Power Output. The RF power amplifier is used to extend the transmission range of the RT The
amplified RF output from the PA is a nominal 50 watts, but may vary from 30 to 70 watts. On the RT, in
the PA setting, a nominal 4 watts is sent from the RT J1 to the PA J2, then amplified and fed to the
antenna via the PA J1. In LO, M, or HI, the output of the RT passes through the PA with no amplification
and is output to the antenna.
RF Power Amplifier Keying Circuit. The PA is keyed by the RT. When the PTT switch on the handset is
pressed, the RT is keyed by placing a ground on pin C of RT AUD/DATA connector. When RF PWR is set
to PA and the RT is keyed, 6.75 V dc is placed on RT, P1, pin H. The presence of 6.75 V dc on PA, P1,
pin N, keys the PA. A power level signal is generated and sent to the RT signal display. All bars of the
signal display light. The RT selects the proper PA filter. Failure of the RT to select the proper band will
cause the PA to fail to transmit.
RF Power Output Filtering. The PA is a linear amplifier across the frequency band of 30 to 87.975 MHz.
The PA has three sets of filters which are selected by the RT.
SELECTOR PIN PA
30.0 to 43.0
43.0 to 61.4
61.4 to 88.0
Voltage Standing Wave Ratio (VSWR). VSWR is the relative value of forward to reverse power, Any of
these faults may cause high reflected power:
Use of an old VRC- 12 antenna or components
Antenna elements broken, dirty or corroded
Lower antenna element connector open
Defective mounting base
Rusty antenna base mounting bolts
Loose or missing ground straps
Wrong type RF cable installed (75 ohm data cable)
Open RF cable grounds
The PA has a VSWR detector used to measure the reflected voltage. It is compared to the forward
voltage to determine the VSWR. When this ratio exceeds 5:1, the PA RF output is automatically
reduced to a safe level. The RF test set can be used to measure RVS PWR. When the 5:1 VSWR is
reached, RT sidetone is disabled (except with the AM-7238B/VRC RF PA).
PA Mount. Some radios use a second PA in a separate mount. The mount is cabled to the VAA, where
internal wiring connects the PA mount to RT-B. Cable CX-13303 is used to provide dc power, and
CX-13291 is used to pass control signals between the RT and PA.