Conference vmszoo::rc

< Note 700.3 by TARKIN::HARTWELL "Dave Hartwell" >
>                              -< Radio questions >-
>    
>    Does anybody have any experiance with sending a radio back to futaba
>    for checkout?  If so, can you fill me in on the details such as
>    local service centers? (besides CA.), cost, time it will be out,
>    plus any other details you wish to share.
>    
>    
>    				Thanks, Dave

I recently sent mine to California.  I called first.  Didn't ask about
local service centers - I assume there are none.  Cost was very
reasonable but if I told you how much you might think it was high.  On
the hand if you saw my receiver before and after you would change your
mind.  It took about a week and a half round trip.  

I was very satisfied and would not hesitate to do it again.

Bye          --+--
Kay R. Fisher  |
---------------O---------------
================================================================================
    

Last nite I replaced the antenna on my Silver Seven receiver.
While it was out of the case, I hooked up my trusty old HP scope
and touched up the tuning.  I managed to get another db or so of
gain out of it, but when I brought the tx close to the antenna I
found a problem.

The waveform out of the decoder should be a square wave.  It is
when the tx is 10 or so feet away, but when it is moved closer,
like onto the bench, the waveform changes to this:



                   |----|
                 |-|    |-|
                 |        |
_________________|        |__________________

What I'm trying to show here is a square wave with a little step
at the top.  Now, I don't know anything about square waves;
whenever I see one I think of calculus and that causes me to go
under my bench and suck my thumb for a while.  So what does this
little step mean when you look at the square wave, in english,
with no transforms please?

I know that this will be of no consequence to the operation of
the radio, the servos are rock steady, I'm just asking for an
interpretation of the waveform.

        Re:      <<< Note 747.1 by CTD024::TAVARES "John -- Stay low, keep moving" >>>

                In the  good  old  days  wqhen  computers were made up of
        discrete components and the most common problem was a bad diode I
        spent some time tracing  signals.  The amazing thing was that the
        so-called square waves that were  so  prettily  diagramed  in the
        timing charts looked absolutely horrible.   I  never saw anything
        as clean as what you describe.   My  advice is that that's what a
        really good square wave looks like.
        
        Anker

No, really, the servos get a pretty nice looking signal; its just
as square as can be.  I've noticed that going for maximum gain on
the signal coming from the receiver (into the decoder) is not
always the hot set-up.  The decoder likes to see the receiver
signal just a little flat-topped.  When I tweaked it last nite, I
made this signal into little spikes -- unfortunately, I had
everything buttoned up when I thought of looking at the servo
signal and found this.  Its not important enough to go back in
for, so I'm only answering a curiousity not a need.  I suspect
that the converter is being overdriven causing it to clip off
some low frequency response, and maybe ring a little too.

    I agree with .2--Rarely do the waveforms looks nice; this is where
    the judgement of the tech comes into play.
    
    Well, unless your working on military equipment where the signals
    always look like the book...or its broke.
    
    :-)
    
    Fred

    Well, I don't know what you "Old Timers" have been working on, but I
    spent a few years programming Automatic Test Equipment, and I saw plenty
    of nice, square, waves ( I did also see plenty with ringing, capacitive
    loading, etc..).  I NEVER saw a squarewave, no matter how messy, with
    another square wave superimposed on it.

    I suggest that the answer lies in the word "Superimposed".  You bring
    the transmitter near the Rx/Servos, and what you end up with is some
    "Noise" from the TX tuned and coupled to the Rx decoder output.  There's
    plenty of different signals inside the Tx that are synchronized with the
    PWM servo signals.

    Re -1
    
    Ole' Timer?  Eh?  What'd you say?  :-)
    
    I had not seen a squarewave with another, or step, superimposed
    on it untill I came to DEC; there are a couple of places in the
    tape drives where we use this type of waveform for adjustment proceedures.
    It looks pretty strange and is sometimes difficult to trigger properly,
    but it works.
    
