[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