What can cause my radio to transmit nothing but a terrible noise?

Question

I have replaced the noise filter and checked the jacks. It does not matter what headset or jacks are used. I replaced the audio panel and had the radio serviced. When I transmit all I hear is a low screech. Others simply reply to transmissions with a simple I cannot understand anything.

Two people can talk inside the airplane with no noise and reception is clear. I have also checked voltage and connections going to the radio as well as installed a new high quality line filter. Both push to talk switches cause the same noise, I have tried flipping things on or off running the trim etc. no change.

Answer 1

Being able to talk inside the airplane is not related to the radios function, the intercom (generally part of the audio panel these days) is what allows cross talk between headsets.

It sounds like you have checked all your connections which is always a good first step but after that:

1. Check your ammeter when the radio is in operation and make sure you have sufficient current to run the radio, if you are showing a heavy discharge on the system you may not have sufficient power to run the radio (the presence of 12V or 24V does not indicate you have enough power to run the radio)
2. Check your radio grounds (also lots of good info here) as they can be the source of a lot of static.
3. If both talk switches cause the issue (and they are wired properly) I would take a look at the radio unit its self. Perhaps have it pulled and bench tested with known good components to isolate any other factors from the aircraft. This may also assure you the radio was serviced properly when it was serviced.

Answer 2

An aircraft radio contains both a receiver and transmitter and some shared components that serve both functions. Normally, when you key TRANSMIT, the receiver is blanked (disabled) and when you release, the transmitter instead is defeated and the receiver is enabled.

The low screech when transmitting might indicate that the receive function is not being blanked, and your transmitted signal is feeding back through the receive section of the radio and making the screech.

The circuitry that does the receiver blanking is generically referred to as the PTT (Push-To-Talk) function; in inexpensive walkie-talkies the PTT is performed by a complex multigang switch which is activated by the push button on the walkie talkie. In more sophisticated radios, the PTT is activated by one switch on the mic which electronically enables or defeats a series of solid-state switches (switching logic) to do the same thing.

But in any case, this is the sort of obvious problem that a radio tech should be able to suss out quickly.