Unreliable Indications

Unreliable Indications

I have a p210 with garmin 530/430.  I have been having an intermitant problem with the vor/glideslope/and localizer.  Both indicators the HSI and the garmin indicator loose signal/flag and or have very unstable needle swings especially while on ils approach.  Both units loose signal and or have needle swings at same time.  Ocassionally will loose indication for radial or ils shown on the window in the 530.  I have a splitter under the instrument panel, I disconnected cables, repluged them in.  Did not seem to work.  I assume since signal is lost to both indicators being powered by seperate radio's, the problem must be from the splitter back to the antenna's.  Any other suggestions on how to look, or how to test.  I do not have a avionics shop handy but my local a&P said he would be happy to try. 

The problem is very noticable during ILS approaches.  When tracking vor radial will intermittantly loose signal, indicator will flag, then come back on.  The cdi indicator sometime will swing and oscillate back and forth almost full scale with the flag poping up during this.

Any suggestions greatly appreciated.

Thank You,
Jay

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Re: Unreliable Indications

Hi Jay,

I'm a freelance Avionics Tech. in southern CA. I find your post interesting. The GS is a separate antenna then the VOR/LOC. Are you sure the GS bug is swinging or just the CDI. If both GPS units are doing same swing than most likely problem is in the antenna, splitter or coax wire in between. Not difficult to test, remove your Garmin units out of rack and use a volt meter to ohm the center conductor of the coax, the resistance should be fairly low. I don't recall the specs from memory but less than 50 ohms rings a bell. Also, you may have a second splitter near the NAV antenna, check that too. Antennas need a good airframe ground to work properly, make sure there is no coorosion under ant base. NAV ant  is located on vertical stab either a blade that sits parallell to the ground or cat wisker type ant. On a 210 the Glide slope antenna is usually mounted inside the airplane, a clear flat plexiglass plate mounted at the top of your windscreen. Don't forget about your static wicks, if they are not working properly the airframe could be developing a charge in flight (that would usually cause poor comm performance too).

Randy Wilson
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Re: Unreliable Indications

Jay,

Do you have a dual blade style antenna?  It could be one side is bad.

Also, make sure your NAV coax is not running near any high power ac sources or an autopilot/trim servo.  These can induce noise in the NAV signal that are near the modulation frequencies of NAV and LOC signals.  In these cases, the reciever will not recognize the signal as being valid and will flag.

Bob
in California

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