George Braly wrote:

> Can you describe in more detail the nature of the problem between the
> alternator and the compass?
>
> Is it the alternator, itself?  ON ? or OFF?  Or is it the wiring passing
> near the compass?

hi George,

I have discovered that have the same set-up that Dave has. I have the TSIO-550
in my ES so the B&C alternator is located on the rear right accessory pad. The
SIRS compass is located in the middle of the panel cover just about directly
over the edge of the panel. The B&C alternator is connected to a B&C voltage
regulator. It is wired to assume control automatically should the primary
alternator experience a problem. The primary alternator is a Continental 100 AMP
wired to a Zeftronics voltage regulator. I have a 28 volt electrical system.

For the longest time I thought I was chasing an HSI problem, but after working
with a good avionics tech we isolated the problem to compass interference from
the B&C unit. When the Master switch is activated you can watch the compass
swing 30' left. Admittedly, this was never part of my start-up check list before
so it took a while to find it. When I pull the B&C breaker the interference is
gone. I can pull the breaker and start the engine and then push the breaker back
in but then higher RPM's will cause it to swing.

I spoke to the tech at B&C and his response was "yes, some of them do that". He
also advised that it is worse on the ground and when facing east or west. This
seems to be true. I have tried using MU Metal to block the magnetic field
interference but it didn't provide much if any relief from the problem. I moved
the compass back to the rear seats and still experienced a 10' error. I also
moved it to the left side of the panel which reduced the error to approximately
15'. SIRS says that a back-up alternator shouldn't cause a swing like I am
experiencing.

The compass is approximately 24" from the B&C unit. I had a discussion with a
good avionics shop and after I explained what I had done to isolate the problem
he seemed to think it was the B&C unit and that I should ask for another unit.
B&C's response is above. He said they put in back up alternators in my location
(on 520's and 550's) all the time and don't experience the problem.

At this point I am planning to try other materials to try to block the magnetic
field that B&C is causing. I plan to start on this "science project" next week.
By the way, both B&C and SIRS are interested in any results I come up with.

Ed Rosiak
Super ES


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