ADF Problem

[tony roberts]

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

[[email protected]]

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

[Mike Spera]

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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[David Megginson]

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

[Teacherjh]

Sounds to me like the loop antenna or the circuitry associated with it is dead.
This leaves only the sense antenna to say "ahead" or "behind".

Jose

--
(for Email, make the obvious changes in my address)

[TaxSrv]

"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.

[Matt Whiting]

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

[Mike Wylde]

"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

[Kevin Chandler]

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

[tony roberts]

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

[TaxSrv]

"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.

[David Megginson]

tony roberts wrote:

> 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.

I had some similar problems with the Narco ADF 841. It wouldn't lock onto
anything more than a few miles away until the radio had been on for an hour
or so. I sent it in for repair, the shop tested it and replaced two
capacitors, and now it works perfectly.

A one-hour bench-test of your ADF at a good avionics shop is probably the
cheapest place to start -- just yank it out yourself and mail or drive it
in. You're right that the problem could be the antenna or connections, but
it will take longer to test those (and most shops don't even have proper
equipment for ramp-testing an ADF antenna and cables the way they can
ramp-test a VOR antenna and cables), so you might as well rule out the radio
first: besides, you won't have to ground your plane while the radio's being
tested, unless you need it for a specific IFR approach.


All the best,


David

[[email protected]]

On Tue, 11 May 2004 02:37:46 GMT, tony roberts >
wrote:

>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

As has alrady been suggested it may be interference, since it works
close to the NDB.

You say ' the needle tracks the NDB if I am near' so have you listened
to the audio out of the ADF receiver? Maybe you can identify some
interference.

Sounds like a similar problem, we had, which was due to alternator
noise (a whining noise which varies with engine RPM). In our case an
alternator decoupling capacitor had not been fitted with the
replacement alternator which did not have an inbuilt suppressor.




E-mail (Remove Space after pilot): pilot

[Everett M. Greene]

writes:
[snip]
> As has alrady been suggested it may be interference, since it works
> close to the NDB.
>
> You say ' the needle tracks the NDB if I am near' so have you listened
> to the audio out of the ADF receiver? Maybe you can identify some
> interference.
>
> Sounds like a similar problem, we had, which was due to alternator
> noise (a whining noise which varies with engine RPM). In our case an
> alternator decoupling capacitor had not been fitted with the
> replacement alternator which did not have an inbuilt suppressor.

Onboard interference source sounds very likely. The ADF
may even be giving a clue as to the general direction of
the interference source from the antennas and may help
isolate it. [Note the "may" in the statement.]

[Mike Spera]

A fellow objected to my posting somewhat strongly as follows:
>
> 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.
>
>

O.K. Chill suit "ON".

I believe we are talking about 25 year old radios. There ain't no
digital ANYTHING in there (except maybe some units that had digital LED
segmented or incandescent segmented frequency displays and some simple
CMOS gate packages to drive them).

As I said, I don't repair radios for a living. Sorry if your opinion is
different. If anyone can explain analog AM radio alignment and theory of
operation in better laymen's terms, I'm all ears.

However, the 3 folks I deal with that do repair old radios for a living
report that, in their experience, tantalum caps ARE more prone to
leakage/failure with age than other types (electrolytics, mylars, etc.).
Your experience may be different. If anyone has actual testing and
failure data, please chime in.

A few titles in the "Maintenance" section for my ADF:
* Oscillator Alignment
* IF Alignment
* Receiver Alignment
* ADF alignment.

Leaking caps are not the problem? Maybe, but the odds are they
contribute to some problems. Maybe I was just on a streak when we had to
replace several of them in my unit to get it to work properly. Sorry,
one was actually shorted, not leaking.

If the caps were not leaking, resistors drifting/burning, and
transistors changing conductance across their junctions (all due to heat
and age), why would any of this be necessary? Does it matter WHICH
component type actually causes the most trouble? I think not.

The fellow was asking why his receiver did not point correctly. I tried
to relay the message that many of these old buzzards needed the
equivalent of a "tune up" to get them working properly. The posting above
suggests that I was trying to sell him some kind of deception. Not so.
Making it up? Let's just say that my efforts may be imperfect in this
case. I cannot be an expert in everything. Hell, I ain't an expert in
ANYTHING that I can think of. I only fixed electronic gear
for a couple of years, and that was a LOOOOONG time ago.

I was not volunteering to fix his receiver. So, I have no duty to be
spot on in any diagnosis or explanation of the theory of operation. I
offered my limited experience with the caveat that this ain't my line of
work any more.

I merely suggested he get the thing fixed. Sorry if my imperfect
explanation of the problem causes anyone heartburn. My advice was worth
every penny he paid (BIG SILLY GRIN)

Good Luck,
Mike


P.S.
Post from another fellow on the thread:
"I had some similar problems with the Narco ADF 841. It wouldn't lock
onto anything more than a few miles away until the radio had been on for
an hour or so. I sent it in for repair, the shop tested it and replaced
two capacitors, and now it works perfectly."

Please note the offending devices - capacitors, I believe. Won't work
unless REALLY warmed up = LEAKING. Maybe I am not that crazy after all...


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