Sierra Video Manzanita 161VS - Manual (Page 6)

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Sierra Video Systems • P.O. Box 2462 • Grass Valley, CA 95945 • (530) 478-1000 • Fax (530) 478-1105
MODEL 161 INSTRUCTION MANUAL
previous state of U8A and U8B. When the con-
trol input is removed the circuit stays in the last
level set at its input. R52 and C13 provide input
protection and noise rejection.
The output of the four latches connect directly to
the video latch/decoder described previously. The
audio switches operate on +/-7V. Q6 is connected
to the junction of U8A and U8F so that Q6's
output is in the same polarity as the video control
output of the latch except that the voltage swing is
+7 to -7V. The latch for the MSB drives two
level shift transistors for the audio because the
audio selectors are two separate 8 by 1's and each
requires a separate enable input.
Power supply. 24VAC center tapped is produced
by an external power transformer. The low
voltage AC enters the module on J3 pins 27 and
28. It is rectified by D6 through D8. C23 and
C24 are the positive and negative primary filters
respectively. U11 produces +12V for the video
circuits and control logic. U9 produces -12V for
the video circuits. U12 produces +12V for the
audio op-amps. R54 and D5 produce +7V for the
audio CMOS crosspoints and their control level
shifters. Similarly U10 and R53/D4 produce
negative voltages for the audio circuits.
LED Control panel.
503138 control panel. The 503138 is a 16 switch
circuit board which uses LED lamps and includes
switch encoding and lamp driver circuits. The
503138 is primarily used with the Model 161, 16
by 1 video plus stereo audio switcher.
Refer to the schematic diagram while reading the
following circuit description. The 16 switches are
connected together in a 4 by 4 matrix configura-
tion. This allows U1, a keypad encoder, to be
used to convert the one-of-sixteen to a four bit
binary code. U1 also provides switch debounce
and key rollover which means that if two buttons
are both selected, only the first switch detected is
encoded until all switches are released. When no
switch is selected the output of U1 is a high
impedance. The latching circuit on the 503135
(the other end of the system), is a 23K ohm source
impedance. This medium impedance holds the
four data lines at either 0 or +12V. When a
switch is pushed the DA output of U1 is inverted
by Q1 and puts a low on the OE input of U1
causing the output of U1 to become a low imped-
ance. This forces the 503135 logic to follow and
hold the selected four bit code.
The lamp circuit of the 503138 consists of U2, a
4 line to 16 line decoder. U2 operates on +5V
while the control lines swing from 0 to +12V.
R1/R2 (typical) attenuate the 12V swing to 5V.
The outputs of U2 drive the 16 LED's in the
switches. R12 limits the current in the LED's to
about 20MA.
The 503138 is power with unregulated DC. U3
provides regulated +12V for U1. U4 provides
+5V for U2 and the LED's.