In this project, we open up a cavity duplexer from a Sprint / Nextel iDEN cellular repeater.  We see the interior where all the black magic happens, attempt a repair and reassemble it against all odds of success.

 This was a fun project!  I was given a defunct industrial cellular repeater that had been out of service for a long time.  Other than a thick layer of dust, the only obvious flaw was an antenna connector that was broken off.  At first, it appeared to have been broken by mechanical force, but as we shall see, it's more likely the result of a lightning strike that partially vaporized the uplink connector.

This unit is obsolete as of 6/30/2013 when Sprint relinquished most of their licensed iDEN frequencies to the FCC be reassigned to other (non-cellular) services.  The duplexers in this unit have little other use.

These little beasts are marvels of RF engineering.  Their purpose is to allow simultaneous transmit and receive on 4 different frequency ranges.  Allowing a transmitter to blast out its signal on one frequency while a receiver listens to another on the same antenna requires a very special set of filters to prevent the transmitter from burning up the receiver.  These units do all that filtering with very precisely machined cavities to selectively resonate the desired frequencies while blocking all others.

Let's look at the broken duplexer as I received it:

This is where the original N connector was mounted.  When I got the repeater, only the connector's flange was attached.  You can see a metal band inside with solder left over.  The connector in the foreground is a new one.

Unfortunately, I couldn't resolder the new connector without opening the box.  I attempted to put enough heat up through the center pin using a wire and a butane torch, but there was too much heat dissipation.

Nonetheless, the unit worked after simply screwing the new connector on.  There was enough physical contact to get the same performance as the repeaters 2nd (undamaged) duplexer.

Click the picture on the right to see a bigger version.

Broken Connector

 

Before Disassembly

(Click for a larger view)

The unit is a beast!  There are 120 screws that just hold the cover on the 4" x 9" base.  These ensure that the cover is absolutely flat against the base.  Even slight gaps will dramatically alter the unit's behavior.

Cover Off

(Click for a larger view)

In addition, there are 28 screws on the bottom to hold interior elements in place, 12 threaded plugs, 12 connector screws and 52 adjustment screws.  That's 224 threaded holes!

Interior View

(Click for a larger view)

Each of the 4 passages is a tuned path, optimized for a specific range of frequencies.  The N connector on the left goes to the exterior antenna to link up with a cell tower.

Output Paths

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Each of the two SMA connectors is tied to two separate paths for uplink and downlink.  The housing is aluminum and the interior appears to have a highly conductive coating.

Front Wall Deposits

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Inside, we can see vapor deposits from what appears to be a lightning strike event.  Material from the original connector has been burned and deposited on the interior wall.  The new connector is in place, but just pressing against the element.

More Vapor Deposits

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Opposite the front wall, you can see additional deposits on one of the internal partitions.

Microscope View

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Under low magnification, you can see where the new connector's center pin is just pressing against the solder blob on the distribution element.

Deposits On The Cover

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And finally, some deposits on the lid, directly above the ill-fated connector.

Reassembly

(Click for a larger view)

After properly resoldering the new connector, we put it back together and hope for the best.

Half the screws go in and get torqued to 32 oz-in.  Then the remainder go in and the all get re-torqued to 2 in-lbs and finally to 5.3 in-lbs.

Results

(Click for a larger view)

It works!  There are 3 lines in the spectrum analysis above.

Purple is the Tx path before the teardown.
Yellow is the Rx path before the teardown.
Blue is the Rx path AFTER the teardown.

Success!!!

Comments   

0 #1 Carriere 2015-12-24 08:14
Sir I would like to ask where I could possibly purchase the ROBOMAST motorized antenna mount.
Donald Carriere
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