Onwa KR-1338 Service Manual page 20

Modulator
PRINCIPLE OF FET SWITCHING MODULATOR
High voltage is charged into C through R while the magnetron is inactive.
When the trigger is applied to the power MOS-FET, the FET turns on and the high
voltage appears at the primary winding of the pulse transformer. This transformer
boots the voltage, which makes the magnetron oscillate.
One advantage of this method is that the magnetron oscillates only when the FET
is conducive, that is the transmission pulsewidth can be changed by the TX trigger
pulsewidth. Therefore, parts such as relay and coil can be eliminated.
HIGH VOLTAGE
MODULATOR BOARD MOD-0904
The main function of the modulator section is to produce high voltage pulses to
drive the magnetron. To produce these pulse, the MODULATOR Board has a
modulator trigger circuit, modulator pulse generator and booster pulse transformer.
The modulator trigger circuit consists of U805 and associated components. This
circuit generates the pulses which cause modulation FETs Q805,Q806 to conduct.
The pulses are produced when the TX TRIG pulses from the display unit is recei-
ved and U805 conducts. The voltage of the pulses is raised at pulse transformer
T802 until it is 3.5 kV. This circuit adjusts the electrical curent flowing into the
magnetron so it is 3 A.
The MODULATOR Board also contains the TX high voltage circuit and the mag-
netron heater circuit. The TX high voltage circuit charges capacitors with 300 V
high voltage produced at the primary windings of T801 and discharges them once
the TX TRIG pulse is received. The magnetron heater circuit produces stable +7.5V.
16
R
PULSE TRANS
Discharge
Charge
C
D
TRIGGER
Modulator section simple block diagram
MAGNETRON
MOST-FET