Stabilised Rail - Sinclair Microvision MTV1 Service Manual

Stabilised rail

An 0.5mA current source is generated and fed into a 5-diode chain to produce a stabilised 3.6V
point. Two emitter followers are connected from this point to produce two 2.9V rails, which are used
throughout the circuitry as a stabilised rail and reference voltage. The 2.9V rail has a -Vbe
temperature coefficient.
Line oscillator
The line oscillator produces a sawtooth voltage across an externally connected capacitor. This is
done by switching a reference voltage in conjunction with a current source. Both scan and flyback
times are caried when changing this line oscillator frequency. The oscillator flyback time is 9µs.
The phase comparator uses a long-tailed pair and compares the phase of the sawtooth voltage
from the oscillator with that of the line sync pulse, during the flyback time.
Outputs from the line oscillator are used to produce an AGC gating pulse (which is also used by the
filament driver circuit), a pulse to reset the EHT converter, a line blanking output, and two pulses
that are used by the line deflection circuitry.
Frame oscillator
The frame oscillator charges an externally connected capacitor for a short period (600µs), via a
resistor to a stabilised rail. This charging current is then switched off and the capacitor is allowed to
discharge via a variable resistance (the frame hold pot), connected across the capacitor. The
switching occurs when the voltage across the capacitor is the same as the reference voltage inside
the IC. This reference voltage is switched between two levels. The oscillator is triggered by a frame
sync pulse.
Outputs from the oscillator are used to produce a frame blanking pulse, and two pulses that are
used by the frame deflection circuitry.
Page 20 of 39
Sinclair MTV1 service manual V2.1