Identical Hitec servos, but one a bit tardy!

[Adrian Smith 1]

I have to fit two identical hitec 645mg servos to the two elevator halves, both operating at 6v have the same torque and speed. I only have the option of using a y-lead as I have run out of channels on my Futaba TX to slave one of them. So a simple resoldering job saw me reverse the direction of one of them to allow me to use a y-lead. However, whilst they seem to move at the same speed one of them "settles" to a stop momentarily after the other…... not what you want on an elevator. Furthermore, I coupled a Futaba servo which has exactly the same speed, torque as the hitec but rotates in an opposite sense to hitec so the two (hitec/futaba) could be used in a y-lead set up. However, these two servos function the same way with one of them "settling" momentarily after the other. Is this a function of the y-lead as I don't understand why they should do that. I am also wary of using a servo reverser for the same reason.

[Stuart C]

Whenever I've had this sort of problem, it has come down to a difference in friction between the two control surfaces - usually the hinges (either out of line or plastered with dope, glue, fuel proofer etc.). A temporary swap-over of the two servos would show if this be the case. Good luck.

[Adrian Smith 1]

Thanks Stuart will give it a try and see.

[Biggles' Elder Brother - Moderator]

Yes that would be a likely explaination. Another thing to try would be to swop the Y-lead around to see if one leg has a slightly higher resistance and so is causing a small voltage drop.

The fault could be in the position sensing of the servo - but your check with an alternative Futaba servo would seem to contradict that.

BEB

[Bob Cotsford]

I bought a batch of 645MGs a couple of months back and find they only centre reliably with a regulated 6v, on unregulated LiFes they acted like ailervators - one up, one down dependent on time of day and phase of the moon.

[Bandit]

Forgive me for asking but isn't the LiFe rated at 6.6V which means possibly the voltage is a bit high for the servos at full charge which could explain the phase of the moon behaviour. Perhaps a 4.8v Nimh could be tried to see if the performance is less phase of the moonish just as a test.

From what I understand those servos are rated to 6V.

I been on the look out for high voltage ones so I can bin the BEC in one plane I have and did look at those.

Regards

Lop-sided Bandit

[Bob Cotsford]

AFIK 6v is usually taken as a nominal voltage, e.g. a 5 cell Nixx pack otherwise why have 4.8 and 6v ratings? 2S LiFe is actually a lower voltage than a fresh 5 cell NiMh pack and gives a more stable voltage.

I use 2s LiFe on nearly everything and these are the ONLY servos that behave this way. Posts on other forums indicate that older 645MGs were ok on unregulated packs, newer ones are not, so maybe they changed the electronics at some point? Whatever, these particular servos do seem to benefit from a regulated supply.

ps - I've seen freshly charged 4 cell Nixx packs showing over 6v!

Edited By Bob Cotsford on 19/12/2013 13:30:31

[Adrian Smith 1]

Firstly Stuart ,I have disconnected the elevators and activated the servos and that lagging finish is still evident. The next course of action is to just swap the y-lead around to test resistance as BEB suggests. The truth is the job I really want avoid is having to take off the horns (of which I only have two suitable for the plane) and re drill the elevator halves to accommodate a couple of Pete Tindal closed loop sets so as the adopt the pull-pull method using just one quarter scale Hitec Servo. This has worked well in the past on planes, albeit fiddly, but trying to drill an accurate perpendicular hole for the bolt through a tapered elevator half is a total pain and I have really ever mastered it! However, I could really use the weight of 2 servos in the tail. Input as usual is gratefully received.

[Adrian Smith 1]

Bob, the setup I have is a bog standard 2700ma 6V Nimh RX pack with an ordinary futaba on/off switch. Should I be looking at doing it differently?

[Bandit]

Yeah they do show over 6v but very quickly come down from my experience.

I bought a LiFe from Hobbyking on a whim, must get it into something, you know how it is they always tell say you have x number of grams left for the shipping. It has receiver pack written on the battery but also says 6.6v so must go play and see what it is all about but wow it seems light!

Maybe they did just change the electronics.

I have to confess to being grumpy with Hitec as I bought some metal geared servos, one stripped so I opened it to find a plastic gear! If it 'sacrificial' then sell spare ones.

