Rev. 1.0
FMX30/FMX100/FMX250 User’s Manual
4—8
The transmitter is controlled from a single, push-and-turn control knob. LEDs
illuminate the ring around the knob to display start-up, on-air or fault conditions.
The motherboard is the large board in the upper chassis interconnecting the audio
processor, stereo generator, RF exciter, and metering boards. The motherboard
provides the interconnections for these boards, eliminating the need for a wiring
harness, and provides input/output filtering.
With Normal-Bypass slide switch, it is possible to bypass the audio processor,
connecting the left and right audio inputs directly to the inputs of the stereo
generator.
If the audio source is already processed, and further processing is not desired, use
the Normal mode instead of Bypass and set processing to “0”.
If it is necessary to provide resistive terminations at the audio inputs (either line-to-
line or line-to-ground), you may place resistors directly into the 8–pin DIP socket,
A501, located between the XLR input connectors.
The voltage regulator board is the longer of two boards mounted under the chassis
toward the front of the unit. The switch-mode voltage regulator pr12, –12,
and 24 volts to the transmitter. It also contains the program detection and
automatic carrier control circuits.
The 38–kHz sine wave from the stereo generator is used for a synchronization pulse.
In the transmitter “T” version, synchronization is not used.
The 24–volt supply is a switching regulator running at about 35 kHz. Output
voltage is controlled with a MOSFET switch.
A single integrated circuit forms the switching regulator for both +12 volts and –12
volts. It operates at about 52 kHz.
The program detection circuit discriminates between normal program material and
white noise (such as might be present from a studio-transmitter link during
program failure) or silence. If program audio is present, a count-down timer is
continuously reset. Red and green LEDs on the board indicate the presence or
absence of program determined by the balance of the detected signals from the two
filters. The time between a program fault and shutdown is selected by jumpering
pins on header JP701.
Caution
In the BYPASS position, the pre-emphasis circuits and the filter
that protect the pilot and stereo subcarrier are bypassed. As a
result, the occupied bandwidth specifications of the transmitter
could be compromised. The 15-Hz high-pass filters are also
bypassed which may mean that modulation with frequencies
below 10 Hz could cause the frequency synthesizer to unlock.