Sunday, August 24, 2014

Not Fixed: Repairing Nx8500 Helicopter (3 of ?): Installing New Servos and programming the radio with t6config

We tested the helicopter, and hit another snag.  The left servo is completely dead, and the right side is really twitchy.  That sucks.  I don't know if it's the new rotor head or damage from the crash, but either way we are going to need some new servos.

I went to our local hobby shop, Family Hobbies, to get some new ones.  Unfortunately I have no idea what the spec on the old servos are.  They are literal black boxes with no specs at all.  I got 3 new 9 gram servos from the shop to try.

Note: You should also buy some small zip ties to keep the cables tidy.

Update: One of these has started twitching, so 9 grams are probably not strong enough.  I'm going back for stronger ones.

Replacing these has two big parts.  Installing the servos (making new servo arms) and re-leveling the swash plate.

The easy part is physically installing the new servos.
1. Unsnap the servo ends of the linkages.
2. Unscrew the screws holding the the servo arms and remove them.
3. Unscrew the two screws  holding each servo on.
4. Remove the wire ties holding the receiver and servo cables in place.
5. Note the channels where the servos plug in.  Mine was 1,2, and 6.
6. Remove the old servos.
7. Make new servo arms.  If your old arms fit the new servos, great!  Skip this step.

My servo arms didn't fit, so I had to mod the arms that came in the box to work.  I cut the new arm to size and drilled the hole out to 1/16th inch.  Finally I rounded off the edges to clean it all up.

8. Move the linkage balls from the old servo arms to the new arms.
9. This is really important.  Hook the servos up to the receiver and center them before you put the arms on.  If you don't do this now, you will probably have to do it later.
10.  Put it back together.
Be careful routing the wiring for the servos.  You want to make sure they don't abrade or get caught on any of the gears.
The next part is getting the new servos to work with the receiver and leveling out the swash plate.  When I tested this, the servos moved backwards.  Up was down and left was right.  That's not great.  As it turns out, not all servos rotate in the same direction.  I had no idea.  This helicopter came with a computer controlled reprogrammable transmitter, so this can be fixed.  I pulled out the USB cable and hooked it up.

Windows Device manager detected the radio as 
todo: verify
a silicon graphics C2100 Serial UART.  You can download the driver for tthat dhere.
todo: driver

Once the driver was installed I went back to device manager and saw that the radio was assigned to COM4.  The radio came with the configuration program t6config on CD.  I installed it.

To run t6config on Windows 7, click start -> run -> Browse and navigate to c:\Program Files(x86)\t6config.  Then select t6config.exe and press Ok Twice.

To run t6config on Windows 8, click the button formerly known as the start button, wait for the start screen to slide up, type t6config, wait for it to find it, and click the program in the search box.

Once t6config is running, click Settings and select the COM port that showed up in device manager.  Once it successsfully connects you should be able to see the slider bars on the screen change when you move the knobs on the controller.  Immediately save the configuration so you'll have a known good/working copy.

Then I opened up the "Revserve" button (This program is full of typos.  It's hilarious) and reversed my servo channels.  Now they move the right way!  Perfect.

I also checked to make sure the swash plate was reasonably level.  It was.  If it wasn't, I could fine tune it in t6config.  I didn't have to change anything here because I was really careful putting the arms on in the right position.

I saved a copy of the new config and went out for our next test flight.

During the test not-flight the new servos worked okay, but one of them has developed a twitch.  I don't think they are strong enough.  That's frustrating.

I think I found the cause of the original crash as well.  The tail rotor isn't turning properly.  When you spin up the main rotor it turns, but any rudder input causes it to stop turning.

Figuring that out will be the next and please-oh-please-oh-please let it be last post in this series.

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