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A Mysterious G 5400 B Problem



I have a most vexing problem with my G5400 B rotor system and am hoping
that one of you have had the same problem, and know the cause.  The
symptoms are as follows:

My G 5400 B is mounted on top of my garage and operates though
approximately 50 feet 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 appears 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 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
warranty service.  This afternoon a quite pleasant and frustrated Yaesu
service manager informed me that they could find absolutely nothing
wrong on the test bench.  They were able to drive a rotor on the bench
with my controller without difficulty.

My problem, ladies and gentlemen, is how to proceed from here.  I could
easily believe the problem was in the rotor if I could see the
resistance change abruptly on my VOM while moving it.  As I do not see
this behavior, if the problem IS in the rotor/cable system then how does
one explain the lack of failure when watching the potentiometer output
with the VOM?  Surely if the ten year old AA cell in my VOM can provide
enough EMF to determine the potentiometer resistance reliably then the
controller should be able to as well.  Note that the problem is
unaffected by removing the external output to the KCT.  I would think
this should eliminate loading from the KCT as the problem.  Also there
does not appear to be any interaction between the intermittent readings
in azimuth and elevation.

I would greatly appreciate advice, experience,  and assistance from
anyone who may have experienced this same difficulty or can help explain
the problem   I do not wish to disassemble the rotor/antenna system
unless I can reasonably be sure of finding the problem.  If there is a
test I could have Yaesu run, it would be helpful to know of it as the
controller is currently still in the shop.

Thanks in advance for the help.


Dean Shutt, AL7CR
al7cr@amsat.org



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