Mackie FAST RECOVERY M-1200, Руководство по обслуживанию

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VERSIO N 1.1
C IRC UIT THEO RY
9
PRO TEC TIO N C IRC UITS
Re fe r to sc he m a tic s o n p a g e s 27-29.
The M•1200/ M•1400 ha s tw o o utp ut p ro te c tio n c irc uits in a d d itio n to the ra il fuse s. The
first m ute s the a m p if the o utp ut d e vic e s a re lo a d e d b e yo nd the ir sa fe o p e ra ting a re a
(SO A). The se c o nd is a c ro w b a r c irc uit tha t e nsure s the ra il fuse s b lo w if the re is DC o n
the o utp ut.
SO A PRO TEC TIO N
The SO A p ro te c tio n c irc uit se nse s the vo lta g e d ro p a c ro ss the e m itte r re sisto rs m uc h like
sta nd a rd VI c urre nt lim iting tra nsisto rs. Q 76 a nd Q 77 w ill turn o n if the e m itte r d ro p
e xc e e d s ~1V. If the y a re o n fo r lo ng e no ug h to c ha rg e C 38 (thro ug h R159), the SO A
p ro te c tio n c irc uit is a c tiva te d b y tra nsisto r sw itc he s Q 79 a nd Q 78.
If SO A d e te c tio n is trip p e d , the c ha nne l is m ute d b y d isc ha rg ing C 53 thro ug h R200 a nd
D65, fa n sp e e d is inc re a se d , a nd the “ SHO RT” LED lig hts.
DC C RO WBARS
The DC C ro w b a r c irc uit p ro te c ts sp e a ke rs in the e ve nt o f a n o utp ut fa ilure b y b lo w ing
a ll ra il fuse s. SC Rs Q 21-24 a re use d to sho rt the +/ - 80V sup p ly sup p lie s to g ro und .
O utp ut sig na l fro m b o th sid e s is fe d thro ug h 100K re sisto rs R90, R89 to C 21, e sse ntia lly
fo rm ing a LPF. DC o n e ithe r o utp ut w ill trig g e r c o m p a ra to r U4A (p o sitive ) o r U4B
(ne g a tive ). The SC Rs a re the n fire d via d rive r tra nsisto rs Q 87, Q 88.
NO TE:
The LM339 c o m p a ra to r IC is use d e xte nsive ly in the
M•1200/ M•1400. It d iffe rs fro m a sta nd a rd o p -a mp
use d a s a c o m p a ra to r in tha t it ha s a n o p e n
c o lle c to r o utp ut. If the vo lta g e a t the inve rting inp ut
e xc e e d s the no n-inve rting , the LM339’ s o utp ut
switc he s to it’ s ne g a tive sup p ly. O the rw ise , the
o utp ut is o p e n.