On 1/8/2015 8:08 PM, Vigilant1 wrote:
Greetings,
I'm not a ham, but I am hoping a radio pro here can offer some advice.
I am a private pilot and the VHF radio in my plane (118-137 MHz) is not
behaving well. Reportedly my transmissions are weak and only partially
intelligible. I sent the unit back to the factory a year ago, they
replaced some bad capacitors and adjusted the TX frequencies, but now
the problem is back.

I bought a handheld unit (Yaesu FTA-550) and am considering just using
that in flight--it is legal, and performance is supposed to be adequate
if used with an external antenna. But, my installed radio (a Val 760
model) is a little more powerful and it is a less "kludgy" fit in my
aircraft's cramped interior.

I'd like to do some testing to find out if my existing radio really has
a problem, or if I've got a problem with my cable or connectors to the
external antenna.
Ideally I'd like to be able to see field strength and look at the
spectrum of the TX. Is there an inexpensive meter for doing this? I've
seen some very impressive add-ons for Android devices or USB devices for
laptops that do analysis of various kinds, and was hoping something
already existed for at least VHF field strength measurement. I won't
use it often, so I don't want to spend a lot of money, but an avionics
shop will probably charge me $100 just for a one-time diagnosis--I'd
rather have the tools to do it myself.

Thanks in advance for any assistance, and accept my apologies if I've
used incorrect terminology, etc.

Mark


Mark,

I am also a pilot. There is very little you can do legally with your
radio. The FAA regulates anything to do with the avionics pretty
closely. Even with a General Radiotelephone license (grandfathered from
a First Class Radiotelephone), I can't work on avionics legally
(although many years ago you had to have at least a Second Class to work
on most transmitters). And if you don't have the technical background,
you're likely to do more harm than good.

For instance, field strength won't do you much good without controlled
conditions and a comparison. Even then, it will only tell you what you
already know - that your signal is weak. It won't fix the problem for you.

Since you mentioned no problems with receiving, chances are it is not
the antenna, and you can ignore Jim's advice (which would be illegal if
you followed it, anyway). I know it hurts - but your best bet is to
cough up the Benjamin and have an avionics shop check it out. They know
what they're doing and can fix it for you.

And I would consider $100 cheap - it costs me more than that to rent a
Cessna 172 for an hour...

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

Jerry Stuckle wrote:

snip

I am also a pilot. There is very little you can do legally with your
radio. The FAA regulates anything to do with the avionics pretty
closely. Even with a General Radiotelephone license (grandfathered from
a First Class Radiotelephone), I can't work on avionics legally
(although many years ago you had to have at least a Second Class to work
on most transmitters). And if you don't have the technical background,
you're likely to do more harm than good.

For instance, field strength won't do you much good without controlled
conditions and a comparison. Even then, it will only tell you what you
already know - that your signal is weak. It won't fix the problem for you.

Since you mentioned no problems with receiving, chances are it is not
the antenna, and you can ignore Jim's advice (which would be illegal if
you followed it, anyway). I know it hurts - but your best bet is to
cough up the Benjamin and have an avionics shop check it out. They know
what they're doing and can fix it for you.

And I would consider $100 cheap - it costs me more than that to rent a
Cessna 172 for an hour...

http://www.flyingmag.com/aircraft/mo...-you-can-do-it

http://avstop.com/technical/maintenance/maintenance.htm

A pilot/owner can legally clean just about anything; replacement is
another issue.



--
Jim Pennino

Jerry, thanks for the suggested course of action. The plane is amateur-built, (an experimental), so I've gotten used to doing everything except the annuals myself, but you are right, this situation more closely parallels the required 24 month transponder check/certification--it affects others (everybody else using the RF spectrum) and needs to be handled by a certified shop.

But the prices for service! Dang. I'll probably be very lucky to get a diagnosis for $100. A new radio would be about $1k, a used version of this one would cost about $400, and if I just use the handheld Yaesu with an external antenna, the cost is zero. I'm having a hard time figuring out why this TSO'd, entirely solid-state aircraft radio would crump out in the first place (e.g. why they needed to replace capacitors) and why that fix didn't work longer than 12 months.

Mark

Vigilant1 wrote:

;833309 Wrote:


An external instrument will tell you very little and the cheapest
inline instrument that would be useful (a SWR meter) is going to
cost at least $50 to buy, which is getting close to the $100 shop
fee without having done anything.
. . .
Jim Pennino
Jim,
Thanks. So, do what I can to improve the integrity of the most
obvious/easily fixed failure points external to the radio. An SWR can
be used to see if what I'm doing is improving things, at least as far as
the antenna, then it's just a check of the physical condition of that
piece. Seems simple enough.

A SWR meter will tell you about the integrity of the cable and antenna
as a system.

Of course the SWR won't tel me anything about the integrity of the
signal itself (am on on frequency, is it spilling over, am I
overmodulating, etc).

That will take a bunch of rather expensive equipment for a one off fix.

And if any of those things are bad, you will HAVE to send the radio
somewhere as there is nothing you can do about any of those things
without the maintenance manual, a pile of test equipment, and an
appropriate piece of paper from the FAA.

Can I learn anything by checking the cable's (cable and connector's)
resistance? It's 50 ohm cable and about 12 feet long.

Not much; if the resistance between the center conductor and shield is
less than infinity you have a short/grundge/corrosion problem.

Again, I appreciate your patience and support.

Mark

One other thing to look at if the problem is transmit audio; check the
mic/headset connector. It too should be clean and shiny.

Mandatory war story...

Back when I worked at the avionics shop, about twice a year some guy (not
the same guy every time) would come in with some piece of avionics in
a cardboard box that was partially disassembled.

My first question was "Engineer at General Dynamics?", which was nearby.

The guy would either nod or mumble yes and I would give him the paper
to fill out and tell him we would call when it was working.

The moral here is that for other than simple stuff, test equipment and
skill are not enough, you also need the manufacturers maintenance manual.

However, a large portion of flaky/intermittant issues were due to
loose/dirty/corroded connections, so start there.

And if you do find the antenna or cable to be toast, you will at least
save the labor of the tech discovering that.


--
Jim Pennino