ESI VideoBridge 2150, Service Manual

Page: 194 / 332

Q2 , R 4 , R 5 , a nd CR l . W h e n t h e I V p- p v i de o i np u t s i g na l , a s app l i ed
to Q l , i s i n i t s nega t i ve p ea k , a mp l i f i e r Q l i s t u r n e d o f f .
c a u s e s t r a n s i s t o r Q l ' s co l l e c tor to r i s e t owa rd + 4 0 V D C supp l y .
Th i s
Th i s
r i s i ng v o l ta g e i s
t h e e m i t t e r o f Q2 .
f e d t h roug h bu f fe r pa i r Q3 a n d Q4 , a n d d i od e CR I to
When t he D C vol tage o n Q2 ' s e m i t ter e x ce e d s t he
ba s e b i a s , de t e rm i ne d by v o l ta g e d i v i de r R4 a n d R 5 , t r a n s i s t or Q2
t u r n s on .
Th i s c u r r e n t c h a r g e s c o up l i n g c a pa c i to r C l p r o v i d i ng b i a s
f o r Q l . Du r i ng t he r e s t o f t h e v i deo i npu t c yc l e , Q l
f o r c i ng i t s ou tpu t be l ow Q2 ' s b i a s po i n t t u r n i ng o f f Q2 .
pa r t o f t h e cy c l e , C l beg i n s to d i s c h a r g e t hr oug h t h e
i s tu rned o n
D u r i ng th i s
B a s e / Em i t t e r
j u n c t i on o f Q l . H owe v e r , be c a u s e the d i s c h a r g e pa t h f o r C l i s t h r oug h
a h i g h i mpeda n c e pa t h o f Q l , C l l o s e s on l y a m i n i m a l amou n t o f c h a r g e
b e f o r e t h e c h a r g i n g c y c l e i s r e p e a t e d .
prov i de d f o r Q l .
T h u s , a c o n s t a n t b i a s i s
Re s i s tor R 6 p r o v i d e s c u r re n t l i m i t i ng f o r Q3 a n d Q4 s h ou l d t h e ou tpu t
a t p i n 7 be s h o r t e d t o g r o u n d .
T h e o u t p u t o f t h e s t a g e c a n h a v e
vo l ta g e sw i ng s i n t h e v i c i n i ty o f OVDC t o 3 0V D C w h i c h a r e t he l e ve l s
requ i re d when c ompo s i te v i d e o i s app l i ed t o t h e c a t h ode o f t h e C R T .
T h e s e v o l t a g e s w i n g s w i l l t u r n t h e C R T f r o m f u l l - o n t o f u l l - o f f
prov i d i ng c r i sp b l a ck a nd g r e e n , a n d some ha l f- tone d i s p l a y s .
3 . 5 . 2 Ve rt i ca l Re trace B l a n k i n g
P301
I
1
; lJlJ�:�1
I ..J " .7
... �
�
V E RT I CA L
4 1
R7 fOI(ll.
SYNC I N
I
�Eln SY":
1
l!i
+-1 -
10" Dill
=
10'1 , oz. ... !>
P30'l.-3
(1IlIl.T �)
R
I
,
"'oo�
I T,
IINIOIIM
B RI G H
TN E
S S
I
GRID (CRT)
P302
F i gure 3-18 . Ve r t i ca l Re t r a ce B lank i ng
e l s l i
3 - 2 9
2 1 5 0 / 2 1 6 0 5 / 8 5