KAT3 Mistuning

Posted by John G3PQA on Sep 06, 2014; 12:06am
URL: http://elecraft.85.s1.nabble.com/KAT3-Mistuning-tp7592847.html

Hi Everyone,
I would appreciate some advice on what might be causing an ATU tuning fault which my K3 developed, possibly after tuning by mistake on an open load.
The KAT3 now mistunes on all LF bands below 18MHz, progressively worse when going lower in frequency.
It tries to tune, sometimes hunting and the display ending in dashes, and other times partially succeeding.
For example, on 1825 kHz it turns a 1:1 swr into 1.5:1, and when going up 100kHz it turns a 4:1 swr into 5:1.
On 80m it turns a 1.8 to 1 swr into 1.5:1,   etc. etc.. up to 18MHz where it tunes more or less well, and appears normal on 28MHz.
The fault is still present when using a dummy load, and when using a 100 ohm dummy load the results are similar (turns 2:1 into 1.5:1 on 160m, and 2:1 into 1.3:1 on 7MHz)
The swr bridge readings of power and swr on the K3 screen appear accurate as previously calibrated.
The LPA and KAT3 screws are properly tightened, and RF was not a problem before the fault occurred.
The KAT3 itself is not the problem as swapping it with another module gives the same fault.
Has anyone seen this problem before, or can give suggestions as to cause?
73
John