Thorp Air Command - T18.net

Hi guys....comms weird with advice needed - with engine off, all comms are loud and clear. Start the forward cooling fan and and things go unreadable quick - I can hear everyone fine and dandy, but am transmitting unreadable. Changed voltage regulator - no joy - found loose connection on P to Talk - resoldered - no joy - checked and upgraded (made mo bedda) connxs to alternator and checked all grounds - no joy - Alternator putting out 14.2V to the battery steady per the voltmeter.

I am beginning to think the alternator is excited and throwing out amps/volts everywhere and mucking up my outgoing signal - there looks to be a "filter" on the alternator - change filter out - alternator is less than a year old? I have also heard that engine/mag grounding issues can send radios and their owners to the asylum.....

This problem has slowly manifested itself over the last few months, as I have been told while in formation that my radio has gotten more "scratchy" as a flight progresses.

Before I pull the KX-155 and take it up to "Bloodsucker Avionics" for bench test, any suggestions?

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Frase,
I put an alternator switch in that airplane for you if I remember correctly .... try shutting the alternator off and see if that changes anything. If so .. double check the shield on your alt line.

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

If you lose a diode in the alternator, you probably wouldn't notice any charging issues, but it gets very noisy electrically.

Frase,
First - do as Lee suggests.
Check your mic and headset wiring shields.
Check antenna connections for corrosion also the base of the antenna(ground plane connection).
Check mag P lead shields, should only be grounded on one end at the mag.
Try transmit during a mag check, rule out or narrow down a magneto as the problem.
Have you tried a different mic?
Get a length of coax and a rubber ducky antenna connect to the radio to rule out antenna wiring.
Have the alternator tested.

It just could be the radio, if one of your local buds has the same radio try swapping units to see if the problems moves or not.

My previous radio(Apollo 704) had a problem when with in 5 miles of an antenna farm on top of one of the local mountains(lots of static), replaced the radio and problem went away. New radio is a GPS/comm that also replaced the Apollo 604 Loran C which the Coast Guard obsoleted.

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

This may not be your problem , but it was mine . I bought a "spare" Microair radio to have as a bench backup should #1 poop the bed . Well...the new radio had a few new features that I liked and since I had the panel tilted to replace a RAC trim indicator I thought I would put in the "new radio ." Got everything connected and driveway tested the radio with the handheld . OPS normal . Off to the airport to fly up to Rosamond the next day and thought I would ground check the radio with the guys in the CAB . I knew right away I had a problem when they said readable , but scratchy ? NEVER had a problem with radio #1 ! Long story short I got out to the hold short and could not contact the Tower ? Taxied back to the run up area and pointed the AC at the tower . GRD asked if I could here them and to flash my taxi light . Got clearance to taxi back to the ramp . Tilted the panel and began checking the wiring . All was secure . Made sure the coax connector was on correctly and tried ground again . Same results . Readable , but scratchy . As I sat in a pool of sweat on the 95 degree ramp , I tried to remember if there was anything I had done differently . Come to find out I had tighten the coax fitting . I removed the 3 piece fitting only to see a single strand of the braided coax resting comfortably against the center pin ! Carefully folded the braid back into position , CORRECTLY tightened the coax fitting (yes there is a proper sequence) , attached it to the radio and got a loud and clear from Grd. Control ! BOOM !

RB

Thanks for the help guys - I just remembered I turned off the alternator to see if that would make a difference while in formation with Homey a couple of weeks ago. I did not, Sorry, forgot to add that little tidbit....and once again, while the engine is not running, the whole system works as advertised...it's only when I start the fan. And even then, the first radio check is OK, but the next one goes south to unreadable. I'm starting to think it maybe an engine/mag ground that I am missing.

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

I had a similar problem.
'Feisy's' radio worked fine on the ground, engine off. But would sometimes give great bursts of static, or not receive at all, although I was told (after flights) that my radio broadcast fine.
It turned out to be my right (impulse) mag. During the last condition inspection, I noticed the timing had slipped, so I reset it, and read about it in the SAK skyranch manual, and a downloaded copy of an old Bendix Magneto rebuild manual. I looked in the engine log and noted this mag had been giving trouble and had minor service. On the next flight, the engine would not fire when cranking. With luck, and a quick release of the key, the fixed mag managed to start the engine. I had 1/2 expected this, but once running, both mags gave an acceptable check. So I took off the mag and sent it to Aircraft Magneto Service. The mag had about 300 hours in service, but was rebuilt in the 1970's. It had condemned coil, condenser & gear and maybe more, so the noise was internal breakdown, the points ran hot and melted the rubbing block which changed the timing.
I decided to install a Pmag rather than rebuild, and noticed the radio noise had gone.
Last week, I noticed a small spark plug noise over the radio, but was mainly concerned with the out of ballance feel of the wood 2 blade propeller. A poor mag check (but perfect Pmag check) lead me to pull the lower spark plugs, which are fired by the remaining Bendix mag. the 2 left bank plugs failed the resistance test (open circuit), so I replaced all four with new Tempest REM37BY plugs.
The radio noise went away, most of the 'prop' vibration went away, and the climb rate increased about 200 FPM.
Go figure.
So I would suggest testing your radio during the mag check. My initial mag generated noise was so severe at cruise RPM it did not sound like ignition noise, just a blast of static that (mostly) continuously operated the auto squelch system on my Narco 810 Comm radio. the times the radio would let the noise thru was painful!

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

Mine had noise going directly through the Mic at cruise, pull the power back and it transmitted just fine, spin the fan up and make some noise and it went right into the Mic, I even heard it in my head set went through that for a long while before figuring out what it was and fixin it.....Dan

Wupps forgot, it was the MIC Gain, a lot of radios have the gain set too high for a cockpit as noisy as mine so the mic picks up the noise,gain is set too high, 10th of a turn on the gain pot screw fixed it.....Dan

Dan..."And the Rest of the Story is ?" Operator error ? Co-Pilot noise in the head set ? Only at Cruise ? What was the cure ? >:( BOOM !

RB

Frase, this is Deans area of expertise, give him a call and pick at him for a bit, he is done painting now and on reassembly, give him a holler......Dan

Wupps forgot again, Boom!!

Bad suppression resistors in your spark plugs can cause radio interference. Sparks are wide band noise emitters, much like lightning in a thunderstorm. Resistors in your spark plugs are used primarily to reduce the electrode erosion effects caused by capacitance after-fire, but also help with radio noise suppression. If your plugs are getting older, it is possible that the noise is increasing as the resistors become less efficient. This may explain why your noise is slower getting worse overtime and why it happens only when your fan is moving.

_________________
Dean Cuke
Visalia, California
N772DM
KVIS

Thanks Dean....I was almost ready to change out the kanooter valve and install a new muffler bearing beside the vacuum suppressor. You may have saved me a bunch of time - while dropping something off at the hangar on Monday after work, I did notice a loose wire off the mag to the P-lead, so that was my next fix - the plugs are not that old, but like me, may have been dropped on the floor right out of the box. Great comment - thanks!!.

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

It is the cratamatic fractionizer and not muffler bearings, while at it , check the grid leak bias. I have a spare jar of it somewhere.