21
Explanation for circuit operation
When B+ 135V is supplied and +5V is applied, approx. 31KV high voltage and 11.6KV focus
voltage are generated automatically. If EHT of HVG is outputted more than 35KV, HVG
module starts over voltage protection to stop the high voltage.
5) Spot killer components
Approx. -220V voltage made by using HVG high voltage output voltage is supplied from HVG
PIN4, rectified and smoothed to D819 and C425 and used as spot killer power. After this
power is divided into R448 and R449 normally, a voltage near to -20V via R465 is supplied
for CRT board GRID 1 via connector PA13 G1. But, at first AC power ON, the spot killer
signal supplied via a connector PA15M in main board is provided for the base of Q412. At
AC power OFF, a voltage near to -220V via Q414 is provided for GRID 1 of CRT board via
G1 of connector PA16M. As a result, the screen gets dark to prohibit the generation of a
spot.
6) Dynamic focus voltage generator
The dynamic focus voltage provides a parabola waveform conforming to CRT spec.
(horizontal:1000V, vertical:200V) for CRT to make the focus of 7” CRT optimum. A D/F
waveform from 1403 pin32 is amplified approx. 100 times in an amplifying circuit composed
of OP AMP and a parabola waveform is amplified approx. 1000Vp-p in an amplifying circuit
composed of QF11, 12, 13. Then, they are input to QF14 base. The horizontal waveform of
1300Vp-p left in T402 terminal No.16 is rectified and smoothed to DF13, 12, RF17, CF11, 12
and supplied for QF14 collector as B+ for dynamic focus.
7) ABL input part
The ABL current supplied from main board via the ABCL terminal of a connector PA401
flows in via FBT PIN2. Then, the current amount is detected, the BCL circuit is operated in
the main board and the maximum current amount is controlled. If a beam current begins to
flow about 1.5mA, the ABL circuit will work and if about 1.8mA, the ABL feedback circuit will
work. Therefore, be careful not to be more than 1.8mA.
8) VM (Velocity Modulation) outline correction part
The outline correction circuit is operated in exclusive correction IC, IV11 (STK396-130). The
VM waveform made in the main PCB is input to the deflection PCB PA401 PIN1. The input
1Vp-p signal is input to IV11 PIN1. The input signal is amplified about 100 to 120Vp and
output to IV11 PIN9. Then, it is input to IV11 PIN10 via the circuit composed of VM coil PV11,
12, 13. The power of +12V and +135V GND are used.
