Repair of the Three Springs Historical Society Sign

The Historical Society purchased a building which used to be a bank. This included external signage which displayed the time and temperature on large LED displays, organized as 7 segment digits.

The sign and controller appear to be fairly old, from a company called WatchFire. Documents and support information doesn't seem to be available from their support web site, nor from searching elsewhere on the Internet. However, the Motorola M12+ GPS receiver documentation is readily available.

After the bank's signage was removed, the electronics of the sign were damaged, and the time displayed would be incorrect after a power loss.

Investigation revealed that the sign controller electronics used a GPS receiver to determine the time, and that the external GPS antenna and associated cabled had been damaged when the bank's signage, located above the time and temperate display was removed.

Examining the control electronics behind two of the large 7-segment digit displays on the south side of the sign, reveals a large printed circuit board.

As can be seen from the photo above, there is a pig tail cable exiting from the box with the cover. This is appears to be RG-174/U type coaxial cable, though the variant with a teflon jacket and dielectric material. The cable seems to have had some sort of coaxial connector on the end, though its seems to have been pulled loose from some mechanical trauma.

Likewise, there is a run of black RG-174/U coaxial cable (not pictured) at the bottom of the enclosure which also seems to have a missing connector. It appears that the other end of the cable used to run to a GPS antenna, mounted on the top of another sign assembly which was removed. Presumably, removal of this upper sign was what pulled the connectors from these coax cables.

The aluminum die-cast box on the left contains a Motorola M12+ GPS receiver, which is used to determine accurate from the GPS navigation satellite constellation. The GPS receiver module is mounted to a carrier board which looks like it has a voltage regulator and cabling to the main sign controller board.

The antenna cable enters the aluminum box through a cable gland, and is connected to the receiver PCB using what appears to be an MMCX connect. This was confirmed by review of the Motorola M12+ GPS receiver manual specifications.

Further review of the manual indicates that the Motorola M12+ GPS receiver requires an “active” external antenna with a low-noise amplifier. The GPS receiver module provides +3 VDC on the antenna coax cable connector to power the external amplifier.

The repair plan is to abandon the existing damaged MMCX pigtail cable, and the existing RG-174/U coaxial able leading to the presumed damaged/missing GPS antenna. A new pigtail cable assembly and active antenna will replace these items. A new MMCX mail to RP-SMA female pigtail cable, 6 inches in length has been identified and ordered.

This will be run through the cable gland (which will provide mechanical strain relief) and connect to the Motorola M12+ GPS receiver's MMCX connector. The other end of the cable will present an RP-SMA female connector, which the antenna and cable will connect to. The intent is that this shorter cable with strain relief provided at the penetration of the aluminum box will continue to protect the relatively fragile printed circuit board from any damaged caused by pulling force on the attached cable. As before, if too strong a pulling force is applied, the in-line RP-SMA connectors will fail first and pull off the cable before coupling any of that force to the printed circuit board assembly.

A replacement GPS antenna has also been identified and ordered. This has a 28dB gain amplifier integral with the antenna, and is attached to a 3 meter long RG-174/U cable. The cable has an RP-SMA male connector on the end, which will connect to the RP-SMA female on the pigtail cable. The GPS antenna itself is a small black box, about 4.4“x3.6” in size. It also has a built-in magnet, usually intended to attach to the roof of a vehicle, that could be used to affix the antenna to the top of the sign.

Both of these items have been ordered, and should arrive by April 10th.