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Electrical Gremlins

dbegeman

Member
I have an issue that has been bugging me since I bought my airplane a year ago. Unfortunately I didn't realize it was an issue until after the pre-purchase inspection was done and the paperwork signed.

What happens is, when ever I push the PTT button (on either stick) my manifold pressure gauge starts decreasing and the ammeter deflects 180 degrees from zero amps. It does it on either radio (ICOM 200 and a Bendix/king nav/comm) and with the engine running or just on battery power. The ICOM makes the ammeter deflect one way and the Bendix/King the other way but both deflect 180 degrees from zero amps. In addition I have a switch labeled AMP that on the right selection says ALT and the left selection is unlabeled and I have no idea what function it serves. I included pics (and hopefully I can lable them). Any help would be appreciated!!

IMG_6018-M.jpg

MAP before pushing PTT

IMG_6019-M.jpg

MAP with PTT pushed

IMG_6020-M.jpg

Ammeter before pressing PTT

IMG_6021-M.jpg

Ammeter with PTT pressed

IMG_6022-M.jpg

Ammeter with PTT held down longer and the "AMP" switch in the upper right hand corner.



Thanks in advance,
Dan
 
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What kind of intercom/audio panel set up do you have? The push to talk switch should just connect the transmit key-line to audio common. There should never be any 12vdc on that circuit.
 
Ken,

The audio panel is a ICOM200 and a Bendix/King Nav com (I'll get the model tomorrow along with the intercom model). There is a switch that toggles between com1 (bendix) and com2 (Icom) for transmission only.

Dan
 
This is 'normal' operation for the unshielded Van's gauges.

:D

A little tongue-in-cheek perhaps, but unfortunately Walt is right. The standard analog gauges are not good at rejecting RFI and this is a common complaint.
 
How well do the radios work? Do people complain that your transmit is garbled, noisy or weak? How does the sidetone sound?
 
Ok I admit I am an electrical neophyte. For the brain trust here what would happen if he re-wired the manifold pressure and ammeter gauges with shielded wire?
 
One word: Grounds

Check ground locations for both antagonists and victims.

Make sure that your antenna coax has solid shielding termination on both ends. Make sure that the antenna(s) are properly and securely grounded to the airframe. Make sure that the ground paths for the radio(s) and the instruments are grounded to a 'single point' on the airframe, preferably near the battery negative terminal (near where the battery is grounded to the airframe).

No doubt others have had issues with those gauges, *but* if not everyone has the issues, then either mfg of the instruments are highly inconsistent, or installation practices are highly inconsistent. Which is more likely?
 
Antenna Grounding

I have an issue that has been bugging me since I bought my airplane a year ago. Unfortunately I didn't realize it was an issue until after the pre-purchase inspection was done and the paperwork signed.

What happens is, when ever I push the PTT button (on either stick) my manifold pressure gauge starts decreasing and the ammeter deflects 180 degrees from zero amps. It does it on either radio (ICOM 200 and a Bendix/king nav/comm) and with the engine running or just on battery power. The ICOM makes the ammeter deflect one way and the Bendix/King the other way but both deflect 180 degrees from zero amps. In addition I have a switch labeled AMP that on the right selection says ALT and the left selection is unlabeled and I have no idea what function it serves. I included pics (and hopefully I can lable them). Any help would be appreciated!!

IMG_6018-M.jpg

MAP before pushing PTT

IMG_6019-M.jpg

MAP with PTT pushed

IMG_6020-M.jpg

Ammeter before pressing PTT

IMG_6021-M.jpg

Ammeter with PTT pressed

IMG_6022-M.jpg

Ammeter with PTT held down longer and the "AMP" switch in the upper right hand corner.



Thanks in advance,
Dan

I previously had my #2 COM radio (SL-30) connected to an Archer wingtip antenna and I experienced similar issues on a GRT avionics Engine Instrumentation System. Pushing the push-to-talk to transmit from this radio would cause large spikes in the readings of various engine parameters. When I switched from the wingtip antenna to a belly mounted bent-whip, the problems went away completely. I suspect that the problem was actually due to poor grounding of the wingtip antenna that caused RF energy to radiate into the aircraft and be picked up by the EIS wiring.

Other things that may cause this is poor quality antenna cable connector installation. The moral here is to check the quality of installation of your antenna and antenna wiring to be sure that the antenna and cable shields are well grounded.

Skylor
 
Also...

There is a section in the Aeroelectric Connection book that may help.

All that happens when you push a PTT is the mic key wire goes to ground. I would start by looking at how it and the affected instruments are grounded. Are you setting up a ground loop when you key the mic?
 
This is not a ground loop problem. This is basic RFI.

Get someone with an antenna analyzer to ring out our feed lines and antennas. If you have a bad connection this will show it. What you can do in the meantime is look at wire bundles - do you have feed lines bundled with coax?

Shielding instrument wires would not hurt. You may need to experiment with small disc capacitors across the analog gauge terminals ls - this will bypass the RFI to ground. This was standard practice in ?back in the day? radio work.

Carl
 
Funny...

Funny, Carl, that you can make that statement with absolute certainty.

I had a similar issue in my C172...and it WAS a ground loop issue. That is why I suggested it...
 
Funny, Carl, that you can make that statement with absolute certainty.

I had a similar issue in my C172...and it WAS a ground loop issue. That is why I suggested it...

Was it though? Or was it just a mechanic telling you it was a ground loop issue because he didn't figure you were smart enough to know the difference?
 
Importantly for analysis, 'ground loop' is only one way (of several ways) to have a ground problem.

Check your grounds and ground paths.
 
It is likely to be a RFI problem as many others have found in the past.
Depending on how much your time is worth it may be easier to install a different (better designed) gauge.
Pete
 
Thank you everyone for the help! The antennas are belly mount so I will check the connections. I tried running an independent ground for the ammeter and it made no difference, I did not try isolating the ground on the manifold pressure gauge but will when the weather gets better. I will look at the wire bundles and try to see if the coax runs with the electrical. I did have a previous owner reach out to me and said he had the same problem and just ignored/dealt with it. The radios did have some issues but that was when the "AMP" switch was in ALT. Also I still have no idea what the "AMP" switch does.

Thanks,
Dan
 
Mine has done it since it was built. Never could figure what causes it in 380 Hours.I always thought it was a bad ground but never figured it out. I guess I am used to it
 
The first thing to check is if there is high SWR on the antenna. if this is high it causes the coax to radiate RF which causes all kinds of problems.

If you have high SWR there's all kinds of things you can do to fix it. I had a bad connector at the antenna end of my coax and it caused **** with my instruments. Fixed that and the problems went away.

However - as others have already posted - if you have instruments that are sensitive to RF you may have to experiment with bypass options at the instruments themselves.
 
Belly Mount

Everything Charlie and Mark X 2. As soon as he said "belly mount" my eyes perked up. Don't think I've ever seen better than 4 to 1 SWR on a Comant
bent whip ( the kind that is parallel to the fuse ) . A place to start. It is 400 coax, right? Dual shielding, it helps. Then start bypassing with disc caps, helps a little more. You will get it, but more than one treatment will be necessary.
John
 
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