Cathode Corner SCTV, Инструкция по сборке и эксплуатации

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SCTV Clock Assembly Manual 1/24/22 6
Theory of Operation
Power Supply
The power supply is of the switching type. 12V DC power is switched through a high-frequency
transformer to produce the necessary operating voltages. The power supply uses a unique topology to
drive the CRT and the logic circuits.
The most common types of switching power supplies are flyback and forward converters. A forward
converter uses the power driven when the primary switch is conducting, and multiplies that voltage by
the turns ratio. A flyback converter stores energy in the transformer while the switch is on, then transfers
that energy to the secondaries when the switch turns off.
This supply is both of these types in one - its outputs are all fed through voltage doublers, so that both
halves of the cycle are used. This is done to allow the voltage multiplier for the high voltage negative
supply to be regulated as well as the lower-voltage supplies.
The reason is that a forward converter develops a secondary voltage proportional to the turns ratio,
whereas a flyback converter develops a secondary voltage proportional to the duty cycle. These two
functions are different with regard to load, so a regulator for one will not be well regulated the other
way.
The switching IC is a National Semiconductor LM2586. It interrupts the current flowing through the
primary winding of T1 at a rate of 100 Kilohertz, and controls the duty cycle of this interruption to
provide regulated voltage outputs.
Moving on to the regulator, the deflection voltage is the one actually regulated. The other voltages
follow this voltage in proportion to their turns ratios. The regulator samples the deflection voltage
through a resistive divider.
The low-voltage supply provides -5V to the deflection amplifiers. The +5V supply is provided by a
linear regulator, because the switching supply doesn’t start up with a solid 5V to bring up the Teensy
computer reliably. This was an engineering change after problems wee found in a few of the kits sent to
early customers.
The filament supply is straight AC, since that heats up a wire as well as DC does. An AC-coupled
connection to the filament supply feeds a doubler which powers the optoisolator used for the modulation
signal. This optoisolator is a special logic-level photodiode unit with sub-microsecond switching time.
The Z modulation supply provides enough voltage to switch the grid on and off (~60VDC) and enough
current to do so quickly. A simple transistor switch controls the grid.
CRT Beam circuits
The CRT requires the correct DC voltages at all of its electrodes in order to form a bright yet small spot
on the screen. The cathode may be considered as the starting point of this system.
The grid requires a negative voltage relative to the cathode. This voltage is adjusted by the Intensity
control.
Blanking is active whenever the beam is being moved from one location to another. The computer tells
the blanking circuit when to blank the beam. Blanking is accomplished by switching the grid to a much
more negative voltage by the Z modulation circuit to cut off the beam.