Here’s a fun one for you guys, more complex than expected, and worthy of a blog post. I’ve been getting back into R/C flying lately with a couple little micro helicopters and quads, and decided to resurrect my old equipment. Way back when I did some flying with a then-excellent, but now archaic, JR XP652 transmitter. After more than a decade away, I’d like to get back into flying and am looking to use my existing gear to get back into the swing of things. Time has marched on, and everybody now uses 2.4ghz in place of my old 72mhz rig. So I have an OrangeRX brand 2.4ghz conversion module on the way to allow me to fly with DSM2/DSMX.
While it’s still en-route from China, I decided to get the transmitter ready for the install. I have some ideas on how I want to proceed, and wanted to get it all ready to drop in the new module and parts.
I started with the basics, pulling off the back cover to expose the RF board which will be replaced with the 2.4mhz module. In theory, I just remove that amber colored board in the middle, connect the new module into the wire harness that’s plugging into the old RF board, and add a couple extra bits.
It was supposed to be an easy conversion….
On closer inspection, I started to notice things weren’t quite as they should be. Now, keep in mind this is a radio that hadn’t been opened since it left the factory. I bought it brand-new from their main US distributor at Horizon Hobbies. So it caught be a bit by surprise to see a jumper wire soldered to the back-side of the RF board connector. It leads to the main processor board, with an additional jumper across the processor. Since I need to tap into that connection on the RF board, and I need to remove the RF board, this poses a problem.
Now, electrically this connection goes a few different places. They both get connected at the RF board pin, so in theory I should be able to move that add-on jumper wire from the RF board to the others side of the harness that plugs into the connector. With me so far? This is where things really started getting tricky. The connectors JR chose for the vast majority of this radio design end up being soldered down, can’t just pull out plugs. There are a lot of small sub-boards in the radio. Instead of de-soldering everything, I opted to basically dismount all the switches and controls in the upper half of the radio. This allowed me to pull the main processor board forward. That’s where I found out the solder joint I needed was on a completely different PCB….
My guess is this rework was done after the radio was assembled, and this was the easiest method to run a ground to where they needed electrically. However it does look like the modification doesn’t affect the RF board at all, it affects the ground between the processor board and the power board. The solder joint on the RF board was just the easiest spot to make the connection. So a quick snip, strip, tin, and solder gets me this connection…
Now I’m ready to add the new 2.4ghz module, right? Wrong! Guess what also appeared under the bottom side of that main processor board…
Oh, goodie, a solder mounted 2032 lithium battery. This is good and bad. The good news is the radio has a backup for memory settings in case you have to remove the main operating battery. The bad news is this sucker is a PCB pin style that’s soldered in place. Even better is that it’s on a 15mm spacing, where everything from my normal suppliers is on a 20mm spacing. So I’m also waiting now on a new battery from E-bay. The old one wasn’t too hard to remove with a tiny dot of flux and my semi-trustworthy Xytronic de-soldering station. Ironically this battery is probably 15 years old and still has a full 3.2vdc on it! Then again, they did use a brand-name Sony battery. Makes sense really, everything in this radio is done with good components and is well assembled. I may have made a few different design choices, but construction and component choice is excellent. I also don’t know what the design constraints were at the time it was originally conceived, so I won’t fault JR too much.
Removing the old battery was pretty easy, though I do always get nervous de-soldering batteries like this as you can possibly get some voltage shot around in places it shouldn’t be. 3.2v would be hard pressed to hurt anything, but you never know.
Now to wait a week or two for all the new parts to arrive, and I can start re-assembling things. So stay tuned for part 2 on this one.