a +20dBu tone on both channels. If the fault is only present with a particular input
impedance setting then RL3, the impedance switching relay is probably the cause. If the
fault is independent of input impedance then the fault is probably RL1 (left channel
faulty) or RL2 (right channel faulty). Figure 10.7 is useful if diagnosing relay problems.
No
RELAY 3, LA102
BOTTOM BOARD
RELAY 1, LA102
BOTTOM BOARD
RELAY 2, LA102
BOTTOM BOARD
No
Plug LA101 left output into LA102 right
input & vice versa (use front sockets)
Fault in LA101
Fault in LA102
USE LA101 XLRs
Right to Right, but
Plug Left to Left &
Switch LA102 i/p Z
Fault
independent
of input Z?
No
Fault
on LA102
Left
Channel?
No
Fault
disappeared?
Fault
originally on
Left Channel?
RELAY 3, LA101
BOTTOM BOARD
No
RELAY 2, LA101
BOTTOM BOARD
RELAY 4, LA101
BOTTOM BOARD
BOTTOM BOARD
RELAY 5, LA101
Fault
originally on
Left Channel?
No
LA102
displays fault
on other
channel
Fault on
both
channels?
RELAY 1, LA101
BOTTOM BOARD
No
Yes
Yes
Yes
Yes
Yes
Yes
Fig. 10.7 Reed Relay Fault Finding Flow Diagram
10.13 Microprocessor Faults Microprocessor faults
If the display shows a random or black pattern when turned on check the +5V supplies as
explained in section 10.7. If both 5V power supplies are operating correctly then the
fault may be with the processor, EPROM, RAM, processor clock, reset circuit, data lines
or address line. Such faults can be difficult to locate but fortunately they are very rare.
Refer to the circuit in appendix D.1 and examine the waveforms on all of the 16 address
and 8 data lines. Most of them should be changing cleanly between the two logic levels,
but if a line is stuck at a particular logic level it suggests that there is a short on the board,
perhaps caused by a faulty component. Similarly a line stuck at a level between 1V and
4V indicates a fault. Disconnect the bottom board and try again, and try removing latch
ICs (40373 chips) until the fault disappears.
166
10. Service Information
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