Mitsubishi Electric 1786FD2 Service Manual page 62

MEA 1786FD2 CRT Monitor Service Guide
When power is initially applied to the circuit, capacitor C626 charges through R624, R623, ZD601.
When the voltage across C607 reaches a level of 16V, IC601 is turn-on the +5Vdc will be
set up at pin8 then R613, C615 generate a fixed frequency sawtooth wave to pin4, at this time
MOSFET will be drived by pin6 with square wave the pulse width of square wave is decided by
pin2 , pin3 is current feedback control, It will to sense MOSFET current. The D613, D612, R614,
C617 are soft start components to avoid the duty too large when power starts up.
3.6 Feedback circuit
This power supply adopt feedback circuit of +45V and +13V. It used IC702 for voltage regulation
and IC602 for primary-secondary isolation, The output voltage differential signal will be detected
and sensed to the pin2 of UC3842 for comparison then the duty cycle of MOSFET will be decided
to control the output voltage.
R616
10K
4
3
IC602
TLP721F
R617
1K
C610
3300P
250V
+6.5VA
R738
82
1/2W
1
R736
2
(SHORT)
C725
0.47U
50V
K
(D)
R
IC702
TL431
A
C611
3300P
250V
Circuit Operation Theory
Q765
(OPEN)
SGND
C730
+45V
(OPEN)
+14VA
R737
R734
20K
39K
1/4W
1/4W
REF
R726
VR701
500
470
R735
1.2K
(OPEN)
R766
(OPEN)
Q766
(OPEN)
R767 +6.5VA
(OPEN)
R768
(OPEN)
D766
(OPEN)
REF
R775
(OPEN)
C766
(OPEN)
16
Q767
(OPEN)
+