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Old Feb 07, 2012, 04:44 PM
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United States, UT, Logan
Joined Feb 2012
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help with UM T-28 aileron servo, and TBL problem

I recently bought a UM T-28, at first it was lots of fun but it's become a real nightmare, the main problem came from the faulty linear servo that controlled the ailerons, it jammed all the time and cause many crashes and soon the plane was unflyable, i contacted customer support, they sent me a new servo and fuse, and i was up in the air, but soon enough even the new servo fell the the same fate, jamming, worse than the first one, i can't even cycle the ailerons 5 times without it jamming and me having to poke at it to get it to unjam, i've looked around inside and it isn't catching on anything that i can see so I'm just out of ideas, i really want to fly this fun little plane but can't because of this terrible little servo, any ideas or help?

on a lighter note, i also installed a set of tiny brite lights, when i first plugged them in i turned them off easily with the gear switch, now they are stuck on and i can't control them, any ideas?
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Old Feb 07, 2012, 05:42 PM
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United States, NY, CENTRAL VLY
Joined May 2011
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I've had only one of those servos out of dozens actually fail.

I have had almost every single one of them at one point jam though. If you take a close look, maybe even under magnification, you'll find a bit of grit somewhere in the gear. They are relatively soft nylon gears and are prone to collecting dirt.

This is compounded if you fly around ballfields, etc where sand can get kicked up. Even though it's in the fuse, the prop is grabbing everything it can and hurling it back. I usually have to clean out my sons' Champs once a week.

Spin the gear by hand and you'll probably feel a spot where it drags. A brush with rubbing alcohol will do the trick.

I'm not a shrill for Horizon, I just found a few steps will keep me in the air more than waiting for replacement parts. Those servos really are amazing for their size, but can get tripped easily.

My tiny brite lights weren't switchable, it went in the aux channel port, but stay on whenever the battery is plugged in, so I can't help with that.
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Old Feb 07, 2012, 07:11 PM
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thanks for the tips for the servo, ill spray some wd-40 in there and blow it out with a compressed air can. As for the lights i figured them out, i was using what i thought was a micro extension cord but it turned out to be a reversing adapter, not sure how or why but that made the lights stick on, one i got the correct extension cable plugged in things worked fine, i have them plugged into the 4 prong gear port in the middle of the Rx, they only take up 3 prongs of course but now work fine, and turn on and off with my gear switch.
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Old Feb 07, 2012, 08:04 PM
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I wouldn't use WD40, you don't want to get any on the contacts below and any type of lube (except dry graphite) will only attract more dirt. Definitely blow it out with compressed air though and roll it back and forth. Sometimes the grit can cause a burr on the gear which can be fixed carefully with an X-acto.
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Old Feb 07, 2012, 08:07 PM
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Quote:
Originally Posted by hermanmango View Post
thanks for the tips for the servo, ill spray some wd-40 in there and blow it out with a compressed air can. As for the lights i figured them out, i was using what i thought was a micro extension cord but it turned out to be a reversing adapter, not sure how or why but that made the lights stick on, one i got the correct extension cable plugged in things worked fine, i have them plugged into the 4 prong gear port in the middle of the Rx, they only take up 3 prongs of course but now work fine, and turn on and off with my gear switch.
Does your light set have red, green, blue and white, with a resistor along the way with heat shrink tubing on it? Are there any visible switches? I wonder if I can get mine to switch on and off too.
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Old Feb 07, 2012, 09:06 PM
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United States, AK, Fairbanks
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Make sure your aileron throws are set below 95%. If set at 100% or higher you risk the servo locking up or servo damage.
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Old Feb 08, 2012, 12:08 AM
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United States, UT, Logan
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my light set is a 6 light, 3 white, one white strobe, and a red and a green all attached to a blue board.

as for lowering the throw in the servo, that can help, but I shouldn't have to resort to that to avoid crashing, the servo should have no problem going 100%, i can understand if pushing it to 125% caused a jam, but i should be able to use the servo to the capacity it was designed.
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Old Feb 08, 2012, 01:00 AM
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Quote:
Originally Posted by hermanmango View Post
i should be able to use the servo to the capacity it was designed.
In theory, yes you should. In reality, you will save yourself from lock ups by backing it off 5 percent from each direction. I have had this same problem with both of my Ts and fixed it by reducing the throws.
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Old Feb 08, 2012, 02:10 AM
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thats what I'll probably end up doing, the little T-28 doesn't have enough servo movement on the aileron anyway, but it's better than crashing, lol
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Old Feb 08, 2012, 01:10 PM
aka: A.Roger Wilfong
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Novi, Michigan, United States
Joined Jan 2001
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I've had what sounds like the same problem with a flat foam Zero that uses a brick out of an Ember. Once in a while, the rudder will stick full over and it just spirals in. It usually happenes at the beginning of a flying session, just after I've unpacked the plane.

Check the keeper on the end of the servo screw shaft. It's the little black thing on the linear pot shaft sticking out of the end opposite the gear. If you wack the correct control surface, you can put enough pressure on the servo that the shaft will slide and push the keeper loose. This allows for end play in the servo and can left the pot gear disengage from the motor pinion. Wiggling the control surface will pull the shaft back enough tfor the gears to engage, so it appears to work ok for a while.

An easy check for this is to look at the servo while gently flexing the cotrol surface. If the servo horn moves, the keeper is too loose. If it moves enough that the gear disengages - it's really too loose.

The fix is to push the keeper further onto the shaft. Hold the gear tight to its end of the pot and use a pair of tweezers or needle nose pliers to push the keeper tight to its end. Don't grab the keeper itself. Put the tweezers/pliers on the distal end of the shaft and press against the keeper. I suppose it is possible to get it too tight, but you'd really have to push hard.

With my Zero, the plane used to rattle around inside the box I carry it in and, since the rudder sticks out the farthest, it keeps getting wacked and keeps pushing on the servo. I added a block of foam to the box to keep the plane in position, but I also check the end to end play on the servos before I begin flying.

- Roger
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Old Feb 08, 2012, 03:24 PM
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Key question is dopes the servo stick along the travel perhaps at various spots, or only at the ends?
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Old Mar 04, 2012, 11:27 AM
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United States, AZ, Chandler
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Fully resolved this issue on my UM T28

I had this same problem. After much fiddling around I figured out that the lip on the base of the motor gear was binding with the other gear. I used a small screwdriver to push the gear as close to the motor housing as possible.

Now the servo travels fully without any glitches whatsoever.

Such an easy fix but not really obvious. The lip is nearly microscopic.

Hope that helps! -Randy
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Old May 04, 2012, 11:58 PM
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United States, WI, Sheboygan
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Quote:
Originally Posted by makg View Post
Make sure your aileron throws are set below 95%. If set at 100% or higher you risk the servo locking up or servo damage.
Do you need a Computer radio to set the throws like that?
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Old May 05, 2012, 04:22 AM
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United States, AK, Fairbanks
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Quote:
Originally Posted by FlyGuy3345 View Post
Do you need a Computer radio to set the throws like that?
Yes.
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