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ADF Problem



 
 
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  #11  
Old May 11th 04, 01:19 PM
TaxSrv
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"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.

  #12  
Old May 11th 04, 02:36 PM
David Megginson
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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
  #13  
Old May 11th 04, 02:39 PM
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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.




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  #15  
Old May 12th 04, 02:59 AM
Mike Spera
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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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