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T-REX 250 › Align 250 SE buring out tail servos
01-15-2010 01:42 AM  8 years agoPost 1
cwright

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Blossvale New York

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I have been working on a friends 250 and am burning out tail servos. This has happened using a 4.8 battery and using the esc set at 5.0 volts. ( not at the same time). The end points are set and the pushrod moves freely with no binding. It will start out working fine then for no apparent reason it fails. The servo becomes very hot and the last one actually melted the servo case. This to me means that there is binding but like I say there is none that I can find. Also I am using a DX7 and using the gear channel for the gain. I have the end points at 25 so I am pretty sure the gain isn't to high. I am at a loss for what is happening. Has anyone got any ideas what might be going on? Thanks in advance.
Chuck

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01-15-2010 04:07 AM  8 years agoPost 2
dansvt

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Miami, Florida USA

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You might have a bad esc.

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01-15-2010 05:02 AM  8 years agoPost 3
dkshema

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Cedar Rapids, IA

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What gyro, what servos?

It sounds as if you are using analog servos with a gyro set to run digital servos and you're frying the servo amps.

A bad ESC won't pick on just the tail rotor servo as long as the voltage is set to 5.0 as indicated in the original post.

Or you are using a digital servo with the servo frame rate set too high at the gyro.

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Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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01-15-2010 11:40 AM  8 years agoPost 4
cwright

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Blossvale New York

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I am using the stock align gp750 gyro and align ds420 servo. Gyro is set for digital servo. Thanks. Chuck

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01-15-2010 03:42 PM  8 years agoPost 5
revomix

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Fontana, CA

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what frame rate is it the gyro set to?

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01-15-2010 07:39 PM  8 years agoPost 6
dkshema

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Cedar Rapids, IA

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I took a quick look at the GP750 specs, I don't think there is a frame rate setting. You can choose the neutral pulse width (1520 vs 760 us), but I think the frame rate is fixed. What that frame rate IS, doesn't appear in the specs.

Align simply says "enable digital servo mode if you have a digital servo" and to not turn it on for an analog servo.

The frame rate crossed my mind as the root cause, but you have to know there are many others out there with the 750/420 combo that aren't smoking servos.

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Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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01-15-2010 09:02 PM  8 years agoPost 7
cwright

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Blossvale New York

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I did double check the gyro and it is set at 1520 frame rate. Thanks Guys. Chuck

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T-REX 250 › Align 250 SE buring out tail servos
01-16-2010 12:18 AM  8 years ago •• Post 8 ••
dkshema

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Cedar Rapids, IA

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1520 is not a frame rate, it's a pulse width the servo sees as neutral.

The frame rate refers to how often the receiver sends position information to the servo.

A normal everyday radio sends out position pulses out at somewhere in the neighborhood of 50 - 60 times per second (50 - 60 Hz). And that is traditionally the rate at which analog servos were designed to expect.

Gyros such as the GY401 have the "Normal/DS" switch. In normal mode, they ship out the servo position at that same 50 - 60 Hz rate. Works great with analog servos. When you flip the switch to "DS" for digital servos, the 401 ups that pulse output rate up to 270 times per second (270 Hz). Digital servos are already designed to work internally at higher rates, this is why their holding torque is much higher than analog servos, and accounts for their higher current drain, as well.

If you try to send position information to analog servos at a 270 Hz rate, the servo amplifier inside can't handle it, and the signal processing circuits heat up, smoke, and die.

The 401 "digital" rate is 270 Hz, some of the newer gyros allow up to 333 Hz frame rate.

As I stated earlier, the Align gyro doesn't spec the frame rate, nor does it allow you to change it.

But the DS420 isn't that great a tail servo, perhaps your heli wants a better servo and is trying to tell you.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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01-16-2010 03:00 AM  8 years agoPost 9
cwright

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Blossvale New York

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I think the owner is going to try a different brand servo. Thanks for your help everyone. Chuck

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01-17-2010 04:44 AM  8 years agoPost 10
rotory

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Los Angeles

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What is /are good tail servo for the stock gyro? and also with 520?

bob

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