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1. 04:46 AM - Re: Garmin GNC300 problem (Brian Lloyd)
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| Subject: | Re: Garmin GNC300 problem |
--> Avionics-List message posted by: Brian Lloyd <brianl@lloyd.com>
Charlie England wrote:
> --> Avionics-List message posted by: Charlie England <ceengland@bellsouth.net>
>
> Posted for a friend:
>
> New install of Garmin GNC300. All functions seem to work normally unless
> the transmit button is held for 10-15 seconds. The display will then go
> dim to dark & the unit hums & groans. This continues after the transmit
> button is released. If power is cycled, the unit will immediately return
> to normal operation. The owner has removed the GPS antenna & has seen
> no change in the symptoms.
What you describe sounds a lot like thermal runaway in one of the power
transistors, either the PA or the modulator output transistors. As the
transistor gets warmer it draws more current which then drags down the
voltage.
Most output devices have a temperature sensing diode physically coupled
to them that varies the bias current to keep the devices from doing this.
If you have an ammeter you can see what the current draw does. Key the
mic and watch the current. If it steadily increases you are probably
seeing a thermal runaway problem.
Another possibility is that the operating point of the transistor
changes with temperature and a parasitic oscillation starts. This would
be readily apparent on a spectrum analyzer.
Oh yeah, if it keeps happening after you shut off the transmitter it
probably is in the audio stage assuming that power to the PA is cut off
during receive.
The bottom line is that it isn't supposed to do that and you need to fix it.
--
Brian Lloyd 6501 Red Hook Plaza
brianl@lloyd.com Suite 201
http://www.lloyd.com St. Thomas, VI 00802
+1.340.998.9447 (voice) +1.270.912.0788 (fax)
There is a time to laud one's country and a time to protest.
A good citizen is prepared to do either as the need arises.
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