Our club repeater also ended up interfering with the local tower.

It seems that the transmitter PLL was unstable, and "hopping" between that
frequency, and ours.
We were clearly audible in their recordings.

Ouch. That'd be categorized as a "double-plus ungood" for certain!

Lest any "experts" step in and claim that you can't receive FM on an AM
receiver, I'd ask them to consider what effect the passband filter of the AM
receiver's IF might have on the FM signal as it deviates from side to
side....

I believe the magic words are "slope detection". The resulting audio
on the AM receiver isn't great (it's often distorted) but it's
certainly there.

I hit the magic codes and took the repeater down, once we determined that
this was indeed the source.

A re-tweak of the transmit PLL, and a stub filter cut to pass 146.730 and
reject the tower frequency, cured the problem, and insured that if it ever
happens again, they probably won't hear us. The tower now has our phone
numbers in their books, in case there is ever another problem. The tower
complimented our rapid and assertive handling of the problem in their
closing letter to the FCC.

Well done!

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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