Troubleshooting - avionics whine on startup

[bd32322]

I did run a search in the forums, but couldnt find the same problem.

I start the engine up, radio master is off. JPI shows 14 volts, then I flip the radio master, weird whine comes from the speakers or somewhere else. I'll try and determine exactly where. The garmin 430 doesnt power up.

Then I turn off the radio master and turn it back on. The noise goes away and stays away for the duration of the flight. The 430 fires up normally.

Alternately, pulling the 430 breaker before turning on the radio master for the first time also leads to no noise. Then I put the breaker back in to power up the 430. And all is well.

Any ideas?

Thanks in advance.

[carusoam]

I would take your description directly to your avionics guy.

Looks like you hit directly on the dirty/oxidized switch description...

How old is the avionics master?

Expect the following...

It may be part of a relay circuit. Relays are know to get dirty and are easy to replace.

Best regards,

-a-

[Piloto]

Looks like you have worn out relay that is bouncing when switched. Some avionics are sensitive to this and fail to properly start. Turn on the 430 by it's own switch after turning on the avionics master. The problem could also be caused by a load on the avionic bus that causes a momentary in-rush current.

 

What is Switch Bounce?

The left-hand image below shows a simple push switch with a pull-up resistor. The right hand image shows the trace at the output terminal, Vout, when the switch is pressed. As can be seen, pressing the switch does not provide a clean edge. If this signal was used as an input to a digital counter, for example, you'd get multiple counts rather than the expected single count.

Note that the same can also occur on the release fo a switch.

The problem is that the contacts within the switch don't make contact cleanly, but actually slightly 'bounce'. The bounce is quite slow, so you can recreate the trace, and the problem quite easily.

 

José

[bd32322]

Thanks guys. Jose did you forget to attach the diagrams?

I'll go talk to the avionics guy but it was invaluable to get the scoop here first! Thanks.

The avionics master is probably 1987 original I have only owned this bird for 4 years.

[carusoam]

Check near your battery for physical relay locations...pilot side...

This is where they are on my O.

Things could be significantly different in the J?

Best regards,

-a-