740b affecting compass


May 25, 2023
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My 740b and my new PAI-700 vertical card compass don't play well together. I am in the process of moving the compass further from the 740b, but have considered another option. I don't use any of the audio function so I am looking at removing the two speakers from the 740b. Has anyone done the successfully? I would put appropriate resistors in place of the speaker coils. Anybody have any experience opening up the 740b?

Dave G
I'd point out that just removing the speakers (to get rid of what I assume are the permanent magnets -- if it uses those type of speakers), will not necessarily remove electro-magnetic interference from the electronics, which could be some/all of the problem. Maybe before opening the case you could try to do some shielding? Like copper tape? (I assume that the obvious answer of relocating the 740b is not an option.) But perhaps this would be unsightly in your particular case.

You've probably already tried this: to figure out whether the speaker magnets (if it has them) are causing the problem, you could try bringing the unplugged 740b closer and closer to your compass and see if it moves it. Of course, if you get it close enough, it probably will regardless. But I mean bring it to its normal mounted position.
Yes, So far I have tried disconnecting the 740 from the ships power, turning off the 740 with ships power disconnected with no change in the affect on the compass. I took the 740 off its mount while flying and the compass becomes accurate with 740 laying in the seat. I am reasonable sure the speeker magnets are the issue. Before go into the 740 case I am going to relocate the compass as far away from it as is possible. Shielding is also an interim possibility.
Thanks for the input,
Dave G
Yes, I saw it in the instructions for PAI's balance balls ( sounds like there could be seals involved). I have just ordered some, supposed to be here Monday. I have hopes that by moving the compass further away from the 740 and using the Mu-Metal I will be able to use the new compass.
Dave G
For anybody that might be interested, here is a rundown on what I have done to mitigate the impact of the 740b on my mechanical compass. 1. I physically moved the compass away from the gps. 2. I applied some Mu-metal to the back of the gps.

At this point it appears that the relocation of the compass made the greatest improvement. The center of the 740 b screen and the center of the compass rose are 10" apart now, vs. approximately 4" originally. The compass now is within <10 degrees of actual heading all the way around. I did not try to cover the entire back cover of the gps, just a couple 2" square patches of Mu-Metal over the speakers, which did not make a measurable difference. Maybe if a Mu-Metal lines shroud was made to wrap around the back and sides of the gps an improvement could be seen.

Thanks to all of you that responded to my post.