Helicopter radio noise

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nhraracer90

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nhraracer90
Hello! I have a Hiller Helicopter with a Franklin engine on it (so not very many wiring or electronics). Helicopter just went through a restoration so mostly everything is new. I have a Becker AR6201 radio installed. I am getting a static noise only when the engine is running. We did install an alternator as part of the restoration. Not running I have no noise at all. The radio and intercom works great other than the static. Avionics shop did the avionics wiring for the radio and transponder.

None of the lights or strobe is on so I don’t believe it’s a light issue and when you turn them on nothing gets worse
A mag check doesn’t change the static noise any
Static seems to get slightly worse with engine rpm increase
Ran engine with p leads disconnected (not sure if that would actually prove anything) but didn’t make any changes
All correct grounds/ground straps are installed per the manual
Next step I was going to remove the field wire from the alternator and see if that helps any?
Noise only when running so that would make me think it’s magneto or alternator? It has Bendix mags

I just don’t want to cut my p leads and start throwing noise filters everywhere

Thank you!
 
What happens, engine running but alternator turned off?
 
Field may be the source. Either dirty sliprings or brushes. Is there a noise suppression capacitor on the field circuit?

One way or another, the alternator is the most likely source of this kind of static. I believe that Dan Thomas has said that rebuild shops sometimes mis lubricate them, and lub gets on the brushes and slip rings.
 
You don't have a separate switch to turn off alternator?
 
geezer said:
Field may be the source. Either dirty sliprings or brushes. Is there a noise suppression capacitor on the field circuit?

One way or another, the alternator is the most likely source of this kind of static. I believe that Dan Thomas has said that rebuild shops sometimes mis lubricate them, and lub gets on the brushes and slip rings.
Click to expand...
There is no noise filter/suppression device on that I am aware of
 
AV8R_87 said:
Have the diodes in the alternator checked as well.
Click to expand...
Depending on how this thing has been wired, pulling the breaker should take the diodes right out of the circuit. Moreover, diode failure is rare. Really rare.

Cessna, among others, wired their alternators so that the regulator got its voltage reference from the output stud on the alternator, and that arrangement also supplied the current that the regulator sent to the field. Pulling the output breaker would not isolate the regulator but could cause serious damage to the regulator as the battery was not in the circuit to damp the wild fluctuations that can occur. So Cessna did not install pullable breakers for the alternator, for good reasons. You shut the alternator off by opening the ALT switch, which shuts the regulator, and thereby the field, off.

Piper, and other, used a regulator that got its voltage reference from the bus, via the ALT switch. The regulator is then protected from fluctuations by the battery if the alternator is disconnected from the bus.

There are several possibilities for the source of the noise here, assuming it's the alternator doing it.

1. Arcing at the field brushes in the alternator. As mentioned, overgreasing the rear bearing can do it, but this applies to the Ford alternators. Many other use a sealed ball bearing rather than the Ford's Torrington needle bearing. So what alternator is it? And is it new or rebuilt?
2. If it's an overhauled alternator, were the slip rings remachined exactly concentric with the rotor shaft? If they're eccentric even a little, the brushes bounce and arc and make noise.
3. Does the helicopter have an ALT switch, and what is it controlling? It should be turning the regulator off. Try turning it off and see if the noise disappears. I do hope the people that did the restoration knew what they were doing and followed the original Hiller wiring diagrams, although they may have been replacing a generator with the alternator and maybe got something wrong. I hope that drew a new wiring diagram. Wiring diagrams can answer so many questions.
4. Is there a filter capacitor in the field and/or output circuits? The field circuit can make plenty of noise. Sure, it's getting nice "smooth" DC via the regulator, but that regulator regulates by rapidly shutting off and turning on the field. It's not smooth at all. It's a buzz. Moreover, the field current gets pushback every time a rotor pole passes a stator pole, causing back-EMF in the field circuit. Manufacturers often had a capacitor mounted to a ground stud on the alternator's aft side, and the wire from it connected to the field terminal, to smooth the field noise that would otherwise go back through the regulator to the bus and make radio noise. And some had another capacitor between the output and ground, sometimes on the alternator and sometimes on the firewall. Some, like Cessna's, was a small steel box that had both a coil and capacitor in it, a true smoothing filter.
5. Both the field wire and output cable to/from the alternator need to be shielded, and the shield grounded at both ends. They will emit radio noise otherwise.
6. Are the headset jacks insulated from the airframe/ instrument panel? Never rely on audio grounding through the jack mounting. Stray alternator currents through the airframe will find their way into the audio that way. They should be insulated, and a dedicated ground wire run to the jack from the radio rack.
7. The noise might be ignition noise. Are the P-leads shielded and the shield grounded at both ends? If you run the engine to about 1500 RPM and shut both mags off (AND DON'T TURN THEM ON AGAIN!) does the noise disappear, or does it continue as the engine spools down?
Turning the mags on again while the engine coasts down will cause a big bang in the exhaust system and maybe damage it. After the engine stops, pull the mixture, crank the engine a bit to clear the exhaust, then start it again.
 
