They should tune the same, which to me should be for sure are not always the same... Rest the memory in the KXAT500, if you not and get a baseline. If you still have a problem then a coil is mostly likely not the proper inductance... or someone put in a wrong value or out of tolerance cap.  I have found the KAT3 and the KAT500 to both work the same. However I have used ATU's since day one when LDG introduced the Z-11 and found that a miss wound coil, followed by a out of tolerance cap followed by one bad relay with bad contacts can screw up a band.  I have one right now in my car a MFJ-928 that will load all bands, BUT 17 meters. Being a MFJ product I am sure it is a missing or cold solder joint, they are clueless when it comes to quality control.

Hope I helped...back under my rock.

73 Harry  K7ZOV

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From: "w2or@... [Elecraft_K3]" <Elecraft_K3@...>
To: Elecraft_K3@...
Sent: Friday, March 20, 2015 2:32 PM
Subject: [Elecraft_K3] KAT500 Antenna Tuner Issue. Compared with KAT3 tuner in the K3.

 

Wondering if anyone has noticed where the KAT500 tuner will not match as well as expected on a particular band, whereas with the same antenna, the KAT3 tuner in the K3 will match that same antenna perfectly.   

In my particular case, the KAT500 operating on 17 Meters will tune the 40m dipole down to 2.0-1, and no lower.  This is so when using either the auto tuner mode, and also when manually tuning the KAT500 with its Utility program.  Whereas, in comparison, the KAT3 tuner inside the K3 (while bypassing the KAT500) will tune the same 40M dipole perfectly, down to 1.0-1.  17 Meters is the only band where there is a significant difference in SWR, such as the above - -  i.e. the KAT500 tunes other bands with this dipole perfectly. 

/Mark