VERSION 1.1
CIRCUIT THEORY
9
PROTECTION CIRCUITS
Refer to schematics on pages 27-29.
The M•1200/M•1400 has two output protection circuits in addition to the rail fuses. The
first mutes the amp if the output devices are loaded beyond their safe operating area
(SOA). The second is a crowbar circuit that ensures the rail fuses blow if there is DC on
the output.
SOA PROTECTION
The SOA protection circuit senses the voltage drop across the emitter resistors much like
standard VI current limiting transistors. Q76 and Q77 will turn on if the emitter drop
exceeds ~1V. If they are on for long enough to charge C38 (through R159), the SOA
protection circuit is activated by transistor switches Q79 and Q78.
If SOA detection is tripped, the channel is muted by discharging C53 through R200 and
D65, fan speed is increased, and the “SHORT” LED lights.
DC CROWBARS
The DC Crowbar circuit protects speakers in the event of an output failure by blowing
all rail fuses. SCRs Q21-24 are used to short the +/- 80V supply supplies to ground.
Output signal from both sides is fed through 100K resistors R90, R89 to C21, essentially
forming a LPF. DC on either output will trigger comparator U4A (positive) or U4B
(negative). The SCRs are then fired via driver transistors Q87, Q88.
NOTE:
The LM339 comparator IC is used extensively in the
M•1200/M•1400. It differs from a standard op-amp
used as a comparator in that it has an open
collector output. If the voltage at the inverting input
exceeds the non-inverting, the LM339’s output
switches to it’s negative supply. Otherwise, the
output is open.
Содержание FR Series M-1200
Страница 1: ...M 1400 M 1400i M 1200 SERVICE MANUAL 1998 MACKIE DESIGNS INC 820 179 00 Version 1 2 ...
Страница 18: ...M 1200 M 1400 SERVICE MANUAL PARTS 16 CHASSIS ASSEMBLY 4 5 8 1 2 3 6 1 7 1 11 9 10 12 13 15 16 17 18 14 ...
Страница 20: ...M 1200 M 1400 SERVICE MANUAL PARTS 18 HEATSINK ASSEMBLY ...
Страница 29: ...M 1200 M 1400 INPUT PCB 055 079 00 REV D 23 ...
Страница 30: ...M 1200 M 1400 INPUT PCB 055 079 00 REV D 24 ...
Страница 31: ...M 1200 M 1400 DISPLAY PCB 055 080 00 REV A 25 ...
Страница 32: ...M 1200 M 1400 DISPLAY PCB 055 080 00 REV A 26 ...
Страница 33: ...M 1200 M 1400 MAIN PCB 055 081 00 REV D 27 ...
Страница 34: ...M 1200 M 1400 MAIN PCB 055 081 00 REV D 28 ...
Страница 35: ...M 1200 M 1400 MAIN PCB 055 081 00 REV D 29 ...
Страница 36: ...M 1200 M 1400 MAIN PCB 055 081 00 REV D 30 ...
Страница 37: ...M 1200 M 1400 SOFT START PCB 055 162 00 REV A 31 ...
Страница 38: ...M 1200 M 1400 SOFT START PCB 055 162 00 REV A 32 ...
