Front Panel Pcb Switches; Orientation Of The Diodes And The Leds; Main Pcb Troubleshooting - Alesis ADAT BRC Service Manual

The switches are arrayed in a 14 column x 8 row matrix. In a similar manner to the method
described for the LEDs, the processor uses U5 and U7 (both HC574 flip flops) to scan each of the
columns for a depressed switch by pulling a low on one column at a time. When a switch is pressed in a
column that is low, it creates a current on that specific row, causing that row (normally high) to pull low.
By sensing which row and column are both low, the processor determines which switch has been
depressed.
As an example, Pin 19 of U5 is low. The RECORD 1 switch is depressed which causes current
to flow through the 4.7K resistor which pulls the voltage on that row down (approximately 0.7V), which
is pin 9 of U6. Since the processor senses that pin 19 of U5 is low (the specific column) and it just
sensed that pin 9 of U6 is low (the specific row), the processor determines that the RECORD 1 switch
was depressed.
As is the case with the LEDs, the service technician must enable both a row and a column
simultaneously in order to activate a specific switch. Moreover, the switch matrix has been divided on
the schematics by PCB. By determining which column or row is effected on one front panel PCB, this
column or row can be found on other front panel PCBs as well as the main PCB.

3.4.3 Orientation of the Diodes and the LEDs

All the diodes are oriented the same on each of the front panel PCBs: Anode on the left and
cathode on the right as shown in Figure 3-2. Likewise, all the surface mount LEDs are oriented the same
on each of the front panel PCBs: Anode on top and cathode on the bottom as depicted in Figure 3-3.
ANODE
Figure 3-2 - Diode Orientation Within the

3.5 Main PCB Troubleshooting

Aside from improperly programmed ICs, most of the problems incurred with the BRC
electronics have been related to workmanship. Prior to beginning any in depth troubleshooting of the
Main PCB, check for loose or improperly connected cables. Next, check the workmanship of the
soldered components. Finally, check the affected part of the PCB for improper contacts, excess solder or
flux, and the like. If none of these steps renders a solution to the current problem, replace ICs U10, U11
and U9, one at a time to verify that the chips have been programmed properly.
CATHODE
BRC
ANODE
CATHODE
Figure 3-3 - LED Orientation Within the
9
BRC