FM30 Series Technical Manual Page 23
10.5FMTXDIS
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The FMTXDIS PCB performs three tasks:- The basic power supply function, a central
point for the interconnection of the other PCB's and indicators and the VSWR cut-back
system.
The AC output from the mains transformer is connected to the bridge rectifier. The
output from the bridge rectifier enters the FMTXDIS board at connector CN1 pins 1 and
2. It is smoothed by C1. The external DC input enters the PCB at connector CN1 pins
3 and 4, reverse voltage protection is provided by D1. The mains derived DC supply is
monitored by TR1 and its associated components. Should this supply drop below about
20V the relay RL1 will be switched off selecting the external DC input. D3 ensures that
power is drawn from the mains derived DC supply if the external supply is lower or
absent. D3 also takes the load current during the relay change-over time.
From this point this unregulated supply feeds the FMTX-PSU via connector CN7 pins 1
and 2, the front panel meter via R2 and connector CN3 pin 3 and IC1 which is a 7818
voltage stabiliser. The output from IC1 supplies most of the FMTXDIS board electronics
and the PLL7 boards via connector CN4 pin 6. IC2 receives a 15V supply from the PLL7
boards via connector CN4 pin 5.
Transistors TR2, TR3 and TR4 provide the PLL LOCK LED drive and the external PLL
LOCK OK indications. TR2 is turned off when the PLL is in lock. This in turn will turn
on TR3 and TR4.
The forward power sensed voltage enters the board via connector CN5 pin 2. It is
buffered and amplified by IC2b. The meter is driven via RV3. IC4 is operated as a
comparator with its threshold set by RV4. Transistor TR9 drives the external forward
power OK indication. Transistors TR10 and TR11 drive the front panel P FWD LED.
When the external mute is applied, the cathode of D9 is grounded, allowing TR11 to
switch off. This causes the P FWD LED to show orange.
The reflected power sensed voltage enters the board via connector CN5 pin 1. It is
buffered and amplified by IC2a. The meter is driven via RV1. C4 is charged by R11,
R12 and D5. This provides a rapid peak hold with a slower decay. IC3 further amplifies
this voltage. The gain of this stage (and therefore the maximum amount of cut-back) is
set by RV2. The output of IC3 drives the control input of the FMTX-PSU unit via LK1,
TR5, D6, R17 and pin 3 of connector CN5. Feeding a positive current into this control
input reduces the power supply output and therefore the RF output power and the
consequent reflected power. The output power potentiometer is connected between
pins 7 and 8 of connector CN2. The purpose of TR5 is to turn on TR6 as soon as the
cut-back system stars to operate. TR6 drives the red part of the P REF LED and turns
on TR7 which will turn off TR8.
(49 pages)
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