APN59 Radar Stab Problem

[BlackBox]

Our shop has been having this radar stab problem on a '63 model since last Friday and we've been working it everyday since. The original write-up was a simple "Radar stab inop".

Right away we tried flying the gyro and got nothing; no gyro-following from the antenna at any point after turn on when it was in STAB. It would scan, sector, tilt and beam change just fine. The first shift to work it changed a gyro: no fix. Then they changed an ECA: no fix. They then changed the antenna: no fix and after a few minutes of power up apparently the new antenna started smoking :eek:, so they QDR'd that one and ordered another antenna for my shift to change. They also replaced the stab gyro cannon plug with a pin-type because the old solder-type plug had two broken wires.

My shift and I changed the second one: no fix. During the test of the second antenna I saw that, after about 3 minutes, the antenna gimbal would "unlock" and just kind of go limp since the gyro was still not spinning up. We also lost the ability to beam change or tilt the antenna at that point but it would still scan. The Nav's scope sweep also stopped but you could tell the antenna was still spinning and getting returns because of the heading flash and indications on that stopped sweep line.

I went and checked the fuses and found fuse F101 (4A 250V) was blown. We had only checked voltages off the RT and hadn't popped out and checked the fuses prior to the run so I wasn't sure if it had already been blown or not; we think most likely not. After replacing that fuse and turning the system back on the antenna will return to the null position. We also found that without the gyro connected the radar works and doesn't blow that fuse.

So we started testing the wiring from the gyro cannon plug to the radar nose junction box (JB2) to be sure the gyro plug was re-pinned correctly and it was. We did find that if you removed the wire from Terminal 19 at the JB2 that goes to Pin K on the gyro plug that the fuse would no longer blow. That wire runs the delayed 115VAC signal to the gyro.

According to the GS and 2-8, the ECA waits 3 minutes before allowing the gyro to apply stabilizing signals to the antenna to allow time for gyro warm-up. That makes sense because the R/T does the same thing for MAG current. But the gyro never feels like it spins up. We'd swapped gyros with a known working plan just to rule out bad parts from supply as well: no fix. Going with the logic that the R/T generates all the power for the parts and maybe something internal was blowing that fuse, weekend duty swapped an R/T: no fix. That fuse F101 stills blows after about 3 minutes of run time. Something is grounding all that power on that circuit but we can't seem to locate it. The gyro never seems to get power even though 115VAC is getting to the plug.

I'll get some wiring diagrams scanned in later today if I can. This has been driving me absolutely nuts because we've checked every wire that has anything to do with stab (even going back to the slaving control in the N-1 Compass, which we swapped with the No.2: no fix) and everything checks good.

[Bods]

I had a stab power issue a few years back on a 63 model, ended up being the cannon plug jack at the pressure disconnect. Difficult to get to cause it's under the floor, have to go behind the co-pilot's c/b panel. Ours was litterally burnt and pins were shorting to eachother. If I was still back there I could look at the old log books and get you more info, sorry. Hope you make some progress.

[INS/Dopplertroop]

Just a word of caution. If you're troubleshooting wiring don't use the the Fault Isolation diagrams solely. Be sure you're using the 2-11 wiring diagrams. The FIs do not show every and all wiring contact, connector, J-Box terminal, or device on a system run. Had an INS heading anomaly being worked on for several days. Found out everyone was using the 2-8 FI flow diagrams. They totally overlooked an untorqued nut (resulting in a loose wire) on a terminal board, in a JBox. The FI didn't show the TB or JBox. Found it for them immediately in the WDs.

[BlackBox]

Thanks for the responses.

Bods, I know which plug you're talking about. I'll pop the panel and look around in there.

Dopplertroop, we've been using mostly the 3 page section in the wiring diagram and only went to the FI for the initial troubleshooting. As most of you know, the F.I. is only good for a booster seat most of the time.

One of the guys on days found a ground in the Nav's JB1, and when removed, the gyro got power and spun up but, of course, the antenna wouldn't follow or stabilize with it. That ground is connected to terminal 13 and both indicator power supplies and the synchronizer have a wire running to it. We're going to trace the wire back through to the gyro tonight. He also found that when they remove 1 of the 3 cannon plugs off the antenna (the big one) the CB didn't pop but I doubt it did much of anything else either.

[BlackBox]

we pulled the floor panels on the co-pilots side to check out the plugs where the wires for everything go through the floor (behind the Lox converter in the wheel well) and they weren't actual cannon plugs but instead were pass-through connections so the wires didn't actually have any soldered connections there. It was pretty dirty but nothing seemed bad enough to cause our problem.

[tinyclark]

That ground is the 115VAC common. no wonder it was causing issues. The largest connector on the antenna does contain all the signals needs for antenna stab. If the gyro seems to spin up now and everything works with that connector removed, then my guess is another bad antenna.

It's sure a good thing the USAF decided to get rid of the back shops, or you could just pull it and take it in for a bench check. That would be too damn simple.

[TSgtRet]

That ground is the 115VAC common. no wonder it was causing issues. The largest connector on the antenna does contain all the signals needs for antenna stab. If the gyro seems to spin up now and everything works with that connector removed, then my guess is another bad antenna.

It's sure a good thing the USAF decided to get rid of the back shops, or you could just pull it and take it in for a bench check. That would be too damn simple.

OMG are they still doing "POMO"? I figured that would have died off long ago! Biggest thing we noted was getting a guy that had only ever been in either AGS or CRS and being assigned to the opposite at his new base. Add to that the experience with the individual units you pick up on the bench being applied on the flightline when troubleshooting something like this.