Theory Of Operation (Detailed Functional Description); Introduction; Radio Power; General - Motorola Handie-Talkie MTX Series Theory/Troubleshooting Manual

THEORY OF OPERATION (DETAILED FUNCTIONAL DESCRIPTION)

I.

INTRODUCTION

In this section of the of the manual, a more detailed
description of the radio and some special circuit is given.
For a better understanding of the circuits descriptions,
and to aid in following the text, refer to the applicable
schematic diagram(s) in the corresponding service man-
ual (Motorola part number 68P81200C25), or previously
68P81200C20.
II. RADIO POWER
A. General
As previously described in the THEORY OF OPER-
ATION (BASIC FUNCTIONAL DESCRIPTION) RADIO
POWER paragraph, power is distributed to four general
combinations of transmitters and controllers:
1. VHF/UHF transceiver with closed architecture
controller,
2. VHF/UHF transceiver with open architecture
controller,
3. 800/900MHz transceiver with closed architecture
controller, and
4. 800/900MHz transceiver with open architecture
controller
Discussing each of the four combinations would be
somewhat redundant, so pairs 1 and 4 were chosen for
explanation in the following paragraphs. Paragraph B
covers the vhf/uhf transceiver and the closed architec-
ture controller; paragraph C covers the 800/900MHz
transceiver and the open architecture controller.
B. B+ Routing and DC Voltage Distribution
(for a Closed Architecture Controller and a
VHF or UHF Transceiver)
Raw B+ (7.5V) from the battery (Batt B+) enters the
radio on the transceiver board through a 3-contact
spring pin arrangement (P404) as B+, where it is routed
directly to the RF PA Module and ALC IC pin 13. Battery
B+ is fused, and then routed through the jumper flex
(P704, pins1 and 20) to the controller board (J704, pins
1 and 20). The B+ supply is routed through the con-
troller board to the on/off/volume control (S403/ R401)
on the controls flex at jack J703, pin 8. With the
mechanical on/off switch (S403) placed in the on posi-
tion, switched B+ (SB+) is routed from the controls flex
at connector plug P703, pin 10 and applied to the con-
troller at connector jack J703, pin10. This signal is also
fed to a resistive divider R708, R709 so that the micro-
computer (U705) can monitor the battery voltage.
The SB+ voltage powers the audio PA (U706) and
its internal 5V regulator booster transistor (Q702). It also
powers a discrete 5V regulator (U709). Regulated 5
volts from module U709 powers the microcomputer
10
(U705) and other digital circuitry. The ASF IC (U701)
obtains its 5V (Vcc) from the AUDIO PA internal 5V reg-
ulator through a booster transistor (Q702)
The switched B+ voltage supplies power to circuits
on the transceiver board. The 5-volt regulator, U202, is
applied this voltage through decoupling component
C125 to produce a stable 5.0 volt output. Raw B+ (7.5V)
which is connected to the ALC IC (U101), is switched
through the output (CATH1) to another 5-volt regulator
(U103).
Regulator U202 supplies those circuits which need
to remain on at all times, such as the reference oscillator
(U203), fractional-N-synthesizer (U204), D/A IC (U102),
and the IF module (U3). The D/A IC controls dc switch-
ing of the transceiver board. The SC1 signal at U102 pin
12 controls transistors Q107, Q104, and the transmit
5 volts (T5). The SC3 signal at U102 pin 14 controls
transistor Q105, and the receive 5 volts (R5). A voltage
on the synthesizer SOUT line at U204 pin 19 supplies
power (Vcc) to the VCO buffer at U201 pin 3.
During the receive mode, via switching transistor
Q105, regulator U103 supplies regulated 5V (R5) to the
receiver front end. In the battery-saver mode, R5 can be
switched on and off by controlling pin 1 of transistor
Q105. Module U103 is not used during the transmit
mode. During the transmit mode, transmit 5 volts (T5)
for the ALC IC and other TX circuitry is obtained from
U202 via switching transistor Q104.
1. Low-Battery Detect Circuit (Controller Board )
The low-battery detect circuit generates an audio
alert when the radio's battery needs recharging. The
implementation of this function takes advantage of
the microcomputer's on-chip, 8-bit, 8-channel, A/D
converter, U705 pins PE0-PE7. The 7.5V (SB+) is
divided down to a nominal 3.92V by resistors R708
and R709, and fed to port PE4 of U705. This volt-
age is converted by the A/D converter to a digital
format. The microcomputer compares this voltage to
a preset low-battery trip threshold, which corre-
sponds to a battery voltage of
~ =
or 6.2V in transmit. If the measured voltage is
lower than either threshold, the low battery alert
tone is generated (if option is enabled) to warn the
user that approximately 20 minutes of usable bat-
tery capacity remains.
2. Power for External Accessories
Via current limiting resistor R733 and associated
isolation and protection components VR715,
VR720, and C709, SB+ is available on the controller
board at connector jack J701 pin 16. From the con-
troller board, SB+ is routed through the front-cover
flex (P701 pin 16 to J403 pin 4) and applied to to
the universal connector at P403 pin 4 as OPT B+.
~ =
7.0V in standby