IM620-1
31
Troubleshooting Power Problems
The MCB receives 18 Vac, center-tapped power from transformer T2. It then distributes both 5 Vdc
and 13 Vdc power to various MicroTech components. A problem that exists in any of these
components can affect the MCB and thus the entire control system. Power problems can be caused
by an external short, which can blow a fuse, or a defective component, which can either blow a fuse or
create an excessive load on the power supply. An excessive load can lower the power supply voltages
to unacceptable levels. Use the following procedure to isolate the problem. This procedure may
require two or three spare MCB fuses (see parts list). Refer to the panel wiring diagram or Figures 15
and 17 as you proceed.
1.
Verify that circuit breaker CB1 is closed.
2.
Remove the MCB Power In connector and check for 9 Vac between the terminals on the plug
corresponding to terminals 2 and 3 on the board (see Figures 2 and 18). Then check for 9 Vac
between the terminals on the plug corresponding to terminals 1 and 3 on the board. (Readings of
9–12 Vac are acceptable.)
a.
If 9 Vac is present between both sets of terminals, go to step 3.
b.
If 9 Vac is not present between both sets of terminals, check transformers T2 and T1 and all
wiring between the 115 Vac source and the Power In plug.
3.
Remove power from the controller by opening circuit breaker CB1. Check the MCB power supply
input fuses (F1 and F2) with an ohmmeter. See Figure 18. A good fuse has negligible resistance
through it (less than 2 ohms).
a.
If either or both fuses are blown, replace them. Go to step 4.
b.
If the fuses are intact, the MCB is defective.
4.
Reconnect the Power In plug and disconnect all other connectors on the MCB. Cycle power to
the controller (close and then open CB1) and check the power fuses.
a.
If both fuses are intact, go to step 5.
b.
If either fuse blows, the MCB is defective.
5.
Reconnect the analog input ribbon cable. Cycle power and check the power fuses.
a.
If both fuses are intact, go to step 7.
b.
If either fuse blows, check the ICM Terminal Board, the connecting ribbon cable, and the
field wiring for shorts. Any of these may be defective.
6.
Reconnect the digital input ribbon cable. Cycle power and check the power fuses.
a.
If both fuses are intact, go to step 8.
b.
If either fuse blows, check the ICM Terminal Board, the connecting ribbon cable, and the
field wiring for shorts. Any of these may be defective.
7.
Reconnect the digital output ribbon cable to t he MCB. Cycle power and check the power fuses.
a.
If both fuses are intact, go to step 9.
b.
If either fuse blows, check Output Board and the connecting ribbon cable. Either of these
may be defective.
8.
If there are any AOX-4 boards, reconnect the expansion bus ribbon cable to the MCB; otherwise,
go to step 10. Cycle power and check the power fuses.
a.
If both fuses are intact, go to step 10.
b.
If either fuse blows, check the Analog Output Expansion Modules (if any), the connecting
ribbon cables, and the field wiring for shorts. Any of these may be defective.
9.
With circuit breaker CB1 open, measure the resistance between field terminals “DC-GRD” and “5
Vdc.” It should be greater than 20 ohms.
If the resistance is greater than 20 ohms, go to step 11 if the controller is equipped with at least
one AOX-4 board or a modem. Otherwise, the problem is indeterminate. Obtain factory service.
