Troubleshooting guide

Cinematronics Vector Monitor Repair Guide v.1.0
Page 8 of 53
Edge Gain Amplifier
It is a property of the CRT that, near the edges of the screen, the amount of current needed to move
the beam, say, an inch, is less than the amount required to deflect the beam an inch off center.
Therefore, less gain is required near the edges of the CRT.
The edge gain amplifier is the final stage before the voltage-to-current conversion process. The
input on pin 3 of IC102 (or IC202) is a waveform that is the composite initial position and final
position data signals. The amplifier has an overall gain of about 2x at output pin 6. The waveform
should resemble the DAC output, but is now bounded between +/- 2 volts, centered about 0 volts.
The TL081 op-amp at IC102 serves a dual purpose: it acts as a buffer between the deflection
amplifiers and the analog switch, as well as an "edge gain" amplifier (i.e., height).
At the output of IC102, there is a resistor diode network consisting of R105, R106, R107, R108 and
D101, D102, D103, D104. This resistor diode network is used to compensate for the non-linear
characteristics of the CRT near the edges of the screen. If this circuit were not used, any object
displayed on the screen would increase in size as it moved closer to the edges of the screen.
The amplifier circuit reduces gain as the output voltage increases, indicating a larger beam
displacement, by switching on diodes D101 and D102 for the upper half of the screen, and D103
and D104 for the lower half. This places R106 and R107 in parallel with R105, reducing its value
and consequently the gain. If a figure increases in size near the display edges, a diode in this
section is open.
Potentiometer R109 is the vertical picture size control. By picking off larger or smaller voltages,
the height of the picture is controlled.
EDGE GAIN AMPLIFIER CIRCUIT