[Paparazzi-devel] Hitec Spectra (PLL) Module and Eclipse 7 Radio

[Matthew Currie]

Group,


I have been away for some time.  I am back and on track to get airborne
within a week or two.  The changes within the project since I last left
off are tremendous.  Great job developers.  The Pyrenees flight is
inspirational.

I have spent most of the day trying to stomp out a serious and
unexplainable "RFI" nightmare in Funjet today.  I have a Tiny 13 v1.1
onboard complete with LEA-4P GPS, XBee Pro (2.4GHz 100mW), and
PPM-tapped Berg 4L receiver.  The problem was simple, or so I thought.
The servos would chatter endlessly and the motor would turn on/off
without command (in manual mode).  It was more obvious when I ran the
motor at 1/4 throttle.  It would actually cut out, speed up, slow down,
etc.  My motor/esc are quite acoustically entertaining so it was easy to
"hear".  I went through a dozen different scenarios trying to resolve.
I added a ton of ferrite chokes, routed all wires to the bottom of the
fuse with metalized tape, covered the receiver in metalized tape,
rerouted the antenna, removed servos, ESC, even soldered up/hacked
another FM dual conversion receiver.  Nothing would remedy it.  Then I
realized I had "NEGATIVE" in my pulse_type of my aircraft config file.
I changed it to positive, reconfigured my setup and voila! or so I
thought.  For about 10 minutes the Funjet didn't glitch a single bit.
It was so quiet and silent I forgot it was even powered on.  I was
bragging about this "simple" fix in IRC but moments later the plane
began violently shaking, throttling, etc.  It was back!

During the "RFI silence" I bumped my Eclipse transmitter and the episode
ceased as quickly as it started.  I began suspecting at this time it
might be the radio but it wasn't so obvious.  I carefully disassembled
my radio.  I had previously modified it internally to have a 3 POS mode
switch conveniently located on the top.  I decided to give the radio
power to see if I could wiggle wires, etc.  Within moments I smelt an
evil scent... I turned to see a puff of smoke leave the back of the
radio.  Heart sunk, I figured I just ruined my radio.  Through some
careful poking around and following my nose I discovered something that
I should also warn the others about.  The Hitec Eclipse 7 is a rats nest
of 30 AWG wires (literally, many dozens) all passing under and over
circuit boards.  What I discovered is that all of the through-hole
components on the multiple different boards have razor-sharp leads
remaining and one of the wires, likely from my last modification attempt
had been pierced by a components lead!  I figured the least I could do
is trim ALL of the through-hole leads on all of the boards flush to the
PCB to prevent this from happening again and so I did.  I carefully
removed EVERY piece of debris and finally blew out the transmitter with
compressed air.  Although I was convinced I destroyed the radio I put it
all back together and powered it up.  It worked... glitches and all!

Stumped, I began searching the web.  I was not happy with the
craftsmanship I saw in the radio so began suspecting a larger overall
problem.  It turns out there is a widely known problem with the Hitec
Spectra PLL synthesized RF module I was using.  Many even made reference
to a date code of "507".  Mine has 507 stamped on a little green sticker
on the back.  The problem is described as two fold.  Some are saying its
simply the electro-mechanical mating which occurs when the module is
placed into the back of the radio.  Hitec has added tape to all repaired
spectra modules to make them fit tighter in the back of the radio.
Apparently this snug fit has been enough for most people and solving an
intermittent electrical mating problem with the radio.  Another group is
complaining about an overall faulty design with "glitching" building the
longer the radio is left on as if it may be thermal.  While some say
static buildup, I am skeptical.

I disassembled the spectra module and carefully reflowed each pin on the
electrical connector.  It should be noted the female connector on the
spectra module is tin plated and the male header on the radio is gold
plated.  I sprayed some heavy duty flux cleaner in the connector and on
my solder job.  I reassembled the module and also wrapped its sides in
fibre-reinforced (thick) tape.  I fit the module into the back of the
radio snuggly.  It has been 30+ minutes of my tx left on.  All is
quiet.  I have to keep getting up and wiggling the sticks, as it is so
calm and motionless I have no way of telling it is powered on.

Apologies for the small novel but there were a few points I wanted to
get out.  It sounds to me that the spectra module is not generally
trusted in the RC community and a common recommendation is to replace it
with a fixed frequency RF module.  I may do exactly this.  The glitching
was so severe, I am convinced it would have been a plane killer.


Matthew Currie  VE7MJC
British Columbia, Canada