ESI VideoBridge 2150, Service Manual

Page: 195 / 332

One o f t h e r e q u i reme n t s o f a v i d e o c i r cu i t s u c h a s the on e u s ed i n t h e
2 1 5 0 / 2 1 6 0 i s b l a n k i ng t h e e l e c t r o n b e a m w he n e v e r one f r a me h a s be e n
c ompl e t ed a n d t h e e l e c t ron b e a m i s go i n g t o be r e tra c e d f r om t h e e nd
o f one f r a me up t o t he beg i nn i ng o f the n e x t one .
T h e Ve r t i c a l S y n c s i g na l i s f e d to i n ve r t e r Q 5 w h i c h i n t u r n fe e d s t he
s i g n a l t o t he ga t e o f a V F E T , Q 6 . T ra n s i s t o r Q6 prov i d e s t h e b l a nk i n g
s i g n a l d u r i ng t he v e r t i c a l re t ra c e . When t h e Ve r t i ca l S y n c s i g n a l i s
i n i t s a c t i v e s ta te a s a l ow- g o i ng p e a k a t P 3 0 1 - P i n 4 , i n ve r te r Q 5
i s I n i t s o f f c o n d i t i o n .
W i t h Q 5 o f f , Q 6 t u r n s o n f o r c i n g t h e
j u n c t i o n o f R I O a n d R l l t o g ro u nd . W i t h R I O a n d R i l a t g ro u n d , t he
po ten t i a l a t t h e w i pe r o f p o te n t i om e t e r R 3 3 i s - 40 V D C a s se t by t h e
v o l t a g e d i v i d e r o f R l l , R 3 3 ,
b r i g h t n e s s g r i d o f t h e C R T ,
a n d R 1 2 . T h i s - 4 0 V D C i s
p r o d u c i n g a b l a c k d i s p l a y .
f e d t o t h e
W h e n t h e
V e r t i c a l S y n c s i g na l i s i n i t s i n a c t i ve s ta t e a s a h i g h l e v e l a t P 3 0 1
-
P i n 4 ,
Q 5 i s tu rned on f o r c i n g Q 6 o f f . W i t h Q6 o f f , t h e j u n c t i o n o f
R I O a n d
R l i be come s more p os i t i ve a s d e t e rm i n e d by t h e d i v i de r s t r i n g
o f
R I O , R l l , R 3 3 a nd R 1 2 . W i t h t he j u n c t i on o f R I O a n d R l i a t a m o r e
pos i t i v e p o t e n t i a l , t h e n t h e w i pe r
o f R 3 3
be come s
l e s s ne g a t i ve
s u c h
t h a t t h e p roper b i a s i s suppl i ed t o t h e b r i g h t n e s s g r i d prod u c i n g a
g r e e n d i s p l a y . Po t e n t i om e t e r R 3 3 i s a va i l a b l e a t t h e rea r o f t h e
2 1 5 0 / 2 1 6 0 f or a d j u s t i n g t h e br i g h t n e s s o f t h e d i s p l a y .
C a pa c i t o r C 3 p ro v i d e s a l ow i mpe da n c e pa t h f o r e l e c t os t a t i c n o i s e
w h i c h c o u l d b e c o u p l e d i n t o t h e b r i g h t n e s s g r i d f r o m t h e f l y b a c k
t ra n s f o rm e r c a u s i ng b r i g h t n e s s va r i a t i on s .
3 - 3 0
e l s l i 2 1 5 0 /2 1 60 5 / 8 5