Oscillator; Start Up - Sony Mavica MVC-FD85 Training Manual

Oscillator

The main backlight oscillator parts are bipolar transistor Q860 and trans-
former T851. Resistors R866 and R867 provide bias to the bases of
Q860. The collectors of Q860 feed 17Vp-p to T851. When this section
receives voltage, it oscillates at about 92kHz. The frequency is not as
important as the voltage that is produced by T851. With an input voltage
of 17Vp-p, T851's output is 750Vp-p, more than enough to start the tube
glowing.

Start Up

Concept
As the ND901 tube gets older, more voltage is required to trigger the tube
into starting, especially at low ambient (surrounding) light levels and/or
low temperatures. To insure reliable starts (and therefore extend the tube's
operating life), an infrared LED is placed next to the tube to increase the
photon activity within the tube when the backlight is powered up. By in-
creasing the photon activity, less voltage is required to start the tube,
offsetting the aging process to a degree.
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Circuit
When the backlight is powered ON, 4.9V is applied via Q861 to R873 to
turn on the infrared start LED D901. When the tube lights up, current is
drawn through ground resistor R872 causing a small voltage of 3.2Vp-p.
This voltage is rectified into DC by D853 and detected by IC852. IC852
turns off Q861 to disconnect voltage to the D901 start LED. Therefore
the infrared start LED is ON only prior to the ND901 tube lighting.