Peter wrote:
. I can measure the frequency in this case with a sound editor and it sounds like 20Hz which could indeed be the ignition.
Why not troubleshoot it the easy way?
Jesse wrote:
Have you tried to override the squelch (pull the volume button on your, push it on some others). Switch off different gear on at a time, listen for any difference.
Have you tried this? You could do the same with ignition and alternator, though it doesn’t sound like an alternator issue.
AnthonyQ wrote:
Sorry, one of those annoying suck eggs questions….But did you have any charger plugged into the cigarette lighter?
This would be an example for switch mode noise. Most wide voltage input avionics also have a switched mode power supply.
I did a flight today with the squelch pulled out and while I could not reproduce the same “static” (a “crackle” really, is what I would call it) as I was getting previously, I got a similar noise which was obviously engine RPM proportional.
The ignition harness was changed with the crank swap in 2008, which is about 1000hrs ago.
I would be interested in any suggestions…
Nothing of significance has been done to the aircraft in recent years.
For a long time I have had this on VHF.
It is obviously worse with weak stations. With a decent transmission it is barely there.
I am getting a new ignition harness, but I wonder if this alternator filter could be causing that.
It is a static like crackle and is rpm dependent in frequency.
Sounds like it could be alternator noise – dirty slip rings or worn brushes?
The American spam-can alternator suppressor is an S1915-1, available from Adams, Airpart etc.
The alternator is being overhauled, so that ought to take care of it, but the brushes were always in spec – checked on every 50hr check.
I have a few videos which show the noise e.g. this one at 8:28 (and many other places)
It does sound like ignition but I wouldn’t know… I did prove it is absolutely RPM related.
Is the above suppressor any good, or should I get something better? It is also 15 years old (plus any number of years in Socata’s stock :smile).
I can’t find anything useful on google, and the TB20 MM shows only this
which is a 2-wire item whereas this one is one wire plus the grounding tab.
There is also a suppressor on the electric fuel pump, but this one was on the front of the engine, right above the alternator.
Check bonding for proper RF bonding. It should be able to work, without such filters (although the filter can take the symptoms away). I would rather take the cause away.
Where should I check?
The plane didn’t have this issue years ago. I have some old videos and they are seemingly clean.
I have a similar issue that is related to RPM and it is suggested that a “condensor” (I think that is what was said) on one magneto will be the cause. Any comments please?
Fenland_Flyer wrote:
I have a similar issue that is related to RPM and it is suggested that a “condensor” (I think that is what was said) on one magneto will be the cause. Any comments please?
You can fit magneto filters in series with the P lead. They should be installed right at the magneto – the existing P lead terminal goes onto the filter and the filter pigtail goes to the magneto terminal. The filter body must have a good ground and is usually bolted to the magneto fixing studs. The common part number is MF3A and was available from Adams, Airpart, Spruce etc.
What I don’t get about that French Eurofarad suppressor is: what does it do? It goes across the alternator output, but the alternator should not be generating RF muck. There can be a little bit of oscillation generated by rectifier diodes as they are turning off (with some inductance present) but nearly all of GA uses these same 24V truck alternators, and most planes don’t have this capacitor there.
BTW that filter can’t be the one in the diagram in post 3 because the latter passes the alternator output current, whereas the Eurofarad one obviously doesn’t.
