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16th Jul 2011, 08:32
Hi folks,
Your learned assistance is requested in attempting to solve a problem with the avionics in my 28v C-172N.
It has been fitted with a Michel TKM-MX 385 (Nav/Comm 1) and a BK KX 125 (Nav/Comm 2) for several years, the MX 385 having been fitted for 10 years and the KX 125 installed brand new about 5 years ago.
Over the past 24 months or so, the Nav output to the CDI from the MX 385 has been failing, GS being the worst, sometimes not displaying a signal until < 5 NM from THR and < 2.0 degrees from CTL. The CDI display when tuned to VOR has varied intermittently throughout that period.
In more recent months the Nav output failed altogether, followed by the Comm output.
The a/c was taken to an avionics workshop who swapped out the MX-385 with a set of their own (not present myself but I presume it was an ARC RT 385 or similar) and everything worked fine so it was assumed (oh oh!) the MX 385 had passed on.
A brand new MX-385 has been purchased and installed (straight swap over)but the problems persist.
The TKM-MX 385 was checked yesterday and I personally stayed with the avionics engineer whilst he checked it.
The 2 x Nav signals from the Triplexer were both OK. The antennas, both Nav and Comm, tested at the rack connectors, received clear, strong signals.
The MX-385 was tested for functionality and passed all tests, displaying positive test signals on the Course Deviation Indicator. It even picked up a local (30NM) VOR/DME on the ground and had a correct indication.
The Comms was tested for receiving and local ATIS and Tower frequencies were clear as a bell, and also tested for transmission on 126.900 MHz, which indicated a Tx frequency of 126.902 MHz, well within acceptable limits I understand.
The KX-125 was put back in and the pilot went to leave. 2 minutes later I got a call from him that the MX-385 had stopped working again but as he had to leave he had to make do with the one box, although that has its own problem.
As for the KX-125, the KhZ Comms frequency selector has failed in that it is nigh on impossible to select frequencies although it rotates OK without slipping, the problem apparently being with a micro-switch behind the frequency selector which I understand is a known failure point on this unit. That's going off for repair.
Could there be some sort of incompatibility problem between the MX-385 and the KX-125, although this has been a progressive problem and I cannot think of anything that seems to be the trigger, unless it has something to do with the failed microswitch.
I'm thinking that the problem could possibly be resolved by having the old MX-385 bench tested and, if it passes all tests, having that installed as Nav/Comm 2 in place of the KX 125. It will require a second CDI (replacing the KX 125 LED CDI) but that may be easier to read in any case.
I am once more in a state of confusion.......nothing new there!! :confused:
If anyone has come across similar problems, your earned advice would be greatly appreciated. Many thanks. :)
Your learned assistance is requested in attempting to solve a problem with the avionics in my 28v C-172N.
It has been fitted with a Michel TKM-MX 385 (Nav/Comm 1) and a BK KX 125 (Nav/Comm 2) for several years, the MX 385 having been fitted for 10 years and the KX 125 installed brand new about 5 years ago.
Over the past 24 months or so, the Nav output to the CDI from the MX 385 has been failing, GS being the worst, sometimes not displaying a signal until < 5 NM from THR and < 2.0 degrees from CTL. The CDI display when tuned to VOR has varied intermittently throughout that period.
In more recent months the Nav output failed altogether, followed by the Comm output.
The a/c was taken to an avionics workshop who swapped out the MX-385 with a set of their own (not present myself but I presume it was an ARC RT 385 or similar) and everything worked fine so it was assumed (oh oh!) the MX 385 had passed on.
A brand new MX-385 has been purchased and installed (straight swap over)but the problems persist.
The TKM-MX 385 was checked yesterday and I personally stayed with the avionics engineer whilst he checked it.
The 2 x Nav signals from the Triplexer were both OK. The antennas, both Nav and Comm, tested at the rack connectors, received clear, strong signals.
The MX-385 was tested for functionality and passed all tests, displaying positive test signals on the Course Deviation Indicator. It even picked up a local (30NM) VOR/DME on the ground and had a correct indication.
The Comms was tested for receiving and local ATIS and Tower frequencies were clear as a bell, and also tested for transmission on 126.900 MHz, which indicated a Tx frequency of 126.902 MHz, well within acceptable limits I understand.
The KX-125 was put back in and the pilot went to leave. 2 minutes later I got a call from him that the MX-385 had stopped working again but as he had to leave he had to make do with the one box, although that has its own problem.
As for the KX-125, the KhZ Comms frequency selector has failed in that it is nigh on impossible to select frequencies although it rotates OK without slipping, the problem apparently being with a micro-switch behind the frequency selector which I understand is a known failure point on this unit. That's going off for repair.
Could there be some sort of incompatibility problem between the MX-385 and the KX-125, although this has been a progressive problem and I cannot think of anything that seems to be the trigger, unless it has something to do with the failed microswitch.
I'm thinking that the problem could possibly be resolved by having the old MX-385 bench tested and, if it passes all tests, having that installed as Nav/Comm 2 in place of the KX 125. It will require a second CDI (replacing the KX 125 LED CDI) but that may be easier to read in any case.
I am once more in a state of confusion.......nothing new there!! :confused:
If anyone has come across similar problems, your earned advice would be greatly appreciated. Many thanks. :)