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The Mysterious AL7CR G 5400B Rotor Problem -- Solved

My thanks to the over two dozen folks who assisted me in troubleshooting
this problem.  The fault was finally discovered to be the rotor cables. 
My several month long series of experiments to locate this seemingly
obvious problem are described below.

My G 5400 B is mounted on top of my garage and operates though two
approximately 50 foot lengths of 8 element rotor cable.  After four
years of trouble free service the elevation indication on the meter, and
the external output, became intermittent.  The elevation position would
indicate a maximum of 45 degrees even with the output voltage adjustment
set to maximum.  As the rotor moved in elevation the output jumped
randomly from zero to 45 degrees.  This behavior was also seen in the
elevation counts as monitored with the KCT setup function in WISP.

The above certainly appeared to be the "old bad variable resistor in the
rotor" problem. However, after a trip to the roof to assure myself that
the wires were not loose at the rotor junction box, I performed the
following test.  Terminals 1, 2, and 3 of the elevation and azimuth
connections on the rotor controller are connected to the respective
potentiometers in the rotor.  I disconnected wires 1, 2, and 3 from the
controller on both the azimuth and elevation channels.  I then monitored
the change in resistance with rotor position across wires 2 and 3 for
each channel as I moved the rotor.  The resistance change with position
as shown on my 20 year old ten dollar Radio Shack VOM was smooth.  The
resistance for each rotor moved smoothly from approximately 3 ohms min
to 500 ohms max as each rotor was moved through it's full range of
travel.  After performing this test several times I reconnected the
elevation and azimuth potentiometer wires to the controller.  The
problem then reoccurred just as before.

Having exhausted my electronic troubleshooting expertise (and feeling
rather good that I did not have to take the boom and antennas down to
service the rotors) I sent the controller to Yaesu for service with the
above problem description.  In due course the controller returned with
the following work having been performed.

"Replaced front panel lamps, Q3, Q4, and C24 for elevation output
problems.  Re-soldered all solder joints in the PCB and aligned unit to
factory specifications."

The above seemed reasonable given the problem I had experienced.  The
difficulty is that when placed in service the unit had the same problem
as before service.  Worse yet the azimuth indication was now
intermittent.  It would fall to zero at random intervals as the rotor
moved. Once again I disconnected the rotor wires and performed  the same
test as above, and once again the resistance changed smoothly and
continuously as the rotor moved. 

After consultation with Yaesu service, I sent the controller back for
additonal service. They were able to drive a rotor on the bench with my
controller without difficulty.  Before the controller was returned I
sent out a plea to the amsat-bb for suggestions as to how to resolve
this problem.  Over two dozen responses were recieved suggesting a
number of possible problems.  VP9MU had twice experienced symptoms
identical to mine.  His failure was due to a faulty controller voltage
regulator.  The Yaesu folks were nice enough to replace the voltage
regulator of my controller at no charge, even though it tested as
functional, in order to help me troubleshoot the problem.

I confidently reconnected the controller when it arrived and ... the
same problem AGAIN! The test that led to resolving the problem was a
simple one.  I had a friend rock my controller wires with a broom from
the driveway while I watched the meter indications in my shack.  To my
surprise, they moved wildly.  Clearly at least the rotor cables were

Today, after a pause in my investigation for several business trips and
a vacation, I replaced both rotor cables from the rotor to the
controller and the system now works perfectly once again.

There is a troubleshooting lesson in all this somewhere I am sure.  One
obvious point is that wires are not immortal and can become
intermittent.  Why the VOM test was successfull and the system failed
when connected to the controller is a mystery to me.  Perhaps I would
have seen a change had I performed the "broom test" while connected to
the VOM.

I hope the above may save someone else from suffering the same problem. 
My thanks again to all the folks who helped out.

Dean Shutt, AL7CR
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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