Comant CI-505 Triplexer -- Potentially Bad?

[MisfitSELF]

Airplane:  1984 M20J with dual VORs (the original set from 1984!).

For a number of months, my VOR reception has been a spotty.  By spotty, I mean at mid ranges (20-40ish) miles, the needles would twitch, sometimes lose lock and display the "nav" flags.  VOR1 was always a bit worse than VOR2.  As I get closer to the stations, it clears up.  As I listen to the stations, they morse code is very hard to hear.

Obviously (or maybe not so obviously), I have some sort of signal loss thing going on.  Where I could, I have unhooked the BNC connections at various points and reconnected them.  But I have gotten no improvement.

So I suspect I have (in no particular order):
1.  Bad antennas (but I have two on the tail so probably not likely)
2.  A Bad combiner at the base of the tail (this is "could" be)
3.  Bad cable from the tail to the triplexer
4.  Bad triplexer
5.  Bad radios or cables from the triplexer (again I have two and both behave this way)

The cable from the tail forward would be a huge pain to replace, so right now I'm focused on the triplexer or the combiner as the likely causes.  Except how often do these fail?  They are ~35 years old, so ok they just might.  The other reason I'm focused on #2 and #4, is that they seem to be the easiest just to replace.  But the triplexer costs ~$230, so before I just start throwing money at this I want to see if anybody else has had similar issues and if they determined their cause(s).

Thanks,

Bruce

[ArtVandelay]

If it were me, I would disconnect cables, clean connectors with electronic cleaner, and reconnect.


Tom

[MisfitSELF]

Done that already.

[takair]

They do fail on occasion.  An easy way to troubleshoot, without buying new, is to bypass it and hook up to one side for a test flight.  Of course, an avionics shop could do this on the ground with a signal generator.

[N201MKTurbo]

Get a male to male BNC and bypass the triplexer. Hook the antenna directly to one radio and see how it works.

 

[EricJ]

10 minutes ago, N201MKTurbo said:

Get a male to male BNC and bypass the triplexer. Hook the antenna directly to one radio and see how it works.

 

I've been trying to sort out why there'd be a triplexer in there...?

[N201MKTurbo]

1 minute ago, EricJ said:

I've been trying to sort out why there'd be a triplexer in there...?

I assume he is talking about the NAV splitter.

[Andy95W]

The "triplexer" (splitter) splits the VOR antenna signal to feed VOR 1, VOR 2, and a glideslope receiver.

[MisfitSELF]

That's precisely right Andy.  The Ant gets split to VOR 1, 2 and Glideslope.

If the rest of you are curious it's a Comant CI-505 as mentioned in the subject line.  I like the idea of getting a male to male adapter and trying one VOR at a time.

 

Bruce

[EricJ]

1 hour ago, Andy95W said:

The "triplexer" (splitter) splits the VOR antenna signal to feed VOR 1, VOR 2, and a glideslope receiver.

It's kind of a misnomer.   It's sort of a hybrid between a splitter and a diplexer.   It's not really a "triplexer" since it doesn't isolate three bands, only two (assuming it even does that and isn't just operating as a splitter).

In addition to replacing the "triplexer" with a barrel connector, you could replace the "triplexer" with a regular three-port splitter (a cable splitter would work) and just drop one of the receiving devices, e.g., connect both VORs but not the glideslope.   If performance comes back for both VORs, it was probably the "triplexer". 

 

12 hours ago, MisfitSELF said:

So I suspect I have (in no particular order):
1.  Bad antennas (but I have two on the tail so probably not likely)
2.  A Bad combiner at the base of the tail (this is "could" be)
3.  Bad cable from the tail to the triplexer
4.  Bad triplexer
5.  Bad radios or cables from the triplexer (again I have two and both behave this way)
 

If the antenna is in mechanically good condition, i.e., nothing loose, broken, or bent, the most likely problem with it might be corrosion of the connection to the ground plane at the base or in the connector. 

If the cable is the original from the factory it may well be the problem.   Sagging over time, chafing or fraying of the shield, kinks or bends, or corrosion in the connectors can all cause issues.   If a pinhole or larger leak developed anywhere in the cover, the shield and center conductor may be corroded or even shorted.   The run is long enough that there may be connector(s) along the way to facilitate assembly/disassembly such that it is not a single piece.   If so, those connector(s) are also potential problem areas for corrosion, etc..

The "combiner" could be replaced with a barrel connector (male-male BNC connector) to just use the VOR antenna for testing if the "triplexer" is ruled out.

If the "triplexer" and combiner are eliminated as candidates, the cable may be the most likely culprit.

How old are the VOR receivers?  Are they the same type?

 

 

Edited February 24, 2019 by EricJ

[67 m20F chump]

I had a blade antenna go bad once.  Water got into it.  It’s an expensive antenna.  The plane had 2 antennas and they had different ohm readings with a multimeter.  Been a few years but I think that’s how I found it.

[Jerry 5TJ]

20 hours ago, MisfitSELF said:

......
So I suspect I have (in no particular order):
1.  Bad antennas (but I have two on the tail so probably not likely)
2.  A Bad combiner at the base of the tail (this is "could" be)
3.  Bad cable from the tail .........

The Nav antenna on the tail is probably one of these:

If so the two rods are part of a single antenna.   At the mid-point the coaxial cable is connected to two screws.  That’s a likely spot for corrosion and for water to enter the coaxial cable.  

