Teac MV1421, Service Manual

Page: 43 / 130

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T h e se c o n d p h a se d e te c to r -2 lo c k s th e p h a s e o f th e h o rizo n ta l d riv e p u ls e s a t ou tp u t p in 4 0 to th e
h o rizo n ta l flyb a c k p u ls e a t in p u t p in 4 1 .
T h is c o m p e n s a te s for th e s to ra g e tim e o f th e ho riz o n ta l d e fle c tio n tra n sisto r.
T h e -2 lo o p filte r (C ) is e x te rn a lly c o n n e cte d to p in 4 2 .
T h e ho riz o n ta l p h a se ca n be g iv e n a s ta tic o ff se t v ia IIC bu s (H S H "h o riz o n ta l sh ift).
A d y na m ic c o rre c tio n is p o ss ib le b y c u rren t fe e d b ac k in to the -2 lo o p filter ca p a c ito r.
To p ro te c t th e h o riz o n ta l d efle c tio n tra n sis tor, th e h o riz o n ta l d rive is s w itc h ed o ff im m e d ia te ly
w h e n a p o w e r fa ilu re ("P o w er-O n R e s e t "b it P O R ) is d e te c te d .
The power failure may have corrupted the contents of the internal data registers, so the TDA8374 should be started up again.
T h e T D A 8 3 7 4 h a s a se p a ra te s u p p ly in p u t (p in 3 7 ) th a t o nly u se d a s a c le a n s u p ply vo lta g e fo r th e ho riz o n ta l
o s cilla to r c ircu its.
7-4. Vertical Synchronization
T h e ve rtic a l sa w to o th g e n e ra to r d riv es th e v e rtic a l o u tp u t.
It u s e s a n e x te rn a l c a p a cito r a t p in 5 1 a n d a c u rre n t re fe re n ce re s is to r a t p in 5 2.
T h e T D A 8 3 7 4 v e rtica l d rive h a s d iffe re n tia l c u rre n t o u tp u ts fo r D C -co u p le d v e rtica l o u tp u t s ta g e , like th e T D A 8 3 5 6 .
A t T D A 8 3 5 6 in p u t p in s 1 a n d p in 2 th is c u rre n t is c o n v erte d in to a d riv e v o lta g e via a re sisto r.
7-5. Geometry processing
W ith th e T D A 8 3 7 4 is p o ss ib le to im p le m e n t a u to m atic g e o m e try alig n m e n t, b e c a u se a ll p a ra m e te rs a re
a d ju s te d v ia th e IIC b u s .
T h e de fle c tio n pro c e ss o r o f th e T D A 8 3 7 4 o ffers th e follo w in g five co n tro ls ;
- H o riz o n ta l s h ift
- Ve rtic a l slo p e
- Ve rtic a l a m p litu de
- Ve rtic a l s-c orre c tio n
- Ve rtic a l sh ift
7-6. Colour decoder
T h e co lo u r d e c o d e r co n ta in s a n a lig n m e nt-fre e X -ta l o s cilla to r, a d u a l kille r a n d d iffe re n c e d e m o d u la tors .
To ge th e r w ith th e T D A 8 3 9 5 S E C A M a d d -o n a m u lti s ta n d a rd PA L /S E C A M /N T S C d e c o d e r ca n b e b u ilt
w ith a u to m a tic re co n itio n .
W h ic h s ta n d a rd c a n b e d e c o d e d d e p e n d s o n th e e xte rn a l X -ta l u s e d .
Tw o X -ta l p in s (3 4 an d 3 6 ) a re p re se n t s o n o rm a lly n o ex te rn a l sw itch in g is re q u ire d .
T h e IC m us t b e to ld w h ic h X -ta ls a re c o n n e cte d (b it X A a n d X B ).
T h is is im p o rta n t, b e c a u se th e X -ta l fre q u e n cy o f th e c o lo u r d e co d e r is a lso u se d to c a lib ra te m a n y
in tern a l circ u it. T h e b u rs t p h a se d e te c to r lo c k s th e X -ta l o scilla to r w ith th e ch ro m a b u rs t sig n a l.
T h e ph a s e d e te cto r o p e ra te s d u ring th e b u rs t ke y p e rio d on ly, to p re ve n t d is tu rb a n ce o f th e P L L b y
th e c h rom a s ig n a l. Tw o g a in m o d e s p ro v id e :
- G oo d ca tch ing ran g e w he n th e P L L is n o t lo ck e d .
- L o w rip p le vo lta g e a n d go o d n o ise im m u nity o n c e the P L L h a s lo ck e d .