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ARN147 VOR/ILS and the digital A/P installed.--not working on final...


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Strange, I came here to ask for help, and found a different post with part of my question.

--So we have a MC-130 with a vor issue. it's not working right. very long story short (day 10) both our vor 1 &2 cut out in 357 to 358 degrees range while turning the course knob on the hsi. the fail nav flag comes in view.--no it's not a loss of signal, and I've r/r receivers, flight director's computer, hsi,adi, bdh'si, Auto pilot processor, swapped out the whole nirp panel, and more. We've done a direct connect with the test set-so the antenna is eliminated as an possibility. We've disconnected just about everything.. none of our other aircraft have this dead spot.

has anyone seen a "dead spot" while adjusting the course heading? what the pilots see is the flag popping in and out during final, so as the aircraft moves, the course heading changes and comes across the dead spot so the flag drops.

--also, we noticed that while the vor was on, and tuned to the test freq or any freq--but with the test set off, we notice a strange thing. the cdi bars wonder a bit. they move back and fourth, with no pattern. They should be dead, and the flag is coming in and out. I believe this is a a bad grn, or some kinda resistor?

has anyone seen this?

we've found lots of small issues and been addressing them as we find them..

Help-- any ideas??

Jeremy

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I'm out of town and don't have my reference materials with me, and I have never seen a real dead spot as you call call it. It is strange that the Nav flag is popping into view.

Of "course", the course set knob sends the information to the VOR receiver so it knows how to send out the display signals to the FDC, ADI and everywhere else they go.

The signals are all referred to as the OBS data. Depending on which signal is lost, it can cause the CDI bar to line up incorrectly on the az pointer, but it will still USUALLY give the two dot deflection for 10 degrees. I would ring out all the OBS lines from the Receivers to the ADIs. Did this thing have any major work done on it right before this started? Something like an ISO, window change, work underdeck or behind the intrument panel? We had a very strange TACAN problem one time, realized that one of the terminal boards was changed. The jumper bars were not reinstalled. AT least they got all the terminals on the right lugs. I may have some drawn up info on those OBS lines to make it easierr to follow, butwon't be back home until Monday. You all are very brave swapping the NIRP panel.

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Found it.... Dam this was butt kicker of an issue. so we found that the VOR recivers connectors had one wire in a unused pin positon. the Little K and little J where swaped. This was not providing the proper ground for the processing of the signal.

-we wern't looking for a missed wiring issue. It worked after all, we just had a small dead spot. we thought we had a bad grn or somthing dragging down the signal. Never thought someone previsly might have missed wired the acft.

--So ask, when did someone rewire the connector? Looks like a long time ago. The wiring job is top noch, and looks like it's been thier a while. No one remembers anyone rewiring the VOR recivers, and I can't think of any recent mod where they would touch this.

--So now ask.. If this was thier for a while, why didn't any one notice the dead spot before? or did they just ignored it. I guess since it was such a small dead spot they ignored it. When we started this journey we had much bigger issues with the VOR system, the Nav flag came into view a lot during final aproch, so it was unreiable , we had a bad connector at the VOR cables for the antenna and some grounding issues and nirp relay issues which when banged on would make the flag pop in and out of view. Everything has been fixed or replaced. Now you can bang on every thing, and nothing flickers. We noticed that the CDI bars and scns out test seem very smoth and flawless.

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  • 2 weeks later...

Pin j is tied to chassis ground in the receiver. It was probably picking up a good enough ground through the other circuitry of the indicator to pull the flag. I don't have a copy of my internal diagrams for the indicators any more, or I'd look.

Good job finding it. I don't know if those MCs had the ARN127 installed before the 147 or not. It has probably been that way for quite some time.

We had a C141B VOR that was intermittently written up as weak, but it always checked fine on the ground with the test set. The localizer had it's own antenna in the nose, so it wasn't affected. I took the test set up the t-tail and connected it to the antenna cable, presto, didn't work. I found weird readings with the TDR as well. When the aircraft was stretched, the mod crew apparently connected the VOR line inside the wing box to the Marker Beacon antenna and vice versa. Thank God we had a real skinny guy in the shop who was able to squirm in there and change the connectors around.

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Flashback's Tiny... the aircraft could have had the OLD ARN-14 that was yanked out when the SCNS was installed... I know that the 127's had been around awhile but can't seem to remember about what time the 147's came out (CRS kicking in?). That's when all of the defective 4100 relays started showing up. The new rcvr's just didn't have the same brute force signals to work on a crappy relay. Good job on the fix CND!

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He mentioned the NIRP panel, so it had the ARN-14. Those really took a beating on the MCs. They were mounted a bit further back than the regular E models, so with the doors opened up a lot, the wind would shake them real good. We'd open the covers on the aircraft to check them for tubes that had fallen out before we took them to the shop. I think it was a good 20% fix rate.

Some of the MCs got the ARN127, 64-0523 64-0561 64-0566 and 64-0555 were on the original TCTO, more may have been added later. Hard to say. They modified all the aircraft that didn't have the 127 first, then went on later and replaced the 127s as well.

Too bad CND doesn't have a profile, or we'd know hwere he was at and what year aircraft it could be.

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Not an ARN 147 issue but at least a NavAid question. Was in the gallery looking at several C-130s at AMARC that had the ARN-14 antennas removed and wondering why. After reading this thread an guessing the AF stopped buying ARN-14 spares in anticipation of the ARN127 and later the 147 modification. And that some acft., prior to upgrade, experienced antenna faiure. With no spares available the only resourse would have been AMARC. Is this a good guess or am I in left field? I know this antenna is a sob to replace because I helped a navaid tech. replace one. Believe the fasteners were installed when the acft. was new. Recall while in RVN we had a Da Nang F-102 land at Phu Cat with a cracked windscreen. When we recieved the replacement it was from an AMARC 102.

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Larry, the VOR antenna was relaced by a new design using two rectangular antenna mounted on a panel. The panels were never manufactured, but were local manufacture. An unbelievably stupid way to handle it. When they came out, they just called it "an alternate method". So, the ARN127 or 147 mods did not include the antenna change.

The major problem with the original antennas, besides removing and reinstalling, was that they had a copper strap on the outer edge that butted up against the aluminum skin. For some reason, aluminum, copper and moisture just don't play well together. In some aircraft pictures, you can see black under the bottom of the antenna on the left side of the tail. The antenna had to be removed during a #4 iso to check and repair the corrosion.

Whether they pulled these out at AMARG because of this, or as supply requests, I don't know.

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.....and the screws (hundreds of them!) were a real bummer too! Those that didn't come out at the first go around usually had jacked up heads and had to be drilled because the last guy was too lazy to come down from the cherry picker and get some new ones from bench stock or order what they didn't have. THEN you had to worry about which screws almost came out just to have the nutplates bust loose. I was real glad to see the blades installed and that big ol' box gotten rid of. That was progress.

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