What do they say about not fiddling…
In my continued attempt to pinpoint the source of my random non rhythmic radio crackle I spent a good couple of hours before Christmas and, in the process removed and fully checked both magneto switches …
I had to cut away some of the very hard sheathing around one of the spades on each Mag switch.. to find that they were OK but probably could benefit from a re crimp with my later better quality crimps
I replaced the 2 switches which are a very tight fit on the right hand side cockpit coving and, conveniently right next to the push button starter… RIGHT next to it
The last thing I checked was the spade connector on the Left and Right magentos in the engine bay. These too were hard sheathed and I found a bad crimp so cut off the spade …
Replacing it today and I hit problems … at first it simply wouldn’t start .. and ALWAYS start first touch … then it fired and turning the left mag off killed the engine completely….
After removing the cowl to re check the new spade .. nothing wrong there … so removed the two mag switches again … it looked like the left one could have touched the rear of the starter button … but couldn’t be sure
Then the starter simply wouldn’t start … and seemed to have lost all power .. I looked at this for a while as I wasn’t sure what was going on here … so .. step at a time .. try the alternate power source .. and pop … the first time I’ve seen one of the aircraft fuses pop out …
So .. this means its something around starter button …
It then dawned on me that if I had managed to get the rear of the mag switch touching the rear of the starter .. then a fuse would most likely pop and the failure of the primary master switch had no doubt blown the car style in line fuse 10 minutes before!
Removed both mags and let them dangle whilst pushing the pop alternate fuse back in and press start … Bingo … absolutely fine … and even got an improved left mag drop … so confirming the bad spade crimp I started to fix with …
It was now too late to fly so put her away all rigged for another day 😉
response from Nigel Snell .. I have pasted it in here as it was really descriptive but couldn’t see it in the blog
Hi David
Have you checked that your shielding on the wiring for comms and radio leads are not earthing out to the airframe, if that happens, you will create what is called a ‘ground loop’, which will result in crackling or buzzing.
The terminals, where they meet a metallic panel need to be isolated by fibre washers, likewise the cable shielding needs to be prevented from earthing out along its length.
Random crackling normally occurs when a flexible, or moveable cable is grounding out due to random contact.
Loose pins in block connectors can also create the same effect, or a loose shielding connection.
Hope this helps.
Kind Regards
Nigel
Hi David
Just ‘re read your reply, do you mean the comms leads are earthed to the airframe individually by small screws ?.
The comms leads shielding should only be earthed through the Radio earth back to the battery to complete the circuit, if it is earthed at more than one point, I.e. additional earthing points, it will create a ground (Earth) loop, if the wiring distance is short length, then it might not matter, but at times of high current draw, it can result in buzzing or crackling.
To explain electrically what happens is very complicated. About the simplest way I can describe it, is that normally the earth (Return), will take the least line of resistance.
When you initially press the comms switch, there will be a spike in flow, then the current will stabilise, this can sometimes be heard as a pop as you press the switch, which would indicate that the inline capacitor is not large enough to absorb the initial spike.
With regards the earth ground loop, because it will at some point, have electrons flowing along it in times of high current flow through the airframe, it back tracks up the earth lead, activating the speaker, hence the crackling.
Now, let’s assume that one, or more of the individual earthing points have corroded, and now the current has further to travel along the completed circuit.
When the earth routes were short, it didn’t matter, because the flow of electrons had completed their return to the battery as you released the switch.
With a much longer path to take, then, as you release the switch, there are still electrons trapped in the circuit, so the next time you press the switch, the trapped electrons flow again, but they want to take the least line of resistance, if that path is actually backwards to the speaker, so be it, the speaker is now getting a feed from both sides, so the speaker will alternate very rapidly between live and earth, the frequency will determine whether it’s a hum, buzz or crackle.
I hope I’ve explained it reasonably well, but, like I said, it is very complicated, and I’ve missed out a he’ll of a lot.
My dad was an electronics engineer before he became a University lecturer, so it’s not really my field.
Sadly, he’s long gone, but I know he would have been more than happy to solve it for you.
I do hope this helps.
Kind Regards
Nigel
Jersey Sherwood Ranger 