Rotec Ignition's Radio Interference

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Rotec Ignition's Radio Interference

Unread post by Cloud Base » Wed Dec 02, 2015 2:02 pm

Some months ago I got the Rotec ignition pack installed to replace my right hand side magneto on my J250 and have been very pleased with the immediate start even on very cold days.
But I soon realised that no-one could hear me when I transmitted while on the ground at idle.
At cruise speeds I was clearly readable, but at 1,000 rpm-only noise.
There was no interference on receiving, none at all which was very weird.
On TX I could hear a clicking that corresponded with the rpm, drop the right mag and it would go away.
So after several different condensers were tried, tieing earths directly to the battery, checking ground plane earths, I decided to isolate the power to the Rotec ignition from the mag switch with a relay-
mag switch operates a relay and that relay takes power to the electronic ignition and the noise has gone.
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Re: Rotec Ignition's Radio Interference

Unread post by T4flyer » Wed Dec 02, 2015 2:37 pm

The standard alternator doesn't produce enough oomph at low RPM for power hungry Jabi's. Efis, Strobes, electronic ignition, fuel injection etc. and a hard pull in the battery on a cold start, these things added together are just to much for the standard alternator set up. The good news is that above 2000 or so RPM the alternator normally copes ok.

The radio problem you describe is not uncommon at low RPM (taxing is normally where one encounters this, I have experienced it in more than one Jabi) As far as I know you have 3 choices, 1. Live with it. 2. there is an alternator mod that you can do that enables the alternator to produce more power at lower RPM and 3. Fit an aftermarket alternator kit (Camit makes one)

Hope that helps, Don.
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Re: Rotec Ignition's Radio Interference

Unread post by scsirob » Thu Dec 03, 2015 9:59 am

Cloud Base wrote:..On TX I could hear a clicking that corresponded with the rpm, drop the right mag and it would go away
As Don explained, it is a lack of electrical power at low rpm. When the engine runs idle, all power needs to be supplied by the battery. A good battery should be able to start the engine and have some charge left to run electrical gear for a while. Not too much or too long though. When you turn on all bells and whistles, the battery voltage will sag and some gear like your radio will not work properly. The Rotec ignition is a fairly big load (3A or so), it will pull your battery voltage down. You should be able to confirm this by looking at your voltmeter.

Things get worse when your battery gets older, and doesn't keep a full charge anymore. It will still start your engine, and the Rotec ignition will make the start cycle short. But after that, the battery has no charge left to run other gear. Any big loads like the Rotec ignition will pull the battery voltage below the limits. So, if your battery is a bit older, get a replacement. You might find the problem fixed.

Last thing to check is bad electrical contacts. A poor central ground or a bad battery cable will also cause losses in the elctrical circuit, with similar effects.
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Re: Rotec Ignition's Radio Interference

Unread post by Cloud Base » Thu Dec 03, 2015 3:00 pm

Thanks scsirob. The clicking has gone with the relay taking power directly from the battery. I still have the 25 micro farad capacitor there too.
All that remains now is a crackle at all rpm and while I am readable it's no where near perfect.
The battery is newish (3 months old) and there should be plenty left in it after a start as the Rotec ignition gets the engine going within about 2 seconds of cranking.
I wonder if I need to recondition the connections on the antennae-particularly the ground plane?
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Re: Rotec Ignition's Radio Interference

Unread post by scsirob » Thu Dec 03, 2015 5:18 pm

So if I hear you correctly, you moved the Rotec power supply from behind the master switch (main bus perhaps?), to a relay directly connected to the battery. If that made the difference, then the power path to (or through) your master switch has a poor connection somewhere. With the engine off or idling, take a Voltmeter, and measure the voltage at the battery, then at the master switch input, then behind the master switch, and then onward to the radio. Use the push-to-talk switch at each position and document the voltage with PTT on and off. You will likely see almost the same voltage along all connections when receiving, but a larger voltage drop somewhere along the path when transmitting. That's the source of the problem and it should be fixed. Even though it may 'work' ok right now, a weak electrical link is a source of fire hazard, and in case of alternator failure, it eats into your electrical reserve. So do fix it.

I wouldn't suspect the ground plane or antenna for now.
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Re: Rotec Ignition's Radio Interference

Unread post by Cloud Base » Fri Dec 04, 2015 9:32 am

Thanks for the advice.
I have checked every connection-cracked them loose and re-tightened them.
Pulled on the lugs to check their connections to the wires. All seems good.
Voltage drop not visible on transmit, but may only be able to see that with a 'scope.
I added an auto condenser to the positive side going to the coil and I now have clear comm's.
So final solution seems to be;
Decoupled with the relay to take the pulsing away and added the condenser to reduce RF noise.

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Re: Rotec Ignition's Radio Interference

Unread post by checkride » Sun Jan 24, 2016 12:14 am

Gents,

Just a question note to all. Most Jabis have Aluminum buss bars, in the console and in the engine bay. If you have this i would suggest changing this to Copper. You might find that a lot of electrical noise picked up from whatever device is gone.Remember Aluminium does corrodes, don't think I have to say anything else.

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