    Fred
                         

Thanks for the answers...I don't think its tx noise into the
decoder since I can tune this out with the antenna coil and IF
cans.   Something is being overdriven/saturated there.  I got to
thinking about it some more, and now I think the antenna matching
coil is being saturated.  There are two coils associated with the
converter stage; the antenna matching coil and the L.O. trimming
coil.

When I first asked the question, I thought it was due to some
change in the frequency/harmonic component of the square wave;
Never thought of it being two square waves superimosed.

Tomorrow it gets flown with the first flight of the Cardboard
Special.  I'm more worried about the tail feathers staying
attached than I am about the radio!

Its gonna be tough when I get my Airtronics PCM outfit and I
can't fool about with the circuits anymore.  Its been very
convenient to nuke the radio in a crash and then put it on the
bench and get it playing again.

    Another thought:
    This could also be caused by reflections being sent back to the source
    caused by impedance mis-match at the load end. If so, there is no
    problem since the reflection coefficient (suspect 1.25-2.0) will be
    multiplied to the step voltage level at the LOAD end producing a nice
    looking square wave.
    What is the step voltage level and also the voltage at the highest
    amplitude?
    

Thanks. I don't remember what the step voltage was, but I can
guess that the square wave is very close to the level of the
battery...4.8V, and the step was at about a volt lower.  It was
quite pronounced.

I measured the output to the servo directly from the receiver,
without the load (servo input) attached.  Perhaps this lack of
load. the scope input being very high impedance, is the cause.
I'll try connecting the servo and looking at it next time I'm in
there.

    Here's a new question to ponder over your bowl of gruel:
    
    Last night I was setting up my Vanguard PCM to run in a new glider,
    using only 3 channels, rud, elev, and flaps.
    Everthing was working fine but I noticed I didn't have any response
    from my flap trim tab. I wanted to use it for flap reflex.
    I checked it with my DVM, and there was no resistance change when
    the trim tab was moved. Since the horizontal trim tab on the flap
    stick was working, I decided to swap trim tabs assemblys as a temp.
    fix since I wouldn't need the left/right channel on the flap stick.
    After swapping and rewiring, all 3 servos now run full travel in
    one direction and stay there when powered up. The sticks and trim
    tabs have no effect. When the servo reversing switches are flipped,
    all 3 servos immediately run to their full opposite travel.
    It acts like I'm getting a full stick deflection pulse to all channels
    all the time. I can't think of any way that wrong wiring on the
    trim tab pots could cause this. It's always possible that I got
    a solder blob in the wrong place but couldn't see anything as of
    last midnight. Can't do any rcvr. swapping as I have only one radio
    on that freq.
    Tonight I'll unsolder the wires to the two trim pots and see if
    at least the sticks will work again.
    
    Any other ideas?
    
    Terry
    

    Just unsolder the center wire and see what happens (assuming that the
    trim pots have 3 wires) The system might need the full resistance value
    across the outer leads as a reference somewhere. The trim pot alignment
    will effect the trim setting since the wiper is on the shaft and the
    "wipeE" is stationary with the case. If the trim pot is way out it
    could smear across multiple pulses in the pulse train. The next thing
    I'd try would be to replace the dead pot with a working one. It seems
    that you're getting a fixed value out of it and it might be full
    deflection in either direction.
    
    Just some thoughts from someone that learned the joys of Tx setup from
    a couple of Heathkits years ago.

    Thanks Jim,
    I'll try unsoldering the center wire first. The bad pot seems to
    be open from center to both outside leads, as if the wiper isn't
    touching the windings, but resistance is normal across the two outside
    leads. This would represent full trim tab deflection to the channel,
    but I don't see how it could represent full stick deflection.
    Also it didn't affect other channels before the swap.
    
    My first radio was a Heath HD-19, 3 channel. Cost as much as a present
    4 channel, and had to build it too ! I guess civilization isn't
    going entirely downhill.
    
