ChrisMallory

ChrisMallory

Active Member
I'm hoping someone has had the same, or similar problem, and can give me some ideas where to start troubleshooting my RFI.

I have a Terra TX760D transceiver (I know it's old, but it takes up very little panel space), and a flightcom 403 intercom.

With the radio off, I get no RFI at all. I tried turning everything else on to see if it showed up. So I don't think it is my wiring to the headset jacks (they are shielded by the way).

Once I turn on the radio, I get the faint whine from the alternator. Turning the alternator off makes it go away. Turning on any of the lights increases the whine, but flap motor, fuel pump, autopilot and other avionics do not increase it. There was also no change when I turned off each mag.

I have a BNC fitting on the antenna cable behind the instrument panel, so I disconnected that, and the whine increased.

The wires between the radio and the intercom are shielded.

Any ideas from the group?

Thanks,
 
You have some AC coming into your bus from the alternator. That means either the regulator is bad or you have a bad diode in the alternator. If the alternator checks out ok, you can put a ferrite core onto the radio power supply wire. The reason why it gets louder when you turn the lights on, is because those longer wires act as an antenna which radiates the noise more effectively. Electrical items that have shorter power wires don't radiate as much.
 
Rocketbob,

Thanks for the words. The alternator is abrand new Plane Power, so maybe I should start with the ferrite core. Now I just need to figure out what that is, where to get it, and how to install it.

Thanks.
 
Ferrite Beads

These ferrite beads are aviable from All Electronics.com catalog no FB-83 at $2.00 each. Be sure to make a couple of raps around the bead before closing it.
 
I tried the ferrite beads. I had three large ones (one from Aircraft Spruce, and two from a local electronics shop that were a little bigger than the Aircraft Spruce one) that I used on the power line, with about 20 wraps total around the three. I got a minor, but noticable, drop in volume of the whine.

I'm thinking the next step is to disconnect the power from all the circuits not required for engine running, radio and audo panel, and see if I still get the whine. Then add the circuits back in, one at a time. This will help isolate the problem, and maybe give me ideas where to go next.

Any other thoughts on how to proceed?

Thanks,
 
Check the shielding on all your audio cables, especially the ones between the radio and intercom, in this case. The shield should only be grounded on one end, preferably the source end.
 
Good idea to try the separate battery for the radio. This is a lot easier than disconnecting everthing else.

I think my shielding is correct and grounded at only one end. I would think if shielding was the issue, I would get the whine even with the radio off.

My intercom is hardwired on, I guess I could disconnect it, and see. I do have a jumper that I can plug into the cable coming out of the back of the intercom, that bypasses the intercom. It will still use all the aircraft wiring, just no intercom. I will give it a try too and see what results I get.
 
New alternator

Not an expert here but you mention that you got a new alternator - did you have this problem with the old alternator? Was anything else 'new' or 'changed' just before the problem started?
 
Good question. The alternator is new, and was installed with the engine. Unfortunately, there is no before to compare to.
 
Troubleshooting update

I finally got around to trying out all the suggestions you guys have provided.

First I disconnected everything electrical except intercom, radio, engine display, alternator, mags, and still had the noise, so I think it isn't RFI from any of the periferal circuits.

Then I tried the ferrite beads with no effect.

Then I disconnected the radio and the intercom from the aircraft system and used an additional battery to run them. I tried all three combinations, radio on the battery, intercom on the battery, and both on the battery. I still got the noise every time the radio was powered up, didn't seem to matter from where.

The wires between radio and intercom and the wires to the headset jacks are all shielded.

It is an older radio, a Terra 760D. So my thought is that the radio unit itself is picking up the RFI.

Any thoughts out there for where to go next?
 
Back in the day...this used to be a big problem on Cessna and ADF sine the alt noise would kill the ADF from pointing. The answer was always a bad capacitor on the alternator. Not that far along on my build so I am not sure what is standard today or if it is built in to the alt these days. Maybe someone else can chime in with further details
 
Disconnect the antenna from the radio (DON'T TRANSMIT!!) and see if the noise is coming thru. Evidently this is a case of radiated EMI. Also how well is your engine grounded?
 
Check your grounds.

Do you have a single point aivionics grounding point?

Is the case/tray of the radio grounded?
 
I have tried disconnecting the antenna without effect.

I have checked all the grounds, I don't have a dedicated ground strap for the radio rack, it should be grounded through the brackets.

But would the problem be ground issues if I still get the RFI with the radio completely disconnected from aircraft power and running on a separate battery?
 
I fixed mine.

I had the same noise in my 6. When I added power the whine increased in pitch. The more things I put on the louder the noise. Though I had an alternator problem. What I did was to bring the alternator power output to a fuse buss on the firewall, and connected to the load side of the main contactor. I used to have it in the cockpit under a circuit breaker on the panel. I also did the common ground setup as Bob Nuckolls suggests. All loads are brought back to the firewall mounted ground strap, inside the cockpit and under the cowl. My plane is now dead quite in the headsets, under all conditions. Where is your alternator power lead connected to the battery?
 
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