this observed screwy change occurs when all comm units are OFF and
their circuit breakers are pulled, and even when the PMA7000B
intercom is also off with its CB pulled.  Note that the observed
effect is the same when everything is OFF or ON, when the mic button
is toggled.

It happens on both joysticks, but appears more pronounced with the
pilot side mic compared to the copilot side mic button.

Jeff,

 

The PTT switch takes the line to ground.  Your audio panel selects which radio will have the transmit pin taken to ground when the PTT is activated.  Does your problem occur regardless of which radio is selected?  The Pilot and copilot stick mounted PTTs should take the lines to the same good ground.  Also, If you have one antenna, the antenna switcher (i.e. COMANT 601), also selected by the audio panel Comm1/Comm2 switch, activates the path to the appropriate radio when the PTT is pushed (grounded) and completely disconnects the non-transmitting radio from the diplexer for the duration of the PTT push. 

 

You might want to check that the ground for the PTTs is a good clean one and not possibly causing a "ground loop" condition with other devices.  Are you running both 14 and 28 VDC buses?  If so, is it possible may there be some cross talk between the different power buses?  Note that some devices can work from 10-30 VDC but the grounds for devices on different + buses should also be to different ground busses.

 

Otherwise, ?????????????????????

 

<<<<<<<<<

Toggling mic button (transmit) on the joystick causes the displayed
TIT2 and (to a lesser extent) TIT1 values (on EAU/Chelton) to
promptly increase and the separate JPI ammeter/voltmeter readings to
go screwy.
>>>>>>>>>>

 

It just appears that the signal grounds or device grounds are affected by the PTT ground.