Mitsubishi DPLUS 74SB -BKB, Service Manual

Page: 135 / 176

7-34
5. Horizontal amplitude control
Circuit Diagram
Circuit Description:
1) Diodes D1 and D2 compose the bipolar modulation circuit, and have the function of controlling the
currents of the coils DY(lpp) and Lm(lm) through voltage modulation carried out by utilizing VM.
B+ = Vm +Vcs Therefore, Vcs = (ly * Ly)/ts
→
ly = (Vcs * ts)/Ly, with B+ fixed.
Such being the case, the horizontal width decreases when Vm
↑ →
Vc
↓ →
ly
↓
Inversely, the horizontal width decreases when Vm
↓ →
Vcs
↑ →
ly
↑
(B+ = (Vp*2Tr)/(
π
*Ts), Tr = LyCt , Tm = LmCm )
2) Q2, Q3 and U1 compose the control circuit of H-WIDTH. Of those devices, the transistors Q2 and Q3
compose the Darlington current amplifier, and the operational amplifier U1 composes the emitter-coupled
circuit, that stabilizes the voltage and controls the current.
3) The horizontal width broadens when Va
↑ →
VbI
↓ →
I1
↑ →
I2
↑ →
Vm
↓ →
. An inference in
the opposite sense is also possible.
Test points for maintenance:
1) C
T
= 31 to 37 kHz = 3.2 us
48 to 64 kHz = 3.0 us
Cm
÷
2.8 us
2) V
A
CENTER: 31 to 64 kHz = 1.91 to 4.06V
V
M
CENTER: 31 to 64 kHz = 11.6 to 27.4V
D2
Q2
CT
D1
CS
B+
Q1
Cm
Csm
Lm
DY
Vm
+28V
H-Width
Q3
U1
+
-
Va
Vb1
I1