Mackie FAST RECOVERY M-1200, Service Manual

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M•1200/M•1400 SERVIC E MANUAL
SERVIC E PRO C EDURES
12
TRO UBLESHO O TING TIPS - O UTPUT FAILURES
Afte r a c a ta stro p hic fa ilure , it is like ly tha t the fo ur +/ -80V sup p ly fuse s w ill b e b lo w n.
Re p la c e fuse s a nd
ve ry slo wly
b ring up the Va ria c w hile m o nito ring line c o nsum p tio n. It
is like ly tha t sub sta ntia l line c urre nt w ill b e p ulle d d ue to sho rte d o utp ut p a rts. At the
sa m e tim e , lo o k fo r DC o ffse t a t the o utp ut. O ffse t o f g re a te r tha n +/ - 2.8V w ill trig g e r
the p ro te c tio n c irc uits a nd re -b lo w the fo ur fuse s. Ke e p the o utp ut o ffse t b e lo w +/ - 2V,
o r te m p o ra rily d e fe a t the DC -o ffse t d e te c to r (Lift o ne sid e o f D56).
Re m o ve the m a in b o a rd fro m the unit a nd c he c k fo r sho rte d o utp ut tra nsisto rs. If o ne
b a d o utp ut is fo und , re p la c e a ll e ig ht in the c ha nne l. Whe n a n o utp ut d e vic e sho rts it
c a n p la c e hig h c urre nt stre sse s o n the o the r o utp ut p a rts. The se o utp ut p a rts c a n fa il
o ve r tim e . Sinc e lo ng te rm re lia b ility is p a ra m o unt, p le a se re p la c e a ll the o utp uts.
All e ig ht o f the 3W 0.22
Ω
e m itte r re sisto rs m ust b e ve rifie d fo r p ro p e r va lue . Any o ff-
to le ra nc e , o r o p e n p a rts, ne e d to b e re p la c e d . An o ff to le ra nc e (hig he r re sista nc e )
e m itte r re sisto r w ill p re ve nt it’ s re la te d o utp ut tra nsisto r fro m “ d o ing it’ s sha re ” a nd w ill
p la c e m o re stre ss o n it’ s m a te s in the o utp ut se c tio n. Also ve rify the 8 b a se d rive re sisto rs
(2.2
Ω
, 1/ 4W, fusib le ) a re a ll O K. Ve rify tha t the d rive rs a nd p re -d rive rs a re no t sho rte d . If
o ne d rive r is sho rte d re p la c e it’ s m a te . Do the sa m e w ith the p re -d rive rs. Also c he c k a ll
the re sisto rs surro und ing the d rive rs a nd p re -d rive rs.
C he c k the VI lim ite rs a nd d e te c to rs. It is no t unc o m m o n to d a m a g e the se p a rts w he n
the a m p lifie r fa ils in a sp e c ta c ula r w a y! Lo o k fo r sho rts o n Q 76, Q 77, Q 94, Q 95, Q 83,
Q 84, Q 96, a nd Q 97. Ve rify p ro p e r va lue o n the re sisto rs tha t g o to the b a se s o f the se
d e vic e s. It is c ritic a l tha t the se se c tio ns a re w o rking c o rre c tly. Sho rte d tra nsisto rs c a n
c a use so m e o d d a sym m e tric a l c lip p ing p ro b le m s. O p e n p a rts w ill no t a llo w the c urre nt
lim iting to o p e ra te e ffe c tive ly. If p ro b le m s e xist in the se se c tio ns the a m p lifie r m ig ht fa il
into a sho rt, o r m ig ht c lip p re m a ture ly w he n lo a d e d to 2
Ω
.
All o f the a b o ve tro ub le sho o ting (No t inc lud ing p a rt re p la c e m e nt) ta ke s p e rha p s 15 to
30 m inute s to d o . If yo u ta ke the tim e , a nd d o a ll tha t is ind ic a te d a b o ve , it w ill a llo w
99% o f the a m p lifie rs to c o m e up the first tim e ! Trying to hurry, a nd skip p ing w ha t is
sug g e ste d , c a n le a d to a frustra ting a nd tim e c o nsum ing re p a ir.
Slo w ly b ring up the sup p ly a nd ve rify tha t line c o nsum p tio n isn’ t e xc e ssive a nd tha t the
o utp ut is c e nte re d (no DC o ffse t). It m ig ht b e d e sira b le to d e fe a t the a m p lifie r m uting
(C o nne c t U5-12, 13, & 14 to g e the r) te m p o ra rily so the a m p lifie rs a re a c tive e ve n w he n
the sup p lie s a re still ve ry lo w . If the a m p sta ys c e nte re d , ve rify tha t it w ill p a ss a nic e
c le a n sine w a ve . Re m o ve a ny te st jum p e rs (I.E. DC -fa ult d e fe a t & m uting d e fe a t) a nd
p ro c e e d to the “ Re lia b ility Ve rific a tio n” se c tio n.