The PL tone in an Icom 706 MkII G is generated by
the CPU. An internal A to D generates a stair
step function creating a low frequency sine wave.
The PL tone is coupled to the transmit audio
circuits through a low pass filter.

I noticed that the step function also generates
other frequency components that pass through the
low pass filter and modulates the FM in the
in the audio range. The frequency components are
present when the PL is turned on and can be heard
using another VHF receiver. Using the oscilloscope
in my service monitor, I could even see the the
stair step transitions in the demodulated audio.

I talked to other hams and received e-mail
claiming that their 706 doesn't have that problem
but if you turn the mike gain all the way down,
with another VHF receiver listen to 706 on a simplex
frequency with the PL on, you will hear the tones.
When the PL is off the tones disappear

In the 1980's I worked for Rockwell Collins as
a Engineering assistant. I worked in a group
that analyzed design flaws in products and I am
confident that my diagnosis is correct.

I run a radio repair shop in my home and a
customer complained that the PL in his 706MkIIG
was off frequency and he couldn't access a local
repeater. After checking his radio I noticed
that it was producing low level audible tones
around 1000Hz. I check my 706 and two others
that were in my shop. They all did the same
thing. Several friends have Icom 706's and
repeating an on the air test I could hear
the tones even through an open repeater.

The step function is so poorly filtered that I
couldn't measure the PL frequency with my HP 5334
frequency counter. I had to used an audio bandpass
filter between the test point on the radio and the
frequency counter.

The beat tone changes when a different PL is selected.
I think it's possible that the right PL and beat tone
can cause problems with PL decoders on some repeaters?

Did I open a can of worms?

Jack WA9FVP
Willco Electronics