Although service policy dictates
board-level troubleshooting, it still becomes necessary to perform some tests and
diagnostics to isolate a potential failure to the G1 board. The possibility of
unavailable replacement boards in the future may require troubleshooting down
to component level.
The LA-2 chassis utilizes several
circuits to monitor various conditions during the operation of the unit for
safety and reliability, along with providing diagnostics indicators to assist
both the customer and the technician in understanding what the problem might
be. Excessive voltages, current and operating temperatures are some examples.
Protection can involve the simple task of shutting down the unit completely to
protect other circuits or simply to notify of a problem such as
a failed lamp. Roughly half of the
protect monitoring circuits can shut the unit down without any visible
diagnostics indicators, while the other half will attempt to display the
problem by flashing the lamp or timer LED on the front panel. Any event that
flashes the timer LED will also store the event into a diagnostics page that
can be called up for analysis providing the unit is able to turn on and display
a picture. Although the unit is
considered board-level only repair, it is important to isolate the cause of a
protect circuit event, whether or not a diagnostics indication is displayed, so
the proper board can be replaced to remedy the problem.
DIRECT RELAY DEACTIVATION PROTECTION
There are several protect lines shown
directly tied to the relay latch circuit and all except for the MS3.3V OVP are
located on the G1 board. All of the lines are isolated by diodes and a high
from any one of them will turn on Q6105, pulling down the “high” command from
PWCTL. The base of PNP transistor Q1604 will also be pulled low, turning it ON
and feeding the remaining pull-up voltage to latch the circuit. The
microprocessor will continue to send the high command. This is why the unit
must be turned off to release the latch before it can be turned ON again. As can be seen, all of the protection
circuits except for one are monitoring for excessive voltages. 11V, D6.5V,
17V-1 and 17V-2 are generated by the secondary lines of the main power supply.
MS3.3V IC8009 is located on the A board.
PH1602 is monitoring for a failure of
inrush current relay RY1602. If this relay does not close, excessive heating of
R1652 will occur along with potential damage to the main converter due to large
current fluctuations. The voltage drop across R1652 will light the LED inside
PH1602, causing the phototransistor to conduct and send a high to D1647. One
would always expect a spike of voltage across R1652 during initial turn-on but
C1620 at the base of Q6105 will delay the
activation of the latch circuit to prevent any random transient voltage
fluctuations from accidentally tripping it.
TROUBLESHOOTING
Obviously, any of the previous
protect circuits will shut the unit down without providing diagnostics
indications. The first item to pay
attention to is the clicking of the main relay. If you do not hear a click,
there is not a protect event occurring. Two relay clicks (main relay turned on,
then off), is a clear indication of a protect event. As mentioned earlier, there are other protect circuits on the C
board that can tell the microprocessor to shut the unit down without diagnostic
indications. Here is a tip to quickly tell if one of the lines on the G1 board
is the cause: If you have to turn the unit off with the remote or the power
button to get it to turn back on, one of the lines on the G1 board has
activated the latch. Troubleshooting
now narrows down to determining which isolation diode is being forward-biased
into the latch circuit. Fortunately, this particular design does not have the
blocking diodes directly connected to the base of Q1605. Otherwise, a peak-hold
meter would be needed to capture the 0.6V, which is the maximum level at the
base of a straight bi-polar transistor with the emitter grounded. Since R1630
is in series with the base, rather large voltage levels will appear at the
cathode of any blocking diode whose monitoring circuit is “firing” into its
anode. Placing a DVM at the anode of each blocking diode will expose the source
of the shutdown.
All of the listed monitoring lines will cause
the unit to power down by removing the relay “high” command (PWCTL) to the main
relay driver on the G1 board. The particular event that triggered the
protection will also cause the timer or lamp LED to flash. The lamp LED will
only flash if a lamp failure is sensed.
The timer LED will flash in designated sequences. As an example: If
excessive temperature is detected by IC47 on the H3 board, the unit will power
down and the timer LED will flash four times at 500ms intervals followed by a
two second separation period and another set of four flashes and so on. The
event will also be stored into a NVM IC on the C2 board for viewing if
necessary. Below is a brief description of each protect item:
TEMP:
A digital thermometer (IC47)
located on the H3 board monitors ambient temperature within the cabinet of the
unit. The IC communicates with IC3208 via the I²C bus. Once the temperature has
exceeded acceptable limits the unit will power down and the timer LED will
flash in sequences of four.
OVP:
There is a power board known as
the G3 board that is an integral part of the DIC2 board. It outputs a 3.3V, 2.5V
and 1.8V source. If either exceeds specified voltage levels, a high is sent to
IC3405. The LED flash sequence will be eight times.
17V OVP:
Although the 17V lines on
the G board are monitored by zener diodes to activate the protect latch, the 17V-1
is redundantly monitored here to provide the only diagnostics indication from a
G1 board protection. The LED will flash six times.
Speaker DC Detect:
To prevent damage
to the speakers if one of the amplifiers should short and place DC onto the
lines, both the main L/R amp (IC47) on the AU board and the woofer amp (IC85)
on the A board are monitored. Any DC
voltage will send a high to IC3405. The LED will flash seven times.
Fan Protect:
Although the schematic
shows five fan drivers, only three fans are used in this model. All are located
in the lamp and optical assemblies. When the fans are rotating normally, a low
is output from the protect line of each. If one or more fail to rotate, a high
will be sent to IC3208. The LED will flash four times.
Lamp Cover Switch:
When the lamp
cover is closed, a switch (S3999 on the T board) grounds this line. If the cover
is not closed or the switch fails, a high is sent to IC3208. The LED flashes
three times.
Lamp Detect:
A high-pressure, mercury
vapor arc lamp is used. As the lamp ages the arc gap increases. Current draw will decrease and when this
threshold is reached, the lamp LED will flash, notifying it is time to replace
the lamp.