I don't know if it is related but a buddy has a big 110cc petrol Slick, one of his ailerons kept drooping down then when the wiring was wiggled it popped back up again. Needless to say much cursing went on about leads etc.

Sounds like a big job Adrian

[Bob Cotsford]

Posted by Adrian Smith 1 on 19/12/2013 13:38:14:

Bob, the setup I have is a bog standard 2700ma 6V Nimh RX pack with an ordinary futaba on/off switch. Should I be looking at doing it differently?

Adrian, it might be worth trying a 4 cell pack if you have one to hand. I'd definitely try BEB's idea of swapping the servos on the Y lead or even try them individually with a straight extension. If the receiver is close enough try the bad servo plugged directly into it.

Reading through the op, you say a Futaba servo plugged into that extension behaved in the same way - that does indicate a problem with the Y lead, possibly a dry joint or poor crimp that's causing electrical noise when you put a load through it.

[Adrian Smith 1]

Hmmmm…. yes Bob I tend to agree, anyway I will try this all out at my workshop tomorrow (3 miles away on a farm) and let you know what I find. Otherwise out comes the trusty drill and plan B.

[Adrian Smith 1]

Well I decided to dig out a "y-lead" except it's not like a conventional y-lead in that it's a single lead with a double socket at one end to take two servos. Sadly the same thing happened and one servo settled fractionally after the other. This occured with either two hitec servos (one reversed) or with one Hitec and one futaba servo which moves in the same sense. I only have 5 cell packs so I wasn't able to test that theory relating to 4 cells. With the more conventional y-lead I swapped servos, both hitec, but still one operates at a fractionally different speed and also with the one hitec & one futaba servo but this produced the same reaction. I couldn't see the benefit of trying one servo in isolation by sight as I couldn't tell which was "slower". Both Hitec servos appear to work fine in isolation. So I think I am now going to charge up my Dewalt drill for plan B!

[A.T.]

No problem with a fully charged 5 cell NiMH battery pack and HS-645MG.

. Servo - Operating Voltage Range - (Hitec) usuage with NiMH, Lipol & A123 batteries.

"Y" leads whether the old three wire type or newer double socket, are notorious for failure after some use or ven new. Another reason being whilst the plastic housings fit, the pins do not, refer center section and picture attached:

. After Market Accessories - Caution re Opti-kill, Duralite, Powerbox, Switches, Leads etc.

with regard to the important plastic primary gear in a metal gear servo, please refer to:

. Servo - FAQ : Metal Gear Servos - Why MG Servo have a Plastic Primary 1st Gear

Much more information available under sub sections
"Battery Care, Performance & DIY Battery Packs."
"ESC & BEC - Setup to prevent majority of all RF Problems, Brownouts & Crashes"
"Glitches & Jitter in Receiver, Servo & ESC - Causes and Cures"
"Servo - Alterations, Calculators, Databases, Leads, Repairs, Convert to an ESC or winch & FAQ."
below
"Radio Systems, Accessories, Alterations and FAQ" at
Alan's Hobby, Model & RC FAQ Web Links

Alan T.

Edited By A.T. on 20/12/2013 20:25:39

[Adrian Smith 1]

Wow! AT plenty of food for thought! I will try and digest all of it in a day or so. One observation that scares me is the bit about failure of the Opto kill switch with ignition powered petrol engines, as that's the methodology I use to cut the engine from my TX albeit a futaba one. Certainly sounds that y-leads are not to be trusted too.

[TheFlyingCrust]

I've just managed to wreck a new servo (not Hitec, though) by stalling it for too long whist trying to set the throws on a closed loop rudder. It was fine when I started. It still works but is sluggish with some twitching, especially when cycling with a servo tester, and it doesn't centre at all well. As these things often use the same chip (so I'm led to believe) I suspect the output stage of the amp on mine is shot. Check to see if your sluggish one centres ok. Maybe its a similar problem.

Just a thought.

Ian

[Adrian Smith 1]

I will dig out my old servo tester Flying Crust and examine what happens to them. Not something I had thought to do which is pretty basic (I must think harder). But what you say is quite interesting.