Hi all,
There is a clue here:
> Mine does it on releasing the button on the Heil
> HM-10 mike. Interestingly enough, there is no spike when using the foot
> switch.
>
I know nothing about the Heil HM-10, so my speculation my be
invalid....nevertheless...
The fact that using the foot switch does not result in the spike
suggests one of the following:
1) A mechanical squeak or vibration produced by the motion of the button
is being acoustically coupled into the microphone.
or
2) Power switching to a preamp, or condenser microphone bias is creating
an electrical pulse that gets into the TX audio input while the rig is
still in transmit mode.
or
3) PTT control switching current is getting coupled into the microphone
audio.
or
4) Even if there is no preamp or condenser microphone bias being
switched, some microphones have either a series switch or a shorting
switch to deactivate the mic while not in transmit mode. If this switch
transition occurs before, or too close to the PTT release, some kind of
a pulse could get into the mic input while the rig is still transmitting.
Microphone PTT switches that switch multiple functions such as preamp
power, mic bias or mic shorting SHOULD have the contacts sequenced so
that the PTT is released long enough before any of those other functions
occur, thus letting the transmitter go off the air before any clunks or
pulses are generated by the other switched functions. In the old days
this was accomplished mechanically by the spacings of the various
"leaves" of the multi pole PTT switch. These days electronic parts and
circuit engineering are often cheaper than mechanical engineering of a
multi pole leaf switch. So there may be a single pole switch that drives
some simple digital circuitry which is used to control the multiple
functions. The digital logic may not have the necessary delays designed
into it to prevent the problem, or it may not be working right. If the
sequencing is working right, possibilities 2 & 4 are ruled out.
DE N6KB
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