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ctfarmer

Garmin SL40 problem

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just an update, I took out the PM3000 intercom box, cleaned pins and checked general area. The green wire (which I assume is gnd) looks like it was an afterthought connection into the PM3000 box and it has been dangling in behind my centre console (garmin 696 etc). Since I have had the console in and out several times  trouble shooting the 696, that green wire may have been pulled or moved sufficiently to lose/loosen the contact. I have place the green wire out of the way and I have actually had a test of the radio with no static noise!! I will have a closer look at that green wire hook-up and re-solder at a later date. Hope this solves the issue, thanks to all for their input... now on to the auto-pilot...

gnd wire behind PFD.jpg

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If it's not receiving power, then it will have no effect.

Also, a ground loop is a circular path to ground. It acts like an antenna, picking up fields and fluctuating ground potential. Most electronics are not designed with a fluctuating ground potential in mind, and thus they amplify this noise.

In addition, ground connections need to have as little resistance as possible, especially if several devices share the same ground. This is another type of ground loop, where the power circuits themselves become the loops. A dirty ground raises the resistance, and when power flows across it, causes a voltage drop. If there's only one device, this isn't as big of a deal. HOWEVER, if multiple devices are using it, and you put a device on it with a fluctuating power load, it will cause ALL devices to fluctuate due to the varying voltage drop.

It's like hooking up a hose to your water system. It's fine if you're just using a hand sprayer, but imagine if you split that hose out into several lawn sprinklers like these:

how-calculate-lawn-irrigation-3.jpg

 

Now the overall pressure will be low, they won't be spraying very well. Now if you have a hand sprayer hooked up too, you squeeze the trigger and you can watch the water pressure on your sprinklers change quite a lot. This is what the electronics see with a dirty ground and a noisy electrical device, it acts like the single hose trying to feed all these sprinklers.

Here is an extreme example of an electrical ground loop noise: https://upload.wikimedia.org/wikipedia/commons/d/d7/Ground_loop_50_Hz_sound.flac

I've said it before and I'll say it again: I am not impressed with flight design's electrical system design. It's like someone opened a book and thumbed through to the "amperage carrying capacity" section for wire sizes, and ignored the rest of the design considerations. There's a reason main aircraft grounds are FREAKING HUGE. Far larger than what is required for just amperage.

 

EDIT: just saw you found the cause of the issue. Great!

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Anticept... thank you for the explanation. I am keeping my fingers crossed and your post has me wondering is it only the ground wires that can create the gnd loop or can powered wires too?... I am sorry but my electrical theory knowledge is not good.

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Ground is also known as "reference". Electronic design assumes reference is 0 volts. It doesn't care what is on the power rail (I will henceforth refer to the power input as the power rail), and is expected to fluctuate.

We can design electronics which have a non-zero reference, but it's quite complicated and unnecessary. It's far easier and cheaper to just design for a 0v reference, which is easy to do (well... it should be anyways).

Why can the power rail fluctuate? Switched mode power supplies, or SMPS! They are designed to conduct power until the voltage is high enough, then it opens the circuit, at which point capacitors and inductors in the system act as the power source until they discharge enough that the voltage drops low enough for the SMPS to close the input and charge the system back up.

Now, electricity works on potential difference. If you have a system supplying 12 volts, and has a ground with 5 volts, then it's the same as a 7 volts input and 0v ground. 12 - 5 = 7. Most aircraft systems would shut down in such a situation.

Now, that said: radio and audio systems are SUPER SENSITIVE because of the amplifiers in the system. Amplifiers take a control signal, like a sound signal, and use it to control a POWER rail, which is needed to drive the speakers in your headset. By controlling that power rail with a control signal, we shape the power rail waveform to took like the control!

Now, SMPS or other voltage regulation will try to keep the power rail steady in the circuit, at least to a tolerable level. However, they do not control ground voltage in any shape or form. Compensating for fluctuating grounds is incredibly difficult, to even impossible if it's great enough. That is why ground loops are so incredibly bad, they cause the ground voltage to fluctuate, which causes the power rail to ground reference potential difference to change. In addition, the amplifier and your headset comes after the SMPS, so by the time the SMPS sees the problem, the amplifier and headset are already affected. That fluctuating ground can even affect the control signal as well, making the whole situation even worse!

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thank you Anticept.... did another 1/2 hour today of flight and no noise on my transmit.... I still may remove the extra gnd wires to #3 cylinder as I can not be sure that they have made any difference to the situation..

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hello Anticept... did not remove the extra gnd wires and static seems to be nil on my transmissions... but I did find from one of your other posts about the need for an audio wire  between the EFIS and intercom for the AOA beep... that is what that green wire is (seen on one of my earlier pictures) that I thought was a gnd wire and an afterthought..thanks

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It's whatever color they decide the wire to be.

Flight design loves to tie the wires in a giant ball of electrical tape (they really shouldn't, vinyl tape is baaaaad and highly toxic when it melts and burns), so often people just attach them to the outside of the wire looms as a result.

I'm still trying to figure out a tool that would make it easy to cut the tape free without cutting wires.

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My aux music jack toggle switch seems to, all of a sudden, be causing background buzzing in a repeating pattern (on off on off) in the headsets.  I don't use the aux music jack at all.  When i turn the toggle switch to off (down), the sound stops but so does the side tone. Flipping it back up causes the buzz again, but sometimes not.  It is becoming more frequent.  The sound does not appear to be going out the radio.  

I guess I want to replace the toggle switch but I can't find a reference as to a part number / source.  Can anybody assist in this resolution?

Thanks

Danny

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You probably had a wire break off from a terminal on the back of it. I've seen that cause issues. The connection to the GPS also can buzz, and the switch can get turned upside down, causing it to actually be the GPS.

But the switch itself being bad is pretty rare.

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21 hours ago, Anticept said:

You probably had a wire break off from a terminal on the back of it. I've seen that cause issues. The connection to the GPS also can buzz, and the switch can get turned upside down, causing it to actually be the GPS.

But the switch itself being bad is pretty rare.

I'll check the wires on back before I proceed further.  Thanks

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i have  the sl40 for a few years and it has been fine i just replaced the voltage regulator and the transmit makes a large noise only on one particular frequency ,if i turn the audio panel on it works fine ,i cant figure out what changed?

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