Mackie FAST RECOVERY M-1200, Руководство по обслуживанию

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M•1200/M•1400 SERVIC E MANUAL
C IRC UIT THEO RY
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C IRC UIT THEO RY
Muc h o f the c irc uitry in the M•1200/ M•1400 is se lf e xp la na to ry fro m the sc he m a tic . This
se c tio n w ill e xp la in the uniq ue c irc uits a nd a rc hite c ture . Sa m p le s in this se c tio n w ill re fe r
to C ha nne l 1 (Le ft) fo r c irc uitry tha t is id e ntic a l o n b o th c ha nne ls.
INPUT C IRC UITRY
Re fe r to sc he m a tic o n p a g e 23.
The sig na l p a th b e g ins w ith the INPUT BO ARD. Fo llo w ing C ha nne l 1’ s inp ut, sig na l is fe d
to a unity g a in d iffe re ntia l o p -a m p , U1A. The sig na l is ne xt se nt to U1B w hic h se rve s a s
b o th HPF a nd C D ho rn EQ . If no t in SUBWO O FER m o d e , the sum m ing a m p (U2B) a nd
LPF’ s (U3A, U3B) a re b yp a sse d . The sig na l is se nt via J11 to the g a in c o ntro l o n the
DISPLAY BO ARD. Afte r the g a in c o ntro l, the sig na l is b uffe re d b y U7A, re turne d to the
INPUT BO ARD a nd ro ute d to the MAIN BO ARD via J16.
C ha nne l 2’ s inp ut sig na l p a th is e le c tric a lly id e ntic a l to C ha nne l 1’ s in STEREO m o d e . In
PARALLEL MO NO m o d e C ha nne l 2’ s inp ut sig na l is ig no re d a nd C ha nne l 1’ s inp ut is se nt
to C ha nne l 2’ s g a in c o ntro l via SW5A. Fo r BRIDG E MO NO o p e ra tio n, C ha nne l 1’ s inp ut
sig na l is inve rte d b y U2A a nd ro ute d to the MAIN BO ARD via SW5C .
PO WER AMPLIFIER C IRC UITRY
Re fe r to sc he m a tic o n p a g e 27.
The M•1200/ M•1400 use a c la ss AB trip le d a rling to n o utp ut w ith c o m p le m e nta ry o utp ut
d e vic e s. The m a in p o w e r sup p ly is +/ - 80V, the re is a lso a +/ - 90V sup p ly fo r the fro nt
e nd c irc uitry. C ha nne ls a re m ute d fo r 4 se c o nd s o n p o w e r up a s C 53 c ha rg e s via R203.
Muting is a c hie ve d b y turning o ff Q 73, re m o ving c urre nt so urc e s fo r the d iffe re ntia l p a irs.
(Wa it a m inute ! PAIRS? Re a d o n…)
An im m e d ia te ly o b vio us d e p a rture fro m sta nd a rd d e sig ns is the Dua l Diffe re ntia ls a nd
Sym m e tric a l Vo lta g e Am p s . The re a so ning b e hind this fro nt e nd a rc hite c ture is
a c tua lly q uite sim p le . Tra nsisto rs tra nsfe r c ha ra c te ristic s a re no t e ntire ly line a r, so e ve n
the b e st c o nve ntio na l fro nt e nd d e sig n w ill intro d uc e so m e d isto rtio n. Mo st a m p lifie rs
use ne g a tive fe e d b a c k to re d uc e this p ro b le m (c re a ting a fe w m o re in the p ro c e ss).
Ma c kie FR Se rie s a m p lifie rs ta ke a d iffe re nt a p p ro a c h. By using tw o c o m p le m e nta ry
“ m irro r im a g e ” fro nt e nd c irc uits a ny d isto rtio n c a use d b y no n-line a r tra nsisto r c urve s is
e ffe c tive ly c a nc e le d o ut in the b ia s string , w itho ut fe e d b a c k!
Ano the r d e sig n fe a ture uniq ue to Ma c kie FR Se rie s Am p lifie rs is the Ba ke r C la m p . The
Ba ke r C la m p ha s tw o func tio ns; 1) Pre ve nt o utp ut/ d rive r tra nsisto rs sa tura ting , 2) Drive
the LIMITER LDR. Re fe rring to the le ft m a in a m p sc he m a tic , Q 27 is a c o m m o n b a se
a m p lifie r, it w ill turn o n if Q 31’ s c o lle c to r rise s m o re tha n tw o d io d e d ro p s a b o ve the
+80V sup p ly, p re ve nting it fro m rising furthe r. The re a re thre e d io d e d ro p s b e tw e e n
Q 31’ s c o lle c to r a nd the b a se o f d rive r tra nsisto r Q 28 (D3, D2, Q 29V
b e
). Q 29 w ill ne ve r
se e a vo lta g e g re a te r tha n o ne d io d e d ro p b e lo w the +80V sup p ly, e ve n if the +80V
sup p ly fluc tua te s. Q 27 a lso d rive s the lim ite r LDR LED U2A. The ne g a tive sid e ha s the
sa m e c irc uit m irro re d .