Ivy Biomedical Systems 101NR, Service Manual

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THEORY OF OPERATION
The vertical raster portion of the display is accomplished by turning the crt beam rapidly on and off as the beam
travels vertically from top to bottom of the display at a frequency of about 16 kHz. At the same time that the beam
is rapidly traveling vertically, it is also traveling in the horizontal direction with a trace time of about 4 ms for the
left side of the crt. The size of the raster display on the right side of the crt differs and, depending on what is
displayed, can take from 1.2 ms to approximately 3 ms. Waveform traces are displayed using stroke writing. Stroke
writing is used because it offers a smooth, continuous, high resolution display. Stroke writing entails turning on the
crt beam and varying the y-axis (vertical) deflection of the beam while the beam travels horizontally across the x
axis. It takes about 2.8 ms to draw one complete waveform trace.
Timer Circuit
The Timer Circuit creates a waveform to be used by the high voltage module in the creation of the high voltages
needed by the crt. It starts with the VERT SYNC input.
The VERT SYNC input is a 16 kHz square wave with a duty cycle of 56% that is created on the Mother Board. The
main purpose of this signal is to synchronize the video circuits on the Mother Board while the crt displays rasters. It
is a steady square wave that can be used as a clock.
The Timer Circuit inverts the VERT SYNC signal with a transistor and runs it to a monostable multivibrator (one-
shot timer). The one-shot changes the duty cycle of the inverted waveform from 44% to between 12% and 60%
(varied by adjusting the EFF potentiometer). The ability to change the duration of this signal allows the operation
of the high voltage module to be fine tuned. A small duty cycle means less current to the high voltage module
(longer battery life), but it also means lower display intensity during battery operation.
From the output of the one-shot, the signal is buffered by a power MOSFET and sent to the high voltage module.
High Voltage Module
The High Voltage Module contains a circuit board that produces several relatively high voltages to be used in the
control of the crt. That circuit board is the Power Module assembly, Part Number 5001-00-XX. More information
about the Power Module is included in a section about that board. For now, be aware that when the High Voltage
Module is supplied with a 16 kHz square wave and the feedback amplifier is working, it produces three voltages: -
100 V, +275 V, and 6,500 V.
Feedback Amplifier
The Feedback Amplifier takes a feedback signal from the High Voltage Module, inverts it, compares it to a voltage
reference (the HV potentiometer), and sends it back to the High Voltage Module. It is used to keep the 6500 volt
output of the High Voltage Module within tolerances. When the 6500 volt output is within tolerance, the -100 and
+275 volt outputs should also be within tolerance.
Model 101R/NR Service Manual 27