Display Circuit Board - Crown FM600 User Manual

4.7 Display Circuit Board

The front-panel LEDs, the numeric display, the slide switches, and the processing and RF
level controls are mounted on the display circuit board. To access the component side of
the board, remove the front panel by removing 12 screws. The board contains circuits for
the digital panel meter, modulation peak detector, and LED display drivers, as well as indi-
cators and switches mentioned above.
Illustration 6–9 and accompanying schematic complement this discussion.
Left and right audio from input stages of the audio processor board (just after the Input Gain
attenuator) go to the L VU and R VU input on the display board. Peak rectifiers U1A and
U1B drive the left and right Audio Input displays. The LED driver gives a 3–dB per step dis-
play. The lowest step of the display driver is not used; rather a red LOW indicator lights
when audio is below the level of the second step. Transistors Q1 and Q2 divert current from
the LOW LEDs when any other LED of the display is lit.
Resolution of the linear displays, High Band, Wide Band, and Modulation, has been im-
proved using dither enhancement. With dither, the brightness of the LED is controlled by
proximity of the input voltage relative to its voltage threshold. The effect is a smooth transi-
tion from step to step as input voltage is changed. U6A, U6B, and associated components
comprise the dither generator. Dither output is a triangular wave.
Composite stereo (or mono) is full-wave detected by diodes D5 and D6, U7, U13, Q3, and
Q4 are components of a peak sample-and-hold circuit.
Oscillator, U9F, supplies a low-frequency square wave to the Fault indicators, causing them
to flash on and off.
Digital multimeter inputs are selected with push buttons located to the right of the multimeter
menu. Signals from the push buttons are conditioned by U9A and U9B. U10 is an up/down
counter. Binary input to U11 from U10 selects a green menu indicator light, and lights the
appropriate decimal point on the numeric readout. The binary lines also go to analog data
selectors on the ALC/ metering board.
Processing control, R50, is part of the audio processor. (See section 4.2.)
The DPM IN and DPM REF lines are analog and reference voltage inputs to digital multime-
ter IC U12. They originate from analog data selectors on the ALC/ metering board.
4-10 FM600 User's Manual