QSC ISA 280, Technical & Service Manual

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14 QSC Audio Products, LLC
TD-000163-00 Rev. B
1.8 Revisions made during
production
ISA750 and ISA800T
April 2002—Zener diodes D107, D110, D207, and D210 changed
from 5.6V to 4.7V. This is to eliminate the tendency of either
channel to hang up when the amplifier is turned on. Channel 1 was
most susceptible, and various circumstances tended to aggravate
the problem, such as having the low-frequency filters engaged,
having an accessory powered by the DataPort, etc. For amps made
before the change went into effect, use service bulletin ISA0001 to
correct related hang-up problems.
DC fault protection
The ISA1350 uses a DC crowbar circuit, similar to that used on the
ISA750 and ISA800Ti, on each channel output.
Figure 1.13