FILAMENT CIRCUIT, CIRCUIT FUNCTION DESCRIPTION 3.0
Refer to figure 2-4. The primary purpose of the filament circuit is to provide a
constant current source for the filament transformers as determined by the selected ma
a. From the constant-voltage transformer (CVT) on lead XSP through fuse F12
to XSPP, 120vac flows to the transformer which is acting as an impedance transformer.
The voltage drop across this transformer varies as the amount of conduction of the
MJ15001 transistor in the cooling chimney changes. The diode bridge on the
secondary of the impedance transformer acts as a variable resistor load. When the
MJ15001 transistor is off and not conducting, the diode bridge acts as an open circuit
and the impedance in the primary winding is high. When the MJ15001 transistor is
conducting, the diode bridge acts a variable load resulting in a low impedance in the
primary with the current being inversely proportional to the impedance.
b. The 115vac supply to the filament drive transformer primary comes from the
circuit breakers. The secondary of this transformer produces 40vac, center tapped, to
diode bridge BR-1 where it is rectified, regulated to positive (+) 15vdc by 7815 regulator
and to negative (-) 15vdc by regulator 7915. The unregulated positive and regulated dc
voltage is also used to supply the power to the amplifying transistors for the cooling
chimney driving circuit.
c. A feedback transformer is used to provide a current feedback source to
sample the amount of current in the filament transformer drive line. This feedback is
rectified by BR-2 and is input into pins 5 and 6 of IC1. Pin 7 (TP10) of IC1 will be a
positive dc voltage which is representative of the amount of current feedback.
d. On lead XDA (TP11) from the filament selection is a voltage representing the
ma station selected. This is the filament set and is input to IC1 at pin 3. Pin 1 of IC1
(TP9) will now output a negative dc voltage which represents the ma station selected.
e. A dc voltage representative of the space charge is on lead XCM from the
space charge circuit. This voltage is negative voltage below 78kvp and a positive
voltage above 78kvp. At 78kvp that voltage is zero volts.
f. At TP14 is the filament-drive feedback voltage which is a sample of the
filament drive voltage. It is this voltage that determines the amount of conduction of the
cooling-chimney transistors and ultimately the amount of impedance of the transformer