My ICOM A200 make a clicking noise (like PTT being repeated pressed) on 123.50. This is the only frequency affected by the interference. I get this all the time, even when the engine and everything else but the EFIS are off. Efis is AFS 3500. So this makes me think it is the EFIS or one of the sensors. I'm hoping somebody else has run into this situation.
This is very common with any EFIS or digital instrument (ours included).
It is the direct result of the many high speed signals traveling around the EFIS and small, unavoidable leakages via the display surface and conducted EMI via the connected cables to the EFIS.
Sometimes, poor or incorrect wiring practises can cause tiny signals to become bothersome as they find easy ways to radiate and "get into" just about anything. The fact that VHF airband radios are very sensitive AM radios does not help one little bit.
You may find several frequencies where you get interference and you may even find it varies depending on what the EFIS is doing (like what kind of information it is displaying or if it is busy accessing a disk or communicating with something). A lot of interference (in particular rhythmic sounding stuff) may be due to connected serial data lines, servo interface cables, cables going to COM equipment and so forth.
Often the best way to trace the exact source is a process of elimination (as you have started doing) - try removing non-essential other connections from the EFIS one by one, perhaps it is using one of those connections to radiate.
If nothing helps or you don't want to tear up your wiring, consider using special ferrites over the critical cables. We stock them and sell them by the boatload. Ours are made in Germany for us and are specially formulated to be most effective in the VHF band. Properly used, they are "silver bullets".
We have two types - one is best used for new wiring jobs while another (more expensive) type can be "clipped" onto existing wires. This latter type is very useful for a "try and see" approach to find out where the ferrites have best effect. Contact Matt at MGL Avionics U.S.A if you need some.
You may be able to find ferrites from other suppliers but if you do, please find out if the ferrites will work at VHF frequencies and ask for the data (don't take anybodies word for it). By far most of these things are made for the amateur radio bands or for FCC compliance at lower frequencies and run out of steam above 30 MHZ and have zero effect at 130 MHZ !
Rainier
CEO MGL Avionics
