
operate at approximately 34 Vdc when ON and 0 Vdc
when OFF. The filament operates at approximately 2.4
Vac rms.
The voltage for the VF Display is generated by a
fixed-frequency, variable duty-cycle, controlled-flyback
voltage converter. The switching frequency is approxi-
mately 210 kHz.
Microcomputer line U1-6 controls the voltage
converter, which provides approximately 37 Vdc to the
VF driver (U101-1) and approximately 2.4 Vac rms
across T60-4 and T60-6 to the VF display (U102-1 and
U102-25).
IV. TRANSCEIVER
A. Receiver Front End
The receiver front end consists of a preselector, a
mixer circuit, and an injection filter.
The mixer is designed for low-side injection. The
receiver injection (first local oscillator) comes from the
doubler output of the VCO assembly (825-831 MHz) via
a coax cable. The injection filter is a printed pattern on
the mixer hybrid substrate and is laser-tuned at the fac-
tory. The output of the filter is connected to the mixer
device.
The preselector is a fixed-tuned ceramic-block filter
that is soldered to the mounting bracket underneath the
mixer hybrid. The receive signal (935-941 MHz) is fed
to the preselector from the antenna switch in the PA.
The signal is then sent up to the mixer hybrid where it is
connected to the mixer transistor (Q9126). The receive
injection is also fed to this point. The mixer output is at
the first IF center frequency of 109.65 MHz. Typical
conversion gain for the entire front-end assembly is 7.5
dB. This signal is sent to the first IF on the RF board via
a coaxial cable.
Bus Activity
(Sending or In
Receiving Data) Reset*
High Low
Low High
Low** High
Low and High High
High and Low Low
Low and High** High
Low and High High
Low High
High Low