MAINTENANCE NOTICE
July, 1999 MN-99-006
DIGITAIR® SBU Model 6695 End Unit
Solid-State Motion Detector Modification
Equipment Affected
This Maintenance Notice applies to SBUs
equipped with BOTH of the hardware items listed
below, or SBUs receiving either of these items
as replacement spare parts:
1. A Solid-State Motion Detector (MD), US&S
p/n: N24610101 as shown in Figure 1, and;
2. A Pressure Transducer date coded in 1998
(e.g., G0198-046, H0798-038)
or later
as
shown in Figure 2.
SBUs with Pressure Transducers date coded
prior to 1998 should not be affected.
Figure 1 – Solid-State Motion Detector
Figure 2 – Transducers (showing date code)
Reason for Modification
US&S has learned that the manufacturer of the
Pressure Transducer implemented a design
change in 1998 that causes an increase in the
startup current when the transducer is powered-
on. The SBU routinely turns the transducer on
and off to conserve battery capacity. The
increased transducer startup current causes a
transient on the power line feeding the Solid-
State Motion Detector’s processor chip which in
turn may cause it to output false motion status.
The problem is clearly visible when observing the
motion indicators on an EOT Front End Unit
such as the CDU Model 6696. With air pressure
applied to the SBU while it is standing still, the
CDU motion indicators will typically cycle from
STOPPED to MOVING (while also showing
either FORWARD or REVERSE), and then back
to STOPPED.
Corrective Action
The problem is corrected by reducing the
transient level as seen by the MD’s processor.
This involves soldering a surface mount
technology (SMT) diode and capacitor to the
backside of the MD board as shown in Figure 3.
The diode and capacitor are available from
US&S as listed below:
1. SMT Schottky Diode (P/N: J726102-0015).
2. SMT Cap, 22uF, 20V (P/N: J709153-0024).
Figure 3 – Parts Soldered to Back of MD Board
Capacitor
Diode
