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Mr.RC-Cam

JR Tx Coin Cell Battery Replacement

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A few days ago I was on my way out the door to visit my local flying site. I turned on my ten year old JR XP783 Tx to check the charge status of the battery. Unfortunately I got the dreaded BACKUP ERR message, followed by the loss of all eight model memories.

I thought I would share the details on how the internal lithium battery is replaced. This is a hidden coin cell that maintains the Tx's CMOS memory when the main battery is not in use. It normally last several years, but when it goes, the Tx will lose all its programmed model memory settings. Plus, the factory programmed settings are reset too.

The example shown is my XP783 (not a XP387). It is very similar to JR's other mid-range transmitters, so the details should be relevant to other JR Tx's.

The first thing to do is to place some sticky scotch tape across the front push buttons so that they won't fall out later on. While at it, loosen the eight (8) gimbal mounting screws using a 3.5mm hex driver. Do not remove the screws at this time -- just get them good and loose.

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Continue by removing the main battery pack. Unscrew the back cover and unplug the cables that tether it to the motherboard. Set all the pieces safely to the side.

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The coin cell is on the backside of the motherboard (at the lower left corner). To make things more interesting, it is soldered in place.

So, remove the motherboard. My Tx had one screw that held it in place. The gimbal assemblies were in the way too, so now is the time to remove the eight screws on the front of the Tx.

Once the motherboard is lifted, a half dozen springs will be set free. Don't lose them. With the scotch tape that was applied earlier, the push-button key caps should not come loose. But if they do, don't lose them.

The coin cell is shown in the photo below. It will need to be unsoldered using safe desoldering practices. Heavy handed rework will damage the PCB traces, so be careful.

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A direct replacement coin cell is available from mouser.com. It is part number 639-CR2032-P5-1. I added some kapton tape to the perimeter of the new cell so that it would not short out to the nearby components.

The coin cell goes back the way the old one was positioned. You remembered to mark the (+) side before removing the old one, right?

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My Tx had a soldered link (bridge) between the coin cell's (+) PCB pad and a nearby trace. This "CPU reset" link is just a blob of solder, which was removed when the cell was unsoldered. Don't forget to restore the solder link later on.

In the photo below, the new coin cell has been installed, but the soldered link is missing. I didn't solder the two pads together until after I re-installed the motherboard.

Speaking of re-installing the board, don't forget all the little springs that came out earlier. Be sure they are on the posts of the keycaps and that they align with the push switches. This takes some patience, but is not all that difficult to do.

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All that remains is to install the back cover and all the other bits that were removed earlier.

When you turn on the Tx it will beep and dispay BACKUP ERR (or some other verbiage like that). Turn off the Tx, wait a few seconds, then turn it back on. It should now look alive and act normal, but all previously programmed model memories will be reset.

At this point the Tx is ready to be re-calibrated. Calibration is needed on the microcontroller's A/D reference so that the battery alarm is correct and the sticks have full range of control. There is a menu entry that is used to perform the A/D reference correction. Hint: it is the battery calibration entry.

To recalibrate the Tx you must enable the hidden service menu. Here are some links that describes the process:

http://www.rcgroups.com/forums/showpost.ph...710&postcount=6

and ...

http://www.rcuniverse.com/forum/fb.asp?m=5257513

Keep in mind that some factory settings will be reset too. For example, things like CCPM may be disabled until you manually enable them in the service menu. The link above offers the information on such things (although it may not be obvious without some head scratching).

Fatherly advice:

If any of this sounds confusing, or if you really like your JR Tx, then put down the soldering iron. Instead, get out the credit card and pay the friendly man at the JR repair station to do the dirty work for you.

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Interesting. I wonder if all computer radios are like this :unsure:

Terry

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Most are, at least the models that are more than 2-3 years old. The latest Futaba ones use flash to store the settings and models now so there's no worry about this anymore (the 14 still has a coin cell though, but it's only to maintain the clock). I guess most newer radios from other manufacturers should do the same.

However, for many the service menu isn't accessible that easily, so even if replacing the cell manually it would need to be sent to service for calibration...

For the older Futabas, there's a company that makes a CAMPAC module with service menu enabler (Futaba uses a special one they simply insert in the slot to access the menu), but if you need to buy one for that you could as well send the radio to the service center, it will cost you less.

Except if you need the CAMPAC too anyway of course :P

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I'm not a big Futaba Tx user, but from what I have been told, their computer models use battery-free E2prom memory for model settings. But I can understand how one would be needed in their Tx's that have a clock/calendar. For sure, the coin cell use is a long standing JR thing. From my experience, the hidden battery will often last 10+ years.

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