Dan Thomas said:
Depending on how this thing has been wired, pulling the breaker should take the diodes right out of the circuit. Moreover, diode failure is rare. Really rare.

Cessna, among others, wired their alternators so that the regulator got its voltage reference from the output stud on the alternator, and that arrangement also supplied the current that the regulator sent to the field. Pulling the output breaker would not isolate the regulator but could cause serious damage to the regulator as the battery was not in the circuit to damp the wild fluctuations that can occur. So Cessna did not install pullable breakers for the alternator, for good reasons. You shut the alternator off by opening the ALT switch, which shuts the regulator, and thereby the field, off.

Piper, and other, used a regulator that got its voltage reference from the bus, via the ALT switch. The regulator is then protected from fluctuations by the battery if the alternator is disconnected from the bus.

There are several possibilities for the source of the noise here, assuming it's the alternator doing it.

1. Arcing at the field brushes in the alternator. As mentioned, overgreasing the rear bearing can do it, but this applies to the Ford alternators. Many other use a sealed ball bearing rather than the Ford's Torrington needle bearing. So what alternator is it? And is it new or rebuilt?
2. If it's an overhauled alternator, were the slip rings remachined exactly concentric with the rotor shaft? If they're eccentric even a little, the brushes bounce and arc and make noise.
3. Does the helicopter have an ALT switch, and what is it controlling? It should be turning the regulator off. Try turning it off and see if the noise disappears. I do hope the people that did the restoration knew what they were doing and followed the original Hiller wiring diagrams, although they may have been replacing a generator with the alternator and maybe got something wrong. I hope that drew a new wiring diagram. Wiring diagrams can answer so many questions.
4. Is there a filter capacitor in the field and/or output circuits? The field circuit can make plenty of noise. Sure, it's getting nice "smooth" DC via the regulator, but that regulator regulates by rapidly shutting off and turning on the field. It's not smooth at all. It's a buzz. Moreover, the field current gets pushback every time a rotor pole passes a stator pole, causing back-EMF in the field circuit. Manufacturers often had a capacitor mounted to a ground stud on the alternator's aft side, and the wire from it connected to the field terminal, to smooth the field noise that would otherwise go back through the regulator to the bus and make radio noise. And some had another capacitor between the output and ground, sometimes on the alternator and sometimes on the firewall. Some, like Cessna's, was a small steel box that had both a coil and capacitor in it, a true smoothing filter.
5. Both the field wire and output cable to/from the alternator need to be shielded, and the shield grounded at both ends. They will emit radio noise otherwise.
6. Are the headset jacks insulated from the airframe/ instrument panel? Never rely on audio grounding through the jack mounting. Stray alternator currents through the airframe will find their way into the audio that way. They should be insulated, and a dedicated ground wire run to the jack from the radio rack.
7. The noise might be ignition noise. Are the P-leads shielded and the shield grounded at both ends? If you run the engine to about 1500 RPM and shut both mags off (AND DON'T TURN THEM ON AGAIN!) does the noise disappear, or does it continue as the engine spools down?
Turning the mags on again while the engine coasts down will cause a big bang in the exhaust system and maybe damage it. After the engine stops, pull the mixture, crank the engine a bit to clear the exhaust, then start it again.
Click to expand...
Thank you for all that information!

It’s a new alternator (Prestolite). Helicopter does not have an ALT switch, just the circuit breaker. Yes we were replacing a generator. The headset jacks all have shielded wiring (avionics shop did all that so I am not 100% on specifics) but I do know all the wiring for those are shielded. Yes the p leads are shielded and the shield is grounded at the mag, the connector for the p leads on the deck and grounded at the switch (like the wiring diagram says).
 
nhraracer90 said:
Thank you for all that information!

It’s a new alternator (Prestolite). Helicopter does not have an ALT switch, just the circuit breaker. Yes we were replacing a generator. The headset jacks all have shielded wiring (avionics shop did all that so I am not 100% on specifics) but I do know all the wiring for those are shielded. Yes the p leads are shielded and the shield is grounded at the mag, the connector for the p leads on the deck and grounded at the switch (like the wiring diagram says).
Click to expand...
The machine should have an ALT switch, to shut off a rogue alternator by shutting off the regulator without having to shut the master off. It sounds to me like the installers just installed an alternator into a generator circuit, and those circuits are not the same. What voltage regulator is in that helicopter?


The headset jacks should be isolated from ground, with these washers:

1721754292046.png

Without that, the shielding of the mic and audio can't do much.

You really need someone with an oscilloscope, and who knows how to use it and interpret the waveforms, to determine where the noise is originating and how it's reaching the headseets.
 
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