ADF Problem

Hi
Today I was flying north and my ADF was tuned to an NDB.
The needle pointed at 330 degrees (or 11 O'clock) until I reached the
NDB, and then it swung until it pointed behind.
I dialed up the next NDB and it pointed at 330 again until passage.
And I dialed up another NDB - same indication - all of them wrong.
On the return trip - same indications.
It seems logical that the nav, ADF head and antenna are working because
I get a reading on station passage, but other than that, if I am tuned
to an NDB anywhere ahead of the aircraft the needle points at 11
O'clock.
Any ideas?

Thanks

Tony
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Almost Instrument
Cessna 172H C-GICE

On Sun, 09 May 2004 05:43:35 GMT, tony roberts
wrote:

Hi
Today I was flying north and my ADF was tuned to an NDB.
The needle pointed at 330 degrees (or 11 O'clock) until I reached the
NDB, and then it swung until it pointed behind.
I dialed up the next NDB and it pointed at 330 again until passage.
And I dialed up another NDB - same indication - all of them wrong.
On the return trip - same indications.
It seems logical that the nav, ADF head and antenna are working because
I get a reading on station passage, but other than that, if I am tuned
to an NDB anywhere ahead of the aircraft the needle points at 11
O'clock.
Any ideas?

Thanks

Tony

What was your crab angle, 30deg?




E-mail (Remove Space after pilot): pilot

Tony,

An ADF is basically an AM radio receiver with some added circuitry that
uses a special antenna that allows us to derive the direction of the
signal. Many of these units use discrete (individual) electronic
components, AND most of these units are quite old. Certain capacitors in
these units shift their electronic values over time, causing changes in
the circuitry's operation. Hence, they need to be realigned every now
and then. If not, over time the value shift in some components may make
the unit not tune a signal or will not point to the station at all.

The difficulty is in finding a radio shop that has the tools and
personnel to align your ADF. This is usually done in the airplane
because the 2 antennas (loop and sense) need to be aligned and "trimmed"
with the unit. So, you need to bring the airplane to the shop for best
results. Depending on where you are located, this may not be easy. Take
a look in Trade-a-Plane or other publications to see if there is a shop
near you.

As an alternate, you can ship the unit out. That may or may not solve
the problem depending on the antennas in your plane. If they happen to
be close to the values of the test antennas at the shop, it will work.
If not, the unit may may not work at its peak performance.

Maybe some radio repair person can comment on the criticality of the
antenna adjustment. I do not have a whole lot of experience with radios.

Good Luck,
Mike

tony roberts wrote:
Hi
Today I was flying north and my ADF was tuned to an NDB.
The needle pointed at 330 degrees (or 11 O'clock) until I reached the
NDB, and then it swung until it pointed behind.
I dialed up the next NDB and it pointed at 330 again until passage.
And I dialed up another NDB - same indication - all of them wrong.
On the return trip - same indications.
It seems logical that the nav, ADF head and antenna are working because
I get a reading on station passage, but other than that, if I am tuned
to an NDB anywhere ahead of the aircraft the needle points at 11
O'clock.
Any ideas?

Thanks

Tony


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tony roberts wrote:

It seems logical that the nav, ADF head and antenna are working because
I get a reading on station passage, but other than that, if I am tuned
to an NDB anywhere ahead of the aircraft the needle points at 11
O'clock.
Any ideas?

When you do a 360-degree turn, does the needle stay at 330 all the way
around, or does it turn as the aircraft turns?


All the best,


David

"Mike Spera" wrote:

... Many of these units use discrete (individual) electronic
components, AND most of these units are quite old. Certain
capacitors in
these units shift their electronic values over time, causing changes
in
the circuitry's operation. Hence, they need to be realigned every
now
and then.

Per three ADF service manuals I have, there actually isn't any
alignment procedure to adjust pointer indication. You are correct in
that the installation may need trimmed out, but for example in one
ADF, you remove a mounting screw on the indicator unit and tweak
something with a supplied tool. In another, you loosen and physically
twist the goniometer (what magnetically makes the needle move). Those
adj. are worth only a few degrees, though.

