Trim Servo "Dead Spot"

I'm wondering if anyone else has any experience with this problem on the RC Allen Trim servo....

I seem to have a "Dead spot" when I am trying to trim nose down. I can hear the relay click, but I get no trim motion, I can trim up a little bit, and then it will trim down again. It will work fine for several more cycles, and then appear to be in a dead spot again. Nose up, then the nose down works again. Sometimes it takes two blips of nose up to get it to work nose down. A very strange problem...I suppose it COULD be a contact problem in the relay as well, but I'm not sure...

Just curious if anyone else has seen this, and if there is a fix other than a new servo.

Paul

Servo Relay

We have had to replace the servo several times due to the same issue. Thay have been very good about it and acknowledge the problem. They have replaced the unit without charge on both occasions.

I had to replace the servo relay deck after about 185 hours of flying. I would start there as they are somewhat fragile and only run about $39 at www.rayallencompany.com. I had identical symptoms until the servo relay deck had a total failure.

Easy enough to check..

Clip a voltmeter across the two white (no stripes) wires, as close to the servo as possible, and hit the trim switches. Assuming all of the wire connections are tight:

If the controls are working (relays etc) you will see the same voltage in either direction when you hit the switch (may be less than full system volts if you have speed reducers installed). The polarity will change with the direction you trim but the volts should be the same.

If you have good volts both ways, the problem is downstream to the motor, possibly a dead spot in the commutator (junk builds up from the brushes or they wear through and that creates hi resistance spots). These things have cheezy little motors in them so commutator wear would not be out of the question.

If the voltage droops in both directions, it may indicate mechanical binding (motor pulls excessive current both ways).

Voltage good in one direction but not in the other may indicate a bad relay / switch contact upstream. The motors will generate a little arc on the switch/relay contacts when you release the button. That can pit the contacts just enough to make the operation intermittant.

Finally, check the coil voltage at the relays themselves. A bad control switch can cause low volts there and the relay operation can be intermittant as a result. Make sure the relay coils are snubbed with diodes to eliminate arcing at the switch contacts when they open.

Good luck!
John

Ironflight said:

I'm wondering if anyone else has any experience with this problem on the RC Allen Trim servo....


Paul

Click to expand...

Paul,

I had the same problem at about 125 hrs. It turned out to be the Ray Allen relay deck.

They replaced it for 1/2 price.

Ben Beaird
Plymouth, WI
-6A

Thanks!

Well, I guess I'm glad I'm not the only one....

I went out again and trimmed up and down a number a of times, and now can't recreate the failure....next time it's stuck, I'll pull the empennage fairing and do the checks to see if it's the relay deck or the servo....I've gotta leave on a trip on Monday, so I guess I'll just keep the trim close to neutral and hope it lasts a few more flights!

Paul

Paul I just replaced mine in the 8 after about 150 hours. It was the Relay Deck.
THey charged me $15. I was satisfied with that. Until this thread, I thought it was a fluke. I guess not.
Best,

Trim servo

I used the Ray Allen stick grip and switches with my servos, therefore no relays. Good for 130 hours.

In the scheme of things these servos are not very sophisticated. I have some servos for my giant RC planes that have more torque with all metal gears that would probably work.

A couple of locals have also had trouble with the relays. I wish I would have installed the rate switch to slow the trim at cruise speeds. Seems I'm always between too much and too little.

Well, I flew 4.5 hours today, Houston to Titusville, and the trim hung in there - but I had to tease it a number of times. As soon as I get back, I'll pull the Emp. fairing and wring out the circuit to see if it's the relay or the servo. From all the votes above, I bet it will be the relay deck - dang, that measn pulling the panel! Oh well.....

I agree with Darwin about the servos - when I first saw them, I thoguht of the sail servo in my R/C Sailboat - just about the same size, with the same amount if torque!

Paul

Hi Paul,

Add another bad relay to the count. I had the same problem around 100hrs with an occasional hang up while using the trim. I could tease it like you said, a little opposite direction then it was fine, but it eventually wouldn't work at all. Give Scott Menzimer at Ray Allen a call (760-599-4720) and he'll probably have a new relay waiting for you when you get home. He'll only charge you half price since it's your second relay, but return your faulty unit once you remove it and he'll refund that money after he tests it and makes sure it was a bad relay and was not fried by the wiring in your plane (which I'm sure it wasn't).

