4.6 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 indicators 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 display.
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 improved
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 transition 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 ap-
propriate decimal point on the numeric readout. The binary lines also go to analog data selec-
tors 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 multimeter
IC U12. They originate from analog data selectors on the ALC/ metering board.
4-10 FM600 User’s Manual
To Next Page
Loading...