    Terry
    

    Terry,
    
    Your description of swapping trim pots affecting all servos resulting
    in full servo travel tells me you have the wire for the trim pot WIPER,
    and one of its ends, interchanged.
    
    I would imagine the stick and trim pots all sit across the same
    reference voltage rail, and the summing amplifier merely adds 1/10th of
    the trim pot wiper voltage (say) to the stick pot...
    
    Anyhoo, such are the hazards of working on your radio at midnight.
    
    Tell us how ya make out
    
    ajai

    If it's truely open then unsoldering the center wire won't buy you
    anything. Keep in mind that the center wire is normally BETWEEN zero
    and total pot ohms and with it open it is effectively infinite (float)
    and could do strange things to the circuit. Before you do anything at
    all you might want to try shorting the center to either side and see if
    that causes you to get the normal full trim deflection in each
    direction. Sounds like you'll be shopping for a new pot anyway so that
    might be a good place to start.
    
    Good luck and let us know what turns out to be the problem.

    re.14
    Your explanation makes sense Ajai. I thought I was careful in keeping
    track of the wire color codes, but dawn often reveals what strange
    things can happen at midnight.
    
    re.15
    On a good pot the center wire was in the ~600-1300 ohm range referenced
    to one of the outside leads as the wiper was moved. On the bad pot
    it was simply off scale on the DVM at the same range setting, and
    I didn't bother to switch ranges so see if it was really doing
    anything.
    Tonight I'll move the RF board out of the way so I can compare the
    wiring on the other stick. 
    One side of all the pots are tied together on a separate little
    junction bd. so there could be a potential for a potential. ;^)
     
    Terry
    

    No joy on my Vanguard PCM problem. I unsoldered all the wires on
    both trim pots on the left stick, and the problem persists.
    So since I'll need to replace the one bad pot anyway, I'll just
    send the whole works back to Airtronics. I did manage to prove that
    it wasn't a rcvr. problem by swapping rcvr xtals with my Vision
    rcvr.
    And speaking of Visions, the power meter on mine just went south.
    It's got a handy plug-in on it, so swapped the meters with the
    Vanguard, and the problem isn't in the meter. I can live with having
    no meter as I have some xmtr. battery packs that came out of some
    King avionics gear, and they will run the Vision for 4.5 hours
    before the low voltage beeper goes off.
    And speaking of the low voltage beeper, Kay and I were wondering
    a while back, why a Vision xmtr makes a clicking sound all the while
    it's turned on. You guessed it, it's the beeper, which happens to
    be mounted right next to where the power meter plugs in.
    
    Terry
    

    I got my Vanguard PCM back from Airtronics last friday. One month
    turnaround time.
    They did the following:
    Replaced bad trim pot in xmtr.
    Found cold/intermittent solder joint in xmtr. , which was the source
    of all the problems.
    Replaced hinged front panel on xmtr.
    Replaced antenna wire on rcvr.
    
    Tuned a second FM rcvr that I had sent.
    
    Total cost: $0.00, and they even paid for the return shipping.
    
    Terry

	Got my Conquest back from Futaba last Thursday. This was a 1991 
	conversion. The transmitter is now gold stickered, and I got a new
	receiver, the R114H 4 channel AM narrow band. By their reckoning, it
	took 8 weeks; the total time it was out of my hands was 10 weeks. 
	Cost about $44.

	One minor bitch: the receiver isn't compatible with the connectors on
	the old system, so they sent adapter cables. If I had used the cables
	they sent along with all the other cables, I would have had a mile of
	wire stuffed into the plane (small glider), so I ended up snipping the
	old connectors off the servos and switch assembly and soldering the 
	new ones on.

	BIG thanks to Jim Reith for loaning me a radio while mine was out. It
	would have been excruciating to have the Predator sitting there all 
	finished with no way to fly it!

	Dave

    No problem, Dave. It was well worth the Predator stick time back that
    day at the beginning of the month when I have my "bad" day.