The coaxed cable passes through the empennage hinge area and is flexed a bit with each adjustment to pitch trim.   Not much movement but in 35 years of flexing it can fatigue.  

Edited February 24, 2019 by Jerry 5TJ

[tigers2007]

For what its worth I've had a cable TV diplexer stop working. I have no clue why as I thought it was a solid-state device but I remember watching the Charter Cable guy replace it and then everything worked normal again. Maybe you can measure the resistance and somehow calculate the SWR? Anyone have an aviation band SWR meter he can borrow?

[MisfitSELF]

 

Jerry,

My antennas are definitely not the "wire" kind.  I've got two big "shark fins" on the top of my tail. 

I have a ten pack of BNC splicers on order from Amazon to help trouble shoot.  My plan is to connect one to splice the ANT to the VOR1 then taxi or fly around an note if anything improves.  Why did I get ten?  It was the smallest set I could find quickly on Amazon and it cost all of $6.99 and they are delivered to my door by Tuesday!  I would have spent more in fuel to drive to Best Buy or where-ever and then spent like $5 to get one!

Bruce

[EricJ]

2 hours ago, tigers2007 said:

For what its worth I've had a cable TV diplexer stop working. I have no clue why as I thought it was a solid-state device but I remember watching the Charter Cable guy replace it and then everything worked normal again. Maybe you can measure the resistance and somehow calculate the SWR? Anyone have an aviation band SWR meter he can borrow?

Fundamentally a splitter (which is not quite the same as an n-plexer), is just a transformer that allows distribution (or combination) of rf signals.   Since nothing is perfect, there are often a few other components in there on a little circuit board to help control impedance, frequency response, etc., etc.   Corrosion on the connections or circuit board, cracked solder due to thermal cycles, etc.,  may happen that occasionally make them go bad or change characteristics.   So even though theoretically there's nothing active or very age-dependant in there, things can happen to them.

Here's a typical cable splitter with a porn shot of the unit.   This one has two transformers, labelled T1 and T2:

https://www.solidsignal.com/pview.asp?p=at-705

Edit:  This one has a few extra components because it passes DC to power a unit (typically an amplifier) on the far end.

Here's a pic of a transformer-only cheapie.   They're usually potted to keep stuff together:

 

Edited February 24, 2019 by EricJ

[skykrawler]

It should be pointed out that the 1-to-2 and 1-to-3 couplers or splitters introduce a (relatively small 1dB) amount of signal loss that is a specification of the device.   They need to work with Frequency 108-118  and 329-335.3 MHz.

Each connector adds a .25 dB loss (for a new connector/crimp) and in my J there are two or more connectors from towel rack antenna just in the tail cone.   I have always felt my VOR reception was a little weak and noisy but have not learned of any good ways to test without specialized equipment.

So I'm interested in what you find.

 

[skykrawler]

Apparently some people call them triplexors or whatever.....

Here is a link to a coupler for $156:

https://www.aircraftspruce.com/catalog/avpages/av571coup.php?clickkey=23595

[Yetti]

There is also most likely a balun in the tail attached to the antenna.   Might start with cleaning all the connections in the tail.

[Aerodon]

Too little information:

1) what antenna do you have on the tail - towel bar, two blades or cat's whiskers?  There's lots of room for antenna failure, balun failure, bad terminals, combiner failure etc.

2) then probably have RG142 Co-ax - time to upgrade to RG400.  

3) what radios do you have?  Some have separate ILS and VOR RF connectors (KX155) and others have internal splitters and 1 RF connector (SL30).  If you've had any sort of upgrade, I would check that you still have the right splitter installed.  Needs to be an exact match, antenna to splitter, ILS to ILS, VOR to VOR and no empty ports.

My bet is on #1 above, easy enough to get all this checked out by an avionics shop.

Don

[JeffreyC]

I've got a similar problem. Dual towel rack antenna through a Comant CI 1125 diplexer to a Garmin 430 and a KX 170B. The VOR's both work today but but only work about 50% of the time. The 430 quits first then a couple weeks later the KX170B's will quit. Then they'll both come back a few months later. Seems like temperature sensitive? The 430 always quits first. Don't use them so haven't been in a hurry to troubleshoot.

[carusoam]

Welcome aboard Jeffrey.

Lots of details on modern cables and cleaning procedures for connectors around here...

Clean them up and use them... 

Best regards,

-a-

[FLYFST]

On 2/23/2019 at 12:06 PM, MisfitSELF said:

For a number of months, my VOR reception has been a spotty.  By spotty, I mean at mid ranges (20-40ish) miles, the needles would twitch, sometimes lose lock and display the "nav" flags.  VOR1 was always a bit worse than VOR2.  As I get closer to the stations, it clears up.  As I listen to the stations, they morse code is very hard to hear
Bruce

Bruce, I realize it has been a while since you posted this, but what did you find?  Did you get the problem resolved? How?  I'm dealing with a very similar situation on my 1990 J to the one you describe.

thanks - Hank

[MisfitSELF]

Some follow up.  Had my VORs adjusted and that help some of my heading error.  My DME is of course on a different antenna and it still sucks even after swapping to a new antenna.  Might be a cable issue, might be a KNS-80 issue.  My solution:  I'm scrapping my KNS-80 and upgrading to a Garmin 750.  

 

Bruce