Conference vmszoo::rc

    
    	Keith,  You and or your wife may not have any choise in the
    matter. You see the old frequencies are being obseleted come dec.
    of this year. You'll have to have your radio changed to a new
    frequency for it to be legal. I would suggest that as you are already
    having radio problems that you send your radio in for a check up
    and a frequency change (if it can be done). This way you'll know
    that your gear is OK. You wouldn't want your gear shooting anybody
    elses planes down. I suggest that you contact;
    
    			Hobby Radio
    		3001-A Kavanaugh Rd.
    		Wilson,NC
    			27893
    		(919)291-8548
    
    
    	They say they will work on any brand of radio system. 
    I frequency conversion and a total systems check up would
    cost you all of $50.00. This is a quoted price from their 1987 price
    list.
    
    
    
    						Tom
    

>    	.........The radio gear is Macgregor Digimac IV and is about
>    12 years old. When last used (10 years ago) it was perfect. The
>    nicads may not be all they were, but they hold charge, voltage is
>    good and I've cycled them a few times. 

	*  Keith, I wouldn' trust a set of 12-yr old batteries as far as I 
	could throw the QE-II.  The VERY FIRST thing I'd do, if I planned to 
	keep the system is relace all nicad packs!

>    i	Anyone know about how to determine if the gear really is out
>	of tune?

	*  Did you do a range check BEFORE you flew??  This is yer' first/BEST
	line of defense against out-of-tune equipment.  Radios differ as to how
	to perform a range check but, rule-of-thumb, with the antenna removed 
	(or disconnected) from the Tx, I'd expect to see a minimum of 60-100'
	distance from the plane/Rx before control is lost.  With my Futaba "G"
	and "J" rigs, I routinely get 45+ paces from the bird before things 
	start to get "jittery" which translates to ~ 120' (35+ meters).  You
	may have to perform this check with no other radios on but it should be
	done "before" the first flight of EVERY flying session.

>    ii	If so, how do I retune it?

	*  I can't speak to how things are in the U.K. but, in the "colonies,"
	there are a multitude of small, privately owned/operated radio repair 
	services in virtually any/all areas of the country and in many larger 
	cities.  If you had to ask the question, I have to assume that you (like
	myself and most R/Cers) don't possess the knowledge/equipment to repair/
	tune the system yer'self...don't fiddle with it, let a professional do 
	it!

>   iii	Can anyone think of a reason other than the tuning which may
>    	have caused the problem?

	*  I'd need a more specific/detailed description of the entire incident,
	from the time you unloaded the bird from the car and assembled it right
	through all events leading to the actual crash but old batteries and old
	system components which have aged (changed value) over a 12-yr period 
	are a good place to start looking.

>    iv	Given the age of the gear, and it's suceptibility (spelling!!)
>    	to ground effect reducing range, what is a reasonable ground range
>    	to expect?

	*  See relpy to (i) above for static ground range check.  In flight, you
	should be able to get out several hundred yards/meters at "low" altitude
	before running into difficulty.
    
>    I really would like to get the gear working properly as a new radio
>    is not on at this time - my wife is already unimpressed at the return
>    for the investment I've already made!!

	*  I hear you about wanting/needing to "make-do" with the older equip-
	ment but I'd strongly recommend you seriously consider "biting the bul-
	let" and purchasing a new rig, for different reasons than Tom gave.  He,
	clearly, didn't realize yer' in the U.K. so the frequency changes we're
	presently going through in the U.S. don't affect you.

	My reasoning is that, as mentioned above, electronic componenets "do" 
	age, change value and deteriorate over time, "especially" when un-used
	for extended periods of time.  You might get the old system working fine
	now, only to have it fail just a few flights down the line.  Also, you 
	should bear in mind how far the old rig has slipped behind the state-of-
	the-art over the past 12-years.  It's, doubtless, "much" more suscepti-
	ble to interference today than it was 12-yr's ago!  Everything else 
	might be fine but you get "shot-down" `cause the filtering/noise-rejec-
	tion circuitry of the older system isn't up to todays standards.

	You could easily throw away "lots" more bucks/pounds in trying to get 
	the old rig serviceable while losing several more birds in the process!
	Yer' spouse (and "you too") will likely be much less enchanted with that
	eventuality than if you bought a new system and got down to the business
	of flying with at least "that" base covered.  Then all you have to make
	excuses to yer' "lovely" for is yer' "dumb-thumbs."  (Hah!)
    
>    Thanks, Keith
    
	* Pornada, amigo, hope some of this helps...adios,	Al

I agree with Al on all points, except one; I think that his
requirement of 120' for a ground check is too stringent.
Generally, I collapse the antenna and walk off about 50 feet.  If
it works there I consider it ok.  This is what most of the folks
at the club do.

