Power HD S090M servo

[samclee]

I connect 2 DS090M servos using a Y Cord. At first they work ok. After one short flight I restart the motor (petrol) one servo stop working and jammed. Swapped the connections. Test ok, restart motor same servo stopped. Change a new servo same thing, same side happened again. This time I left the servo in the jammed position and connected to another receiver. The servo won't work until I twist the servo horn and it works ok ever since.
I tried to recreate the above problem. I marked the servos A ans B, connect them thru a Y cord. I discover servo A works all the time and B won't work. Leaving the power on, Servo B won't work even if I disconnect servo A. Servo B will work if I disconnect and connect with servo A connected. Both servos work ok if connected on its own.
What I can't recreate is (1) when both are working and make one not working. (2) to get one not working when connected on its own. Note the above test the servo won't move only, but bot jammed to one side as in the plane.
I assume that when the servos power up servo A cause servo B to lock up.
I used to work on RC gears long time ago, but with the advancement in RC electronics I was far behind this new technology.
I had no idea what's going on with digital servos when they first start up.
Any suggestion ?
I can only concluded that these servos can't use a Y cord as one reviewer in Hobby City forum said.

[RCModelReviews]

You don't say what receiver you're using, which can have some bearing.

One problem with modern 2.4GHz systems is that they generally use 3.3V logic, which means that the pulses which are sent to the servos are slightly under 3.3V. In the past, these pulses were usually at least 4.5V or more.

When you hook up a servo that expects a voltage of 4.5V on the signal line to one of these receivers, they'll usually work but sometimes, when you hook two in parallel, the extra load of the second servo will drag the voltage down just low enough that one (or both) of the servos won't be driven reliably.

Hitec had to modify some of their servos a while back to cope with the lower output voltage delivered by Futaba's new FASST receivers or they would not work when paralleled through a Y lead.

This could be what's happening to your setup but it's impossible to say without actually throwing an oscilloscope on the signal lead.

Now I can't tell from your posting whether it's always the same servo that stops or jams. If so, I would suspect that this servo is faulty. Certainly the fact that, when plugged into another receiver (without the Y) it won't then work until you manually turn the shaft. No servo should exhibit that behavior.

[samclee]

I use the JR921, Futaba-R103F, Spektrum 6110 and even tried on old JR FM Receiver (Ner-J128M) they all behave the same way.
Unfortunately I can't repeat the servo jammed with my test. May be need some vibration from the motor.
From my test I think it is true the receiver cannot drive 2 HD S090M servos to start up.
Since there is a micro processor in digital servo, does it need to boot up like the receiver does ?

[RCModelReviews]

Yes, digital servos have a tiny microcontroller that needs to boot up when power is applied. Some early digital servos (like those from Hitec) actually took almost a second to boot up but these days most digital servos boot in a tiny fraction of a second.

I would suspect, from what you've said, that you have a faulty servo.

[samclee]

While with the subject of using Y cord. I come across another strange behavior which is opposite of servos. I was planning to use twin brushless motors. Naturally I think of using separate channel for each ESC and radio mix to control them. That won't work. Only one motor will initialize and run. I have to connect the two ESCs using a Y cord.

[RCModelReviews]

I suspect your mix may be wrong. Have you tried plugging servos in instead of the ESC and checking to see that they both rotate back to the full "off" position. Chances are that your mixed channel isn't going low enough to allow the ESC to arm. Tweak the mix, the EPA or the subtrim as necessary to get the channel low enough to arm. Also check that it doesn't need to be reversed!

[samclee]

Back to the servos. Actually 3 servos are involved. In my first quote I said I changed a new servo. The third servo now called servo C.
Servo B cannot co-exist with servo A. (ie servo B not working) Servo B can co-exit with servo C. (Both working ) Servo A cannot co-exit with servo C. (ie servo A not working)
Which one is faulty ? Or all faulty ? The fact is they all work on it's own.