Sierra Video Manzanita 161VS - Manual (Page 4)

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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
off the two transistors in the signal path are both
off and the diode between them is on. This
provides very high attenuation (over 100db) of an
off inputs signal through the crosspoint. When a
crosspoint is on, both transistors in the signal path
are on and the diode between them is on. Thus
the desired video signal is allowed to pass on to
the common bus. The control of the crosspoints is
from the output of a 1 of 16 decoder. One output
of the decoder is always high, which is +12 Volts.
The decoder is powered by +12 V and -2 V,
which means the 15 outputs that are not at +12V
are at -2V.
When crosspoint 1 is off the control side of R4 is
at -2V. Because the anode of D1 is connected to
ground and D1 is in series with R4, their junction
will be at about -0.7V and D1 will be on. The
average voltage at the base of Q1 is 0.0V while its
emitter is at -0.7V. This causes the base emitters
junction of Q1 to be reverse biased and therefore
present a high impedance to the incoming signal
while D1 is a low impedance to the signal. This
produces a fairly high attenuation of the video
signal. Q2, as will be explained later, is also off
and further isolates the input signal when the
crosspoint is in the off state.
To turn on crosspoint one, the first output of the
control decoder changes from -2V to +12V. This
causes D1 to now be reverse biased and become a
high impedance. With 0.0V at the base of Q1 and
a positive voltage on R4, Q1 becomes an emitter
follower, buffering the input signal and raising its
DC level to +0.7V. Q2 is another emitter fol-
lower. R13 is its emitter resistor. Q2 lowers the
incoming signal level DC by the same amount it
was raised by Q1. This means the signal DC level
at the common bus of the output of the 16
crosspoints is approximately 0.0V. For an off
crosspoint where D1 is forward biased to -0.7V
the emitter base junction of Q2 is also reverse
biased by 0.7V. In this way Q2 is used in an off
crosspoint as the second series off switch, attenu-
ating the input signal.
Control panels.
The local control panel if ordered with the unit
will be installed at the factory in place of the
blank front cover. If it is installed later, remove
the blank cover, connect one end of the short
ribbon cable with 9 pin D miniature connector to
the mating connector on the front edge of the
Model 161 circuit board. Attach the other end of
the ribbon cable to the D connector on the rear of
the control panel. Being careful not to pinch the
ribbon cable, slide the local control panel between
the top and bottom cover until it stops against the
two magnets at each end of the frame.
The remote control panel if ordered is a separate 1
3/4" high rack mount unit plus separate intercon-
nect cable. After mounting the remote panel at
the desired location connect one end on the cable
to the connector labeled "REMOTE CONTROL"
at the rear of the Model 161 frame. Connect the
other end to the connector on the rear of the
remote panel.
CIRCUIT DESCRIPTIONS
Switching electronics.
The electronics for the Model 161 are all on a
single plug in module, the 503135. Refer to the
schematic diagram while reading the following
circuit description. The drawing is divided into 3
pages by circuit function. The following text is
organized the same way.
Video circuits (sheet 1 of 3 of the schematic
diagram). The 503135 has 16 identical video
crosspoints. The same component designators
and values are used for all 16 crosspoints. Only
one crosspoint is described here. Video input 1
enters the 503135 on edge connector J1 pin 1.
From there it is AC coupled through C1. C1
protects the circuitry of the 503135 from possible
damage if the incoming video signal were to have
an excessive DC component on it. R1, after C1,
provides a ground reference to the AC coupled
signal. The video crosspoint which follows is a
"T" circuit configuration. When a crossspoint is