UPDATE #3 -- Headset Noise

It's not the regulator.

I'm sure that most of the time, what mikem and Jonathan are saying is true.
But my symptoms and yours sound exactly alike. I checked connections and
grounds then had the alternator rebuilt. The rebuild cost about $275. $450
total with labor. No more whine. If this were a $10,000 rebuild, I would
have spent more time diagnosing. As it was, I'm glad to spend the money and
be able to fly with confidence.

--
-------------------------------
Travis
Lake N3094P
PWK

"Jonathan Goodish" wrote in message
...
In article . com,
"mikem" wrote:

I have no explanation for the 32V reading.


I had an opportunity to poke around some more this evening.

Despite Greenlee's insistence that my meter blocks DC when measuring AC
volts (and vice versa), that appears to not be the case. The meter
reads 26 volts AC when measured on the straight battery--no alternator.
It appears to read AC correctly, and DC correctly, but not AC ripple on
a DC bus.

I have a 20 year old Beckman meter that was out of calibration, but I
calibrated for AC and DC volts against the new Greenlee. The Beckman
appears to block DC when reading AC volts.

Both meters show around 12.8v at the battery, and almost exactly 14.0
volts DC when the alternator is charging, both at the battery and the
bus. I've determined that the Greenlee's AC reading on the DC bus is
highly inaccurate, but the Beckman shows almost zero AC volts (0.008) on
the DC bus--I assume because the battery is absorbing the ripple.
However, at the alternator, the Beckman shows approximately 0.30 volts
AC, which increases to a maximum of 0.65 volts AC under full load and
max static RPM. These look more like reasonable numbers.

Could the voltage regulator be at fault? Load seems to amplify the
noise, but the bus voltage is almost exactly 14.0 volts, regardless of
load or RPM. I would think if the regulator was failing, I'd see some
fluctuation, especially under full load. Could I be missing something?

I know that it may very well still be a ground loop. There are some
oddities that I still can't explain, however, like the intermittent
static/crackling when the copilot keys the PTT and transmits. The
landing light causes occasional crackle in the audio, but a corroded
connector may be contributing to that problem, not sure.



JKG

In article ,
David Lesher wrote:

"mikem" writes:


Jonathan Goodish wrote:
I put a voltmeter on the alternator output and measured between 32-35 AC
volts at moderate load and moderate RPM. Looks like some diodes have
decided to give up.

Do you have access to an oscilloscope? That will tell you the "truth"
about what
is happening on the bus. Go look at the waveforms I posted in your
earlier thread...

What He Said. Can you finagle any scope at all? That, and a major
load will reveal bad diodes.

Also, look at mikem's diagrams. Note how he put values in for all
the ground connections. Maybe he'll do them again with 'bad ground'
values.

Poor Ground Shall Cause Grief.

I just don't think that I have poor grounds. I've checked just about
every ground I could find--alt to engine, engine to aiframe, battery to
airframe, avionics, intercom, etc. They're all good, no corrosion. I
cleaned up the big ones before reassembly.

Ground loop could be an issue, but I'm still not 100% convinced that the
alternator isn't at fault. Unfortunately, I've not been able to find
anyone who has a scope, for I know that it would tell more conclusively
what is happening.



JKG

Ground loop could be an issue, but I'm still not 100% convinced that the
alternator isn't at fault. Unfortunately, I've not been able to find
anyone who has a scope, for I know that it would tell more conclusively
what is happening.

Do you know any amateur radio operators?
One of them will either have one or know someone who has one.

Make sure you go back and read the stuff I recently posted in your
UPDATE 2 thread

Then you just need to put a good alternator noise filter in line with
the +12v power line to your audio intercom. You've got alternator whine
riding along the power line into your intercom system and it's being
amplified into your headphones.

Take a look at the Lonestar model 122253-10A filter.

http://www.sacskyranch.com/acatalog/...ilters_65.html

I used to fly a Cherokee that had one of those skinny Terra radios in
it, that would pick up alternator whine really bad, and the Lonestar
filter placed inline with the radio's power wire fixed it up perfectly.
The Lonestar filter was only about $80 back then, and now costs about
$110.

A quick way to tell a bad diode is the use of a clip on amp meter that
will read AC and DC amps. Only the Hall sensor ones will do this. Load
the output of the alternator to at least 50% of rated output and
measure the AC and DC amps. If the AC amps exceed 15% of the DC amps
you have a bad diode or stator winding. This works on all 3 phase DC
output alternators.
John

On Wed, 19 Apr 2006 22:13:56 -0400, Jonathan Goodish
wrote:

In article . com,
"mikem" wrote:

I have no explanation for the 32V reading.


I had an opportunity to poke around some more this evening.

Despite Greenlee's insistence that my meter blocks DC when measuring AC
volts (and vice versa), that appears to not be the case. The meter
reads 26 volts AC when measured on the straight battery--no alternator.
It appears to read AC correctly, and DC correctly, but not AC ripple on
a DC bus.

I have a 20 year old Beckman meter that was out of calibration, but I
calibrated for AC and DC volts against the new Greenlee. The Beckman
appears to block DC when reading AC volts.

Both meters show around 12.8v at the battery, and almost exactly 14.0
volts DC when the alternator is charging, both at the battery and the
bus. I've determined that the Greenlee's AC reading on the DC bus is
highly inaccurate, but the Beckman shows almost zero AC volts (0.008) on
the DC bus--I assume because the battery is absorbing the ripple.
However, at the alternator, the Beckman shows approximately 0.30 volts
AC, which increases to a maximum of 0.65 volts AC under full load and
max static RPM. These look more like reasonable numbers.

Could the voltage regulator be at fault? Load seems to amplify the
noise, but the bus voltage is almost exactly 14.0 volts, regardless of
load or RPM. I would think if the regulator was failing, I'd see some
fluctuation, especially under full load. Could I be missing something?

I know that it may very well still be a ground loop. There are some
oddities that I still can't explain, however, like the intermittent
static/crackling when the copilot keys the PTT and transmits. The
landing light causes occasional crackle in the audio, but a corroded
connector may be contributing to that problem, not sure.



JKG