ALLEN & HEATH GL 4 Service manual

TECHNICAL BULLETIN
To:
From: ARJ
Date: 22 December, 1999
Subject: GL4 Mute Group Lock-Up
Due to a change in specification to the BC549 transistors, some production
consoles will have these devices fitted reversed on the MONO PCB,
AG2005A. These transistors, Q2 and Q14, are part of the power-on reset
circuit for the mute processor, and although they will function in the incorrect
position, they can cause the mute systems in certain consoles to lock up
when first switched on from cold. Normally they will resume correct operation
within a few minutes. This may be more noticeable if the DC supply voltage is
lower than the +/- 16V specified.
If startup problems are encountered, then check these transistors for
orientation, and reverse or replace them if necessary. With the console
inverted on a suitably sized clean work surface, clear of parts and debris, and
with a cloth to protect the console cosmetics, remove the base and locate the
+5V regulator IC12, (identified by the black finned heatsink fitted to it) on the
MONO PCB. The two transistors are to one end of the heatsink, next to a
330uF capacitor (C5). Check with the diagram and correct if needed.
NOTE The overlay in the service manual shows the incorrect orientation for
the BC549 transistors. They should be positioned as the other transistors on
the assembly.
BC549
transistors
üCORRECT
ûINCORRECT

STEP 1. For Direct out 
Pre-fade cut link J37 add 
a link at J38
STEP 2. All pre-fade signal is pre-EQ 
by default.
All pre-fade signal is post-mute by 
default.
To get pre-fade direct out, pre-mute 
(unaffected by channel mutes) cut 
J35, add a link at J36