For sure, if there's any way to replace the gear, do so; that
stuff is just plain obsolete by today's standards.  Also, I
question that the nicads checked ok.  They are most certainly
bad.  There is only one way to check nicads (how's this for being
opinonated?).  Get a voltmeter and put it across the nicad with a
full charge.  Assuming your batteries are 500 milliampers rated,
you want to check them at one-half the load.  If you have a
receiver pack, put a 20 ohm load on the pack; if you have a
transmitter pack put approximately 37 ohms on the pack. 
This corresponds to a 250 mA load.  Any other pack should be
checked with a similar procedure using one-half the rated load of
the pack.  The time to discharge the pack; that is from the time
the load is applied, to the time it reaches 4.4 volts for a
receiver pack and 8.8 volts for a transmitter pack should be
approximately 2 hours.  Any less than one hour is cause for deep
suspicion in the pack (actually, an hour and a half would be
cause for concern).

So much for the batteries; if they pass that test I'd be very
surprised.   As for tuning, you should have it professionally
done. But for getting in the ballpark, you will need an
oscilloscope on the receiver output, just before the input to the
decoder.  You can also monitor the AGC, but its a little harder
to find (its near that point somewhere).  Tune the IFs in
rotation from the first through the detector to peak the signal
(or dip the AGC voltage).  That's it.  Forget about tuning the
transimitter, there are too many variables.

Hope this helps.

Wow! 

Response: informed, fast and good! I'm impressed. Thanks for your input. I
was afraid I'd get the "buy a new one" because in my heart-of-hearts I know
thats whats required. 

re .1 - I've done a quick check of the ads. and asked the manufacturers
themselves about tuning/repair but there doesn't seem to be anyone. They'll
sell me the circuit diagrams though...MUST be old gear!! If I was in the
'States, I'd try that address - thanks. 

re .2 - I'll DEFINITELY throw away the old RX nicads, but the TX nicads
took 2-1/2 hours to discharge (this is an ok test isn't it, leaving the TX
switched on ariel up?) so they would *appear* ok. I did range check with
the ariel up and got what I thought was reasonable range, remembering the
problems the older sets had with ground effect reducing range. I estimate
about 200-250 yds on the ground which I understood tripled when in the air?
I haven't tried with the ariel down. I have MUCH respect for your opinions
based on the notes you've added - I'll buy new somehow! I actually handled
a new Futaba set when last at the field - it handled well and the
resolution on the servos was about 200% better than on mine! At this point
I'd better defend my good lady wife (she may read this sometime!) and say
that she's been very patient with me to date and I hope to get the cash
together fairly soon. My fault, I set her expectations too cheap to start
with (they matched mine actually). 

re .3 - I'll try the 50 feet check in open space and let you know what
happens. I'm no electrical wizzo so I'm not sure what some of the words you
use even mean!! If I can't find someone else to check out the gear, I'll
"bin it" (maybe put it in a little boat?). What are your comments on the TX
nicads though; I was really pleased with their behaviour? Whilst I'm here,
John, thanks a lot for the design details. Snail-mail ==> Hare-mail!! I'll
be in touch. 

I'm trying hard to play cheap and loosing. Ah well, it may just have been
something simple... 

Thanks again, 

Keith. 

    Sorry - meant antenna not ariel. Not receiving anything at the TX
    am I?

I'd say that 2 1/2 hours with the transmitter on is good,
assuming that the batteries read ok after that time.  I've never
been able to check the batteries with the transmitter on; the RF
boogers up my meter.  That's why the 250 mA load check is so
important.

Also, while I'm here, I did make a mistake.  I said to dip the
AGC, but this is not necessarily true; it depends on the radio.
Actually I haven't used the AGC method for quite a while, because
I like the scope.  With it I can tune the pulses so they look
nice with little square tops.  I don't think this makes a
difference, it might even degrade performance a little, but its
asthetically pleasing!  Also, for the record, when you get a peak
on the IF's you should go back and detune each can a little each
way and try to touch up the others.  This is to be sure that the
receiver is aligned to the correct frequency.

    Keith,
    
    I agree with John that 2 1/2 hr.'s is "only" a good indication if
    the pack is "still up."  If it's dead after that time, I'd believe
    the pack is weak.  The ONLY way to accurately assess nicad-pack
    condition is to discharge "under load" using either John's method
    of hooking a load resistor across the pack or, much easier, by using
    a discharge cycler like tose available from L.R. Taylor, Ace and
    many other U.S. mfgr.'s.  I'm sure similar equipment must be available
    in the U.K.  I'd still scrap the "dinosaur" and enhance my peace-of-
    mind by getting into a new radio system, were I you.  Futaba's great,
    as are most of the popular brands of currently available systems.
    
    Let us know how you make out..., adios,	Al

    
    One thing I might add about the range check is to try it with your
    engine running.  Electrical noise and vibration can reduce the
    effective range of the system, especially the older ones.
    
    JUST MAKE SURE YOU HAVE SOMEONE ELSE HOLDING THE PLANE AT ALL TIMES
    OR YOU'LL END UP WITH A RUNAWAY!
    
    I agree with the recommendations of buying a new system also.  Put
    the old one in a car or boat as you mentioned, as long as your
    regulations allow that.  Don't feel bad, I think a lot of us have
    the same problem justifying our hobby to our spouses - "you paid
    THAT MUCH for THIS???".  Good luck!
    
    Bill