my fresh JR tranny
There are advantages in running websites for the odd flying club. Last week I spotted an for sale article from an old friend (yes, we both have many a grey hair). He was just selling the thing I was looking for, a JR Tranny.
My ‘new’ Multiplex 4000 is showing the strains of 11 years use, the case starts to crack. It’s one of these ‘known’ defects of this series. I’ve already repaired it before, but there will be a time in the future, when I will have to accept the inevitable. So, I was on the lookout for a successor. Now, JR radios are supposed to be the best £$E can buy, but before believing any of the advertising blurb, I wanted to have a dig around in a Tx for myself, even if it is in an older model. By and large the technology will be the same in newer models. Cries of ‘it has 2048 bits of resolution’ are mostly met by my ‘so, can you show me a servo that can position itself with that accuracy? And this is 2048 bits for what range? 360 degrees rotation of the output arm? 60 degrees rotation? Stickresolution? or simply the resolution of the A/D coverter that happens to be built into the chip that drives the Tx. In 99% of the cases it is the latter, and it means zilch in the scheme of things if the rest of the system is not able to do anything with it. (Do the math: Superduper servo, input pulse runs from 1 to 2 mSec. Resolution (deadband) is 2microsecs. (this is a very good servo, 90% of the servo’s out there have 10 micro secs resolution/deadband). 1 msec in 2 microsecs steps is 500. The best your SuperDuper servo can ever hope to do is 500 individual steps. Ah, but I need 2048 bits to control these 500 steps accurately. Sure you do. 
But it’s faster.. Been there and yes, the answer is that the message is delivered faster (and repeated often) on 2.4 GHz, but building the message takes as much time as ever. (email might be faster than snailmail, but composing the message takes as long.) Latency is the magic word and defined by most manuacturers as ‘one frame’ and that has traditionally been 20 ms. A change in stick postion is transmitted every 20 ms.
So far only Spektrum is trying to improve on this (there is apparently a Heli-specific tranny that rund a 10 ms framerate. It’s 2 times as fast we shout enthousiastically. (True, 2 times as fast as how slow?) But kidding aside, in the near future someone will start to market a radio that will run on a faster rate. Most 2.4 modules already do send the same message many times more than strictly needed for a 20 ms framerate. At present it’s mostly to create redundancy, to make sure the message is transferred correct, and to be sure that even if we loose half of them the other half is still good enough to reconstruct a valid ‘frame’.
Anyway, I can now say I am the proud second owner of a nice PCM9X in superb condition. And since I know Tony, I know it’s never been maltreated.
Any further abuse will be mine and mine alone! I mean, it has a NiMH battery (for a short time more) (update: Tony put the battery form his new tranny in this one, so we’ll leave it for a bit. And since there is nothing that regulates batteyr voltage in this Tx, I’ll have to add a proper reg to make 10 volts. Not hard to do, just put it on the bottom of the list and it will get done someday. The antenna wire is external to the case, and that RF-connector on the back of the module is an accident waitng to happen, even for me. There’s got to be a better way. (There is, it’s now mounted properly inside the case!) I’m not sure I can handle the myriad of switches, they will confuse me when I least need confusement.
Pictures to follow, after I have a look at the innards..