Mark 12D Problem???

If I get scratchy to loss of reception, that's going through one
circuit path in the audio panel. No transmission and side-tone loss is
taking another route in the audio panel. So what I think you are
saying is that it is not likely that both circuits went bad at the same
time?

That is correct. The odds of BOTH circuits failing at the same time comes
somewhere around the odds of winning an argument with an FAA inspector.

Receive audio comes in one pin, goes to an audio processor, and out to a
phones/speaker amplifier. That's one channel. Sidetone also goes through
that channel.

Microphone audio comes in one pin, mic key comes in another pin, both go
pretty much unimpeded (except for a little diode switching) back to the
transmitter.

BTW, how do you KNOW you aren't transmitting? Are you transmitting carrier
but no audio? Or no carrier at all?

Does this happen on the ground? It seems to take about a half hour for the
symptoms to appear. Does it happen with just the radios on and the engine
stopped? You need to keep the battery up to voltage while checking it
static, so get one of those little 5-10 amp battery chargers to keep the
bats batting while you do the test.



The antenna was just replaced with a brand new one and my old one is
currently on eBay to minimize my losses! That would leave the
feedline... Is that something I could easily replace on my own? Would
that be a legal thing for me to repair or should I just point my
mechanic in that direction and eat the cost of the feedline and
installation labor?

Always look at the LAST thing you did before a failure to localize the
problem. How do you KNOW the antenna is good? Can you get at the connector
at the base of the antenna to look at the possible corrosion and/or tarnish
of the silver plated body? And is the center pin pushed back into the body
of the connector? The tip of the gold plated center pin ought to be nearly
flush with the bottom plane of the connector. Me? I'd put a bidirectional
("Bird") wattmeter in the line and see what was happening when the transmit
failure mode happens.

Simply replacing the feedline is doing what is called "shotgun analysis".
That is, start replacing parts until it works right. That's lousy practice.
Besides, what makes you think a regular mechanic knows how to put an RF
connector onto coax? Most mechanics have less knowledge of electronics than
you do. Some are great. Others are machinists, engine gurus, sheetmetal
wizards, etc.. We all have our specialties.

As to the legality ... hmmm. Ron Natalie, do you know if the radio coax is
connected to the landing light circuit {;-) THe point being, what is
"easily" replaceable? Most aircraft run the coax along the instrument
panel, up the door post, across the headliner, through the aft bulkhead,
along the fuselage formers until it comes to the antenna. Other than
tearing the airplane completely apart, it is fairly simple to replace. Like
I said, go to all that work just to find that it isn't the problem? Not me.




Sorry for being such a knuckle-head... I guess I should have paid more
attention in those electonics classes? I do remember something like
"Bad Boys Rape Our Young Girls But Violet Gives Willingly"??? ;-)

"Better Be Ready Or Your Great Big Venture Goes West".


Jim

On Mon, 11 Jul 2005 10:09:30 -0700, "RST Engineering"
wrote:

. . .THe point being, what is
"easily" replaceable? Most aircraft run the coax along the instrument
panel, up the door post, across the headliner, through the aft bulkhead,
along the fuselage formers until it comes to the antenna. Other than
tearing the airplane completely apart, it is fairly simple to replace. Like
I said, go to all that work just to find that it isn't the problem? Not me.

Looking at the antenna connector is a good idea. But for shotgun
troubleshooting, I wouldn't be averse to temporarily running a length
of coax from the back of the panel through the cabin to the antenna
just to get a data point..

Don

There's an idea. If the antenna has a connector on both ends (radio as well
as antenna) I've sure got enough 50' cables that we could string one on a
temporary basis between radio and antenna just to eliminate that
possibility. While we are at it, we could patch in a Bird bidirectional
wattmeter and the fellow would at least know if he was putting juice into
the antenna. I'd be happy to lend him both, but NOT between now and
Oshkosh.

Jim




"Don Tuite" wrote in message
...
On Mon, 11 Jul 2005 10:09:30 -0700, "RST Engineering"
wrote:

. . .THe point being, what is
"easily" replaceable? Most aircraft run the coax along the instrument
panel, up the door post, across the headliner, through the aft bulkhead,
along the fuselage formers until it comes to the antenna. Other than
tearing the airplane completely apart, it is fairly simple to replace.
Like
I said, go to all that work just to find that it isn't the problem? Not
me.

Looking at the antenna connector is a good idea. But for shotgun
troubleshooting, I wouldn't be averse to temporarily running a length
of coax from the back of the panel through the cabin to the antenna
just to get a data point..

Don

Hi i am Robert
I juste looks at the first letter of your email ,it about the Mark 12d first check if you have good electrical connection in your airplane , you can take another mark 12d an put. It instead of your if it work well into the other airplane an has some problem in yours then you know where to start . It's your airplane your your radio this is one way to solve part of it.

Robert

Thank you

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