Have a safe return flight!

Paul has been uncovering all sorts of gremlins. First the dimmer and now the relay decks. How come nobody complained about these lousy parts before? I would have bought something else had I known in advance.

I also find it odd that everybody is happy to buy another cruddy relay for 1/2 price after the first one goes TU after only 125 hours. I'd call that lousy customer service.

Good Point!

Davepar said:

Paul has been uncovering all sorts of gremlins. First the dimmer and now the relay decks. How come nobody complained about these lousy parts before? I would have bought something else had I known in advance.

I also find it odd that everybody is happy to buy another cruddy relay for 1/2 price after the first one goes TU after only 125 hours. I'd call that lousy customer service.

Click to expand...

You make a good point Dave - I think part of it is that I've put 225 hours on in 10 months, so I am kind of doing accelerated lifetime testing for the fleet....

Paul

Good Service!

Just to follow up, I called RC Allen today and told them about my problem. We chatted for awhile about my troubleshooting so far, and what I had read here, and he agreed that it was the relay - and offered to send one out free of charge. An interesting point that he made is that people who have relay failures frequently have additional relay failures....in other words, he suggested that there must be something about the installation that could be a problem. That actually isn't surprising - mine is screwed on to the bulkhead ahead of the instrument panel, and I have often wondered if that might not be a vibration test bed....I am going to install the new one by simply wire-tying it to a cable harness, and see if that isolates it.

At the same time, I mentioned that my roll trim indicator hasn't worked since my first flight - you can wiggle the wires in back, and make it come on or not. Since I installed it with little bitty #2 screws, I didn't have any kind of nut plates for it, which means I have nuts on the back of the panel....which means to replace it, I have to pull the panel. I have put off doing anything about it for that reason. Long story short, one of those is on the way for half price, and they'll dissect the failed unit when I send it back and make good on that as well if it is justified.

Since I was on a roll, and have to pull the panel to fix the trim (I make a big deal of it - it takes all of ten minutes!), I also got on the web to the folks who make the LC-40 (Flight Data Systems), and told Chuck about my shorted board. After a quick exchange, I think that I might have induced the problem by letting my service loops rub up against the exposed components. He offered to send me the newer model (enclosed case) for basically half price....

Dealing with companies that serve homebuilders - and are run by homebuilders - is really great, isn't it!

Paul

Prolly overkill for what you are doing

I used a couple of these for my trim functions. Solid state relays (DC motor chips) should last for a really long time...

http://www.f1-rocketboy.com/tcm.html

Ironflight said:

...That actually isn't surprising - mine is screwed on to the bulkhead ahead of the instrument panel, and I have often wondered if that might not be a vibration test bed....I am going to install the new one by simply wire-tying it to a cable harness, and see if that isolates it.

Click to expand...

I was just debating about whether to attach the relays to the plane structure or just wire tie them to the cable bundle. I'll go with the later. It's easier and may isolate vibration.

It's good to hear your dealings with both companies turned out well. My dimmer has the enclosure, so hopefully that will prevent any problems.

Thanks for the follow up.
Dave

Kraft RC

Ironflight said:

I agree with Darwin about the servos - when I first saw them, I thoguht of the sail servo in my R/C Sailboat - just about the same size, with the same amount if torque!

Paul

Click to expand...

Jim... I believe there is a relationship.

MAC (the original company) was based in Vista, CA... which was also the home of Kraft RC - one of the last US producers of RC equipment.
When they first started out, they told me the family relationship that existed with Kraft employees....

Noticeable, since Vista isn't exactly one of the industrial areas of CA.

It's just a special RC servo...

Who's old enough to remember Kraft RC products?....

gil in Tucson

I have the same problem with my elevator relay presently.

Scares the heck out of me. i have to go full up trim, in flight, and toggle it sometimes up to a min. to get it to go back down again. Probably 30# of force on the stick to keep from looping if I let go.