[Elecraft] KPA500` (flux leakage)

[Tom W8JI]

Some years ago I received a complaint from W4ZV about 
spurious signals offset about 1 kHz or so on my Icom 751A. 
There was no amplitude modulation of the carrier visible on 
scope, yet Bill could easily hear the spurs.

It turned out the toroidal transformer in the 751A internal 
power supply had enough flux leakage to change the reactance 
of the VCO coils that were about 3 inches from the toroid. 
The rig was being FM modulated. I rotated the power supply 
90 or 180 degrees and the problem went away.

At radio frequencies using a red "-2" mix, there can be 
almost 30% change in inductance from spreading or bunching 
turns in a coil. This is from core flux leakage. I've had 
low pass filters put ripples in an adjacent filter when the 
toroids get too close. These cores are largely touted as 
"self-shielding" in our handbooks.

I'd be very careful accepting broad general statements 
without looking at the specific case in question.

73 Tom