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OVERALL  FUNCTIONAL  DESCRIPTION.  Continued
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TM-11-5820-890-30-5 Ground Non-Icom Radio Sets Consisting of: Receiver-Transmitter Radio RT-1439/VRC (NSN 5896-01-195-0827) Manual
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Section  II.  REPAIR  PARTS,  SPECIAL  TOOLS,  TMDE, AND  SUPPORT  EQUIPMENT (cont)


TM 11-5820-890-30-5 5-4. RF SIGNAL PATHS. a. High Power Transmit Path.  When the HIGH POWER XMT control line is at logic 1, the control circuits set the RF switches in the output filter switch (6A1A1) to the XMT position. See figure FO-16. The RF signal is input at connector J2. It passes through E1 to the first RF switch. With the switch as shown in figure FO-16, the signal  is  output  at  E3.  It  travels  through  cables  W1  and  W3  to  the  power  amplifier  heat  sink  (6A3)  at  E1.  The power  amplifier  heat  sink  provides  12  dB  of  RF  gain.  (1  dB  is  lost  in  the  filters  and  switches.)  It  is  a  push-pull amplifier with input and output impedance matching. The output at E5 goes through cables W4 and W2 into the input filter switch at E1. It switches the signal through one of three low-pass filters in filter FL14. The low-pass filter attenuates  any  harmonics  or  spurious  signals  outside  the  filter’s  frequency  range.  The  output  of  the  low-pass filter is routed to the second RF switch and out to connector J1 through the power detector. The power detector measures  the  output  power  level  to  provide  an  ALC  feedback  signal.  It  also  measures  the  reflected  power  to provide  a  VSWR  feedback  signal. b. Low Power Transmit Path and Receive Path. When the RT RF switch is set to LO, M, or Hl, the HIGH POWER XMT input line is held at logic 0. The HIGH POWER XMT line will also be at logic 0 any time the radio is not transmitting. This causes the two electronic RF switches in the output filter switch (6A1A1) to be set to the RCV position. This basically connects connector J1 to connector J2 so that signals can pass between the RT and the antenna. 5-5. FEEDBACK  AND  OUTPUT  SIGNAL  PATHS. The  two  main  feedback  signals  originate  at  the  power  detector  in  the  output  filter  switch  (6A1A1).  See  figure FO-16.  The  forward  power  is  checked  and  is  used  for  the  automatic  level  control  (ALC)  signal.  The  reverse  or reflected power is checked and is used for the VSWR signal. Both of these are 0 to 4 V dc signals and are fed back to the decoder control (6A2). The ALC signal is used to provide the POWER LEVEL signal that goes back to the RT to drive the SIG display. The VSWR and the ALC signals are compared. If the VSWR is greater than 5:1, the SIDETONE  DISABLE  line  is  set  to  6.5  V  dc.  Adjustments  are  provided  on  the  decoder  control  to  adjust  the sidetone  disable  limits. The power amplifier heat sink (6A3) uses the 27 V dc power from the constant current source circuit of 6A1Q1 and 6A1Q2. Most of the current is supplied from Q1 through filter FL1. It is used to regulate the RF gain of the power  amplifier  heat  sink.  The  gain  is  reduced  if  any  of  the  following  occurs: The  RF  power  output  exceeds  its  maximum  limit. The  VSWR  exceeds  5:1. The  temperature  of  the  power  amplifier  exceeds  70°  C  (160°  F). The temperature of the power amplifier is sensed by two thermistors in the power amplifier heat sink. They are connected  in  series  between  E4  and  ground.  The  decoder  control  monitors  the  resistance  at  pin  24  of  J1. 5-3


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