Rectifier A Nd Power Storage Ci Rcuit; Tr Igger Circu It; Auto Ci Rcu It And Bounce Compe Nsa Tor Ci Rcuit; Quench Circuit - Vivitar 283 Service Manual

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2.5
Automatic Electronic Flash
Model 283
When 01 stops functioning, C3 begins to discharge via R4, T1 and D2, lowering the positive potential on
the anode of SCR 1. When this potential drops to approximately 1.2 volts, the SCR switches off, forward
bias is reestablished, and 01 oscillates once more .
RECTIFIER AND POWER STORAGE CIRCUIT
The diodes D3 and D4 rectify the voltage produced by the oscillator and transformer T1, and the power
is stored in the main capacitor C1.
The pulsating DC produced by the oscillator is stepped up by T1. The diodes D3 and D4 chop the nega-
tive excursions off the resultant AC, that is, these two power rectifiers allow current flow in one direction
only. The main capacitor C1 charges up to the preset voltage. When an AC power source is used to operate
the unit, such as the Vivitar SB-4 cord, 200VDC is applied to this circuit at the junction between D3 and
D4. D3 blocks T1 from this high voltage, while D4, in conjunction with C1, raises the potential to approx-
imately 320V.
2.6 TRIGGER CIRCUIT
When C1 is charged, the trigger capacitor C7 is also charged to the same potential via R 19 and R 12. When
the synch or open flash contacts are closed, via the open flash button, camera shutter, or PC cord, C7 is
connected via D5 across the primary of T2. The discharge of C7 produces a momentary high voltage pulse
in the secondary of T2, which ionizes the gas in the flashtube, providing a path for C1 to discharge across
the flashtube. C6 and R 13 increase the trigger efficiency of this circuit. Before the xenon tube can fully
ioni ze and permit C1 to discharge through it, SCR2 must be gated in the following manner.
The trigger pulse applied from T2 secondary to the flashtube ionizes the gas in the xenon tube FT1. This
causes the 300V potential of C1 to appear at the anode of SCR2 and the anode of DB. Current flows
through DB, C11, L2, C 13, R 17 and R 1 B, and the voltage pulse appearing at the junction of R 17 and R 1 B
is applied to the gate of SCR2. This causes SCR2 to conduct, and the main capacitor now discharges across
FT1, through SCR2 anode to cathode, and back to C1 lower plate. In manual mode, C1 fully discharges
and SCR2 reverts to "open" circuit condition upon deionization of FT1. For auto function, see Section 2.B,
Quench Circuit.
2.7 AUTO CIRCUIT AND BOUNCE COMPENSATOR CIRCUIT (BCC)
2.8
The light from the flash, reflected by the subject, strikes the photo-transistor 04, causing
a
current change
(increase) through R20, this results in a voltage change across R20 and R21, and the base of 03 becomes
negative. Capacitor C16 integrates the voltage across R20, which upon reaching the required potential, drives
03 into forward bias. Collector current now passes through R23 and R24. The voltage developed at the
junction of R23 and R24 is applied to the gate of SCR3, which, on reaching +0.6V, switches SCR3 on.
When this occurs, C19 discharges through SCR3, which acts like a low resistance, through the primary
winding of T3, back to the opposite plate of the capacitor. T3 will now have a high voltage induced into its
secondary winding providing the trigger pulse necessary to fire OT1.
The bounce compensator circuit is automatically switched on when the unit is set at any of the three bounce
angle positions. It increases the light output to compensate for light loss due to reflection by approximately
one-half f/stop. This is accomplished by C15 and C16 being connected in parallel, thereby increasing the
time constant of the auto circuit.
QUENCH CIRCUIT
When the quench tube OT1 fires, capacitor C11 discharges through it. With FT1 firing, SCR2 is also con-
ducting, C11 discharge current passes through SCR2 on its return to C11 left hand plate. This current flows
through SCR2 in the opposite direction of that coming through FT1 and causes SCR2 to switch off ex·
tinguishing the xenon tube FT1.
Vivitar 37460598 (1 1/80)
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