RADIO INTERFERENCE WITH TRIM LED's

Don Patterson · #1 03-12-2008, 05:08 AM

I just mounted a new SL-30 in my 7a and find that when I hit the PTT switch, both panel mounted LED'S used for my trim system go nuts. When I have my headphones on and blow into the mic, it lights up all of the led's like a power meter. I did a search and found some builders ran shielded wire to the indicators, and the problem was resolved. No luck in my case. It seems to be an intermittent problem as well. I can key the mic one minute and it works fine. Then key the mic again, and watch the led's move erratically.

All of the RG 400 antenna wire, runs through a separate conduit as far as possible from the led wiring. I have a comant bent whip antenna mounted just behind the spar under the pilot seat. The wings are not mounted at this time. Just wondering if any other builders had the same problems and what they did to resolve it.....

647jc · #2 03-12-2008, 07:39 AM

Don,

What I know about the Ray Allen bar graph position indicator is that it supplies a small voltage (I think 5 volts DC) back to the MAC Servo on two of the Orange, Green and Blue wires. That voltage is applied across a linear pot in the servo and the voltage returned back to the indicator is proportional to the servo position. The voltage sent back to the pot is low and of low current. It sounds like your problem may be caused by induced voltage into the cable going back to the Servo. I know you stated your antenna coax is routed through a seperate conduit away from the LED wiring but I can't think of any other cause. The servo cable is typically a 5 conductor cable and if not shielded can pickup stray signals and voltage, especially if ran near an antenna or some high current signal. Unfortunately the best solution is to avoid the situation by using a shielded 5 conductor cable for the Servo and / or to insure you route the cable away from any source of induced voltage or current. Are you using shielded cable for the trim servo's.

rglick · #3 03-12-2008, 08:23 AM

Don,

This happened to me. I have two panel mounted RA LED indicators for my TRIM and they went nuts when the PTT was activated. I replaced the wires from my Indicator junction box to the panel mounted indicators with shielded twisted pair and for the most part the issue cleared up.

Good luck

Richard Glick
RV7-A Slinger, WI
Waiting on DAR Inspection Date

fl-mike · #4 03-12-2008, 10:45 AM

I'm guessing the indicator uses something like the LM3916 chip, since the original indicators were meters. If so, you are probably picking up RF in the leads and confusing the chip. Does it only happen when the radio is keyed? If so, you might be able to fix it with a clamp on ferrite bead around the cable at the indicator. It's a quick and easy experiment. Get a sample of one with a big enough hole so you can run the cable through twice if necessary (loop it).
http://www.steward.com/web_parts.asp...r+Round+Cables

If you can't get one easily, I can drop one in the mail to you.

Stabird · #5 03-12-2008, 05:14 PM

We noted a similar RF interference with the Ray Allen position indicators in our RV9A. The first fix involved putting bypass capacitors to ground on the position sensor pins at the indicator. No help. Second solution was to install ferrite clamps across all leads at the indicator. No help. The final fix was to install shielded leads to the indicator from terminal connectors under the cockpit floorboards. Note that these were the only leads not shielded. All others were shielded. This fixed the problem.

Ironflight · #6 03-12-2008, 05:24 PM

One other thing to check is to make sure that you have a really good shield on your antenna cable, from the radio connection to the antenna connection. A bad connection at either end could lead to an RF leak in the metal cockpit, and really drive those sensitive indicators nuts.

(I initially used one of those inexpensive bent-wire whip antennas where you connect the cable using a ring lug. Lit things up like a Christmas tree! Changed to a better antenna with a BNC connector, and the problem was solved. )

Don Patterson · #7 03-12-2008, 06:46 PM

I checked all antenna connections and all looked good. I found that the wire used for the ptt switch was run through the same conduit as all antenna wires were run. I removed this wire and ran it another direction. As far from any antenna wires as possible. This made a big improvement. This is the wire that runs from the grip to the tip of the mic jac.
Thanks to everyone who helped in my dilemma.

rong · #8 03-13-2008, 01:35 AM

I had the same problem with my led indicators, plus the extra distraction of the Vans ammeter going full-scale (in the 'charge' direction) when I keyed the mic.

Turned out it was because I had the battery charger sitting on the ground with the leads going straight up to the engine bay, and they were picking up enough RF to upset the trim indicators and the ammeter.

Took me a while to figure that one out!!

Ron Graham (F1 #105, Sydney, Oz)

Doug · #9 03-13-2008, 03:49 AM

Also, check that the antenna shield/braid is a secure electrical connection to the fuselage skin so it acts as a proper ground plane. This is achieved with the antenna mounting hardware. Otherwise the shield will radiate rather than shield.
Doug

airguy · #10 03-13-2008, 09:39 AM

Quote:

Originally Posted by rong

Turned out it was because I had the battery charger sitting on the ground with the leads going straight up to the engine bay, and they were picking up enough RF to upset the trim indicators and the ammeter.

Much more likely the induced RF confused the voltage regulator on the battery charger and caused the charger to go to maximum output, thinking the voltage was low. An analog ammeter steam-gauge is immune to RF spikes.