An unusual fault occurred today (unusual for me anyway) and was hoping that the techy minded folks on this forum might suggest where to look for the cause and hence a remedy. Aircraft type is a Zenair 701 with a Rotax 912 80hp engine.
Radio is a Bendix/King KY97A. Intercom is a PM3000. Symptoms are:
Flying normally we hear intermittent crackles from both headsets. May last for a few seconds, clears for a few seconds and returns.
The crackles interfere with the intercom and our deliberate transmissions in that the crackles actually interrupt speach between P1 & P2 and also to/from tower.
At the same time we see “TX” on the radio display showing that as the crackles happen, we are transmitting; as yet to be confirmed by the tower (which we will do Sunday).
Turning the radio off we can still hear the intermittent crackles through our headsets which we have swapped between P1 & P2 and also a spare headset; all to no avail.
We have adjusted squelch and volume controls to see if there may be dust on the contacts but no change.
Any ideas will be gratefully received and acted upon.
Richard
By: RichardF - 14th July 2013 at 20:23
Update: Took all the tools in creation to the airfield today. Found that the headset jack plug sockets are fitted into an aluminium box section, which is itself pop rivetted to the airframe. Of course the only two tools I didn’t take with me were the electric drill and the pop rivetter – sod’s law!
Anyway, we all seem to agree that the crackling is a loose or dry solder joint or one that is shorting to where it shouldn’t.
Today we flew circuits for 15 mins and all was quiet, landed and then flew again. Approximately 15 mins later whilst flying the dreaded crackling occurred with “TX” flickering in time on the radio display. This suggests that the problem could be heat induced (taking time to get to the temperature that the fault likes). Again, during flight we turned the radio off, strobe off and transponder off – no change. Removing either P1 or P2 heatset jack plugs whilst in flight seem to cure the problem but only for a short time and then starts again.
After landing we realized we had not turned off the avionics isolator switch in flight so did a few circuits for the test. The first 10 mins were quiet then on the final down-wing leg the crackling started again. We then turned the radio off and also the avionics isolator switch but could still hear the crackling in our headsets.
We conclude so far (cautiously) that with the avionics all off and only the master switch on (engine running or not), the fault seems to be within the intercomm and/or the jack plug sockets as nothing else is switched on.
Next trip to the airfield is planned for wednesday 16 July. Will keep you posted.
Richard
By: Arabella-Cox - 13th July 2013 at 23:55
Crackling is usually a bad connection or dry joint on a circuit board.
By: RichardF - 13th July 2013 at 10:01
Folks, Many thanks for the suggestions; many we probably would not have thought of. I am listing all in a sequential order on a word document and will be carrying out the checks on sunday (tomorrow). Also, now as an afterthought Dragonfly, when trying to seat the jack plugs for the P1, the crackling did increase, so we will be taking a very close look at them; this ties in with Tony’s suggestions aswell
Alan, not something we have tried yet but will include it in our checks tomorrow.
Again, many thanks
Richard
By: AlanR - 12th July 2013 at 23:02
Are the symptoms still there when the engine and all other electronics are turned off ?
By: DragonflyDH90 - 12th July 2013 at 22:55
Just another comment on the headset/intercom jacks, don’t just tighten them, check that they have isolation washers on them as they are not supposed to be grounded to the airframe.
The isolation washers are small plastic stepped washers that completely isolate the body of the jack from the airframe. If the jack is lose it may be contacting the airframe or if these isolation washers are not present the jacks have likely worn through the paint and are now grounding on the airframe.
To activate most push to talk/transmit circuits the mic socket is grounded to the airframe via the PTT button, this could easily explain why you sometimes see TX on the radio if the mic socket (smaller of the two plugs) is grounding to the airframe. Check the mic sockets first and you might save yourself a lot of time and effort in the other areas.
By: TonyT - 12th July 2013 at 22:19
Sounds like an earthing / bad connection problem, first check the intercom jacks and make sure one hasn’t become loose, if it has tighten it ( make sure you hold the plug as you do it up to stop it turning).
Other things that can cause it, check the actual plugs on the headsets, if they are dulled clean them, I normally use a bit of fine scotch brite, another problem can be the comms box itself, you may find its the contacts on the back of that, a cure can be take it out, a squirt of contact cleaner and refit, or even simply see if its fully home.
Other reasons can be the braiding on the plug leads breaking as its there to suppress it, ( though to be honest not had a lot of probs in that area even when they are poor), also check the Alternator, often there is a Capacitor on the back as a filter ( Lycoming / Continentals do ) to stop crackling, the leads can go on them..
Failing that may be a light with a bad earth etc, try turning off the beacons / Nav’s etc to see if it cures it.
Again may be a headset, unplug one see if it cures it.
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July 12, 2013 at 8:27 pm