My son just picked up a new plane. The radio has a loud crackle on transmit. If you reduce power to idle it goes away. Tried a handheld and it gets the same interference.
O-200, mags recently rebuilt, Ford alternator.

Any ideas where to start looking before taking it to avionics shop?

Turn the field off on the alternator to see if it goes away. If so, look at the altenator rectifying diodes. Usually makes a sizzle sound.

Could be alternator diodes, could also be brushes. If the problem goes away when you turn off the alternator, get it rebuilt, don't try to fix it with a filter because that will be temporary at best.

Spark plugs=. Are they resistance plugs?

MTV

mtv wrote:Spark plugs=. Are they resistance plugs?

MTV

I don't know. I assume REM40? but have not looked yet. Could it be a cracked resistor/dropped plug?

daedaluscan wrote:

mtv wrote:Spark plugs=. Are they resistance plugs?

MTV

I don't know. I assume REM40? but have not looked yet. Could it be a cracked resistor/dropped plug?

Could be a cracked resistor, but then usually that plug won't fire properly either. Ignition does weird things though. Could be a bad condenser in the mag as well. I'd do as the other Saud and isolate the alternator out if the circuit and see if that changes anything. Then try to isolate it mag by mag. If it is a mag or plug issue then the noise should go away when you turn that mag off.

There are still a few originally non electric planes out there wearing non resistor spark plugs. My PA-11 had them (no electrics, no radio, nada). I rigged up a handheld with a battery pack in the cockpit and external antenna, and had precisely the problem you describe. Mechanic took one look and said swap out the plugs. problem solved in that case.

Of course, it could be other things in the ignition system as others have noted. Plug check is easy, though.

MTV

Thanks for all the advice. It’s had a radio in it for ten years, and this is a new thing apparently. Runs good on both mags. We will try each mag separately and switching off alt field.