Lcd Ribbon - QSC K.2 Series Service Manual

Applying an external DC supply
When disconnected from the AMP/PSU board, you can power up the input/DSP and
LCD boards using an external DC power supply set to +15V. Using this tip, you can
quickly determine whether the input/DSP or LCD board has failed by isolating them
from the AMP/PSU board. This tip will also give you insight about the ±15V and ± 5V
AUX rails on the SMPS secondary side, particularly in a loaded (ribbon cable connected)
or unloaded (ribbon cable disconnected) scenario.
You can power up the boards either inside or outside the amplifier chassis. It's best to
make a 13-pin wiring harness. You can re-use an old 13-pin AMP/PSU ribbon cable by
cutting it short and stripping the wires on pin4 and pin13 for hook-clip connection to the
external power supply. See Figure 8.8.4 for a custom wiring harness example.
When you're ready to start testing, use a single-rail power supply set to +15V. Remove
the AMP/PSU ribbon cable from the input/DSP board and connect pin4 (+15V) and
pin13 (GND) at J11. See Figure 8.8.5 for example connections.
Notes on external DC supply testing:
• Do not have AC connected to the amplifier module when using
an external supply!
• For safety, set the current trip of the DC supply to 250▲mA.
• When the LCD backlight is on, the approximate current draw
from the 15V external supply is 140 mA. When the backlight is
off, the current draw is 130 mA.
• If the MCU is running, blue LED LD7 will be steady.
• Major circuits on the input/DSP board should work when
powering up. The MCU, DSP, LCD, and push buttons should all
be functional because they are powered from the +3.3VD rail,
which is derived from the +15V auxiliary rail.
• Because only the +15V supply is being applied, the input/DSP
board is not 100% functional. Several op-amps, buffers, and the
audio codec run off different voltage rails, which are not present
using the external DC supply.
• Updating the firmware on the input/DSP board with the external DC supply connected is okay!

8.9 LCD ribbon

The LCD screen is mounted onto the LCD board, with the control and data lines coming through a ribbon cable at J2. A flat
style ribbon with copper pins is used. The connector at J2 is the push-to-lock type, which lends itself to potential assem-
bly error when connecting the ribbon to the board. The ribbon must be near perfectly straight in the connector when it is
locked down. If the individual pins do not make good contact, the LCD screen will not work properly.
Symptoms
• LCD does not display anything on screen
• LCD works, but does not display anything distinguishable
• LCD intermittently works
• LCD backlight never turns on, or is flashing (this could also be related to a broken LCD at the solder pins).
Repair notes
• If you find any issues with the LCD ribbon or connector, it's best to just replace the entire LCD board with a new one.
• Do not try to fix the ribbon cable or connector
TD-001517-01
K.2 Series Service Manual
Figure 8.8.4 - Custom 13-pin wire
harness for external DC supply
Figure 8.8.5 - Applying +15V using an external supply
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