[Elecraft] OT: troubleshooting "gotcha's"

[Edward R Cole]

AS I read "Re: [Elecraft] KAT100 project - removing the control 
board", Stan related his troubleshooting adventure and found that his 
assumptions led him astray.  That is one of the cardinal issues with 
troubleshooting - do not assume.  As soon as you assume soemthing is 
OK that will be what you eventually will find wrong.

Case in point was a problem I was seeing with SWR read by the KX3 at 
some frequencies in the HB TR switch that I was testing.  On HF I 
figured there would be little distributed inductance and reactance in 
the pc board.  I tested at 10w using the KX3 on the work bench and 
got excellent SWR and transmission loss thru the pc board relays 
until I got to 17m, 15m, and 12m.  Strangely, 10m tested good?

On those three bands the SWR went to heck.  Trying several things to 
compensate got me nowhere.  I finally reconnected the input coax at 
the same land as the output coax and still was seeing this effect - 
huh?  I began to suspect the KX3 internal circuits had gone bad on 
those bands.  So I connected a coax direct to the power meter and 
50-ohm load (SWR still high).  Then is dawned on me.  I was running 
with the ATU enabled and it had memory of previous loads that were 
not 50-ohm.....duh!  I switched to BYP and retested and the SWR 
problem went away for nearly all bands.  There was a little creep up 
on the highest frequency (12m and 10m), but that is expected as 
circuit dimensions become significant in wavelength.

 From the workbench,
73, Ed - KL7UW