If there's a combined loop/sense antenna, it may have active circuitry
inside which can also have a defective component, but it may not be
field repairable. However, cleanliness and security of the loop
antenna connections is one thing to check in any case. Beyond that,
some component(s) somewhere is likely defective if a 30-deg error.

Fred F.

tony roberts wrote:

Hi
Today I was flying north and my ADF was tuned to an NDB.
The needle pointed at 330 degrees (or 11 O'clock) until I reached the
NDB, and then it swung until it pointed behind.
I dialed up the next NDB and it pointed at 330 again until passage.
And I dialed up another NDB - same indication - all of them wrong.
On the return trip - same indications.
It seems logical that the nav, ADF head and antenna are working because
I get a reading on station passage, but other than that, if I am tuned
to an NDB anywhere ahead of the aircraft the needle points at 11
O'clock.
Any ideas?

Thanks

Tony

Did you have any portable electronic devices with you that you haven't
carried before? If so, could be interference. If not, then it sounds
like your ADF is going bad.


Matt

"tony roberts" wrote in message
...
Hi
Today I was flying north and my ADF was tuned to an NDB.
The needle pointed at 330 degrees (or 11 O'clock) until I reached the
NDB, and then it swung until it pointed behind.
I dialed up the next NDB and it pointed at 330 again until passage.
And I dialed up another NDB - same indication - all of them wrong.
On the return trip - same indications.
It seems logical that the nav, ADF head and antenna are working because
I get a reading on station passage, but other than that, if I am tuned
to an NDB anywhere ahead of the aircraft the needle points at 11
O'clock.
Any ideas?

Thanks

Tony
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Almost Instrument
Cessna 172H C-GICE

I had a remarkably similar problem with an old Bendix unit.
It turned out to be a short inside a connector, grounding one point of the
loop aerial. (The connector had been put together with no washer under one
of the screws holding the body and the slight extra projection had brought
the screw in contact with a sleeved soldered terminal pin and over the years
it had rubbed its way through the sleeve until it shorted!)
I suggest you carefully check everything on the loop antenna circuitry
outside the unit before sending it off for checking.
Mike

I had a similar problem. In my case, the needle would always point at 60
degrees until I was on top of the NDB then the needle would "seek" the
station. Once I got beyond 2-3 miles from the NDB, it would go back to 60
degrees. I later determined that it only happened when the strobes were on.
Come to find out, the power supply for the right strobe (located in the
front of the right wing) was leaking interference. The ADF seeked the
interference unless the NDB signal was very strong. I suspect was you have
a similar problem. It should not be much of a problem for a radio shop to
figure it out. Muncie (MIE) Indiana found my problem in about 15 minutes.

Kevin Chandler

Thanks very much for all the advice.
It looks as though loop antenna is most likely, with radio interference
as the second most likely. I'll fly it with everything shut off to see
if the problem persists, to help narrow it further.

To respond to the questions that were in some responses,
if a fly a 360 the needle tracks the NDB if I am near it, but not if I
am several miles away.

Nospam asked if my crab angle was 30 degrees. No. If the NDB were ahead
of the nose the needle pointed at 330, regardless of whether the NDB was
at 300 degrees or 40 degrees.

Thanks again for all the advice - I'll post the solution when I have it

Tony

--

Tony Roberts
PP-ASEL
VFR OTT
Night
Almost Instrument
Cessna 172H C-GICE

"Mike Spera" wrote:
That was my point, although I may not have been clear. There is no
"adjustment" for the pointer as you said. However, there is an
alignment
process that tunes the oscillators and other procedures. Once done,
any
drift due to aging of the (usually tantalum) capacitors may be
worked
out so that the unit will operate and point properly.

That's just not how ADF circuits typically work. I think you're
making all this up. Otherwise, you'd understand how the "oscillators"
work when they're commonly digital PLL's, and that if a receiver did
alignment, it sure ain't gonna be due to "drifting tantalum caps," as
if they are especially prone to do that in the first place.

Fred F.