Discussion in 'Instruments / Avionics / Electrical System' started by BBerson, Aug 28, 2009.

1. Sep 6, 2009

### djschwartz

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None that I'm aware of

1) A lot of expensive, specialized test equipment.
Avionics Test Solutions - Aeroflex

2) The knowledge to use it.

3) Factory authorization as a repair station for their radios.

4) FAA approval as a repair station.

Without the last two you are unlikely to get the factory authorization and without the factory authorization you won't have access to the information needed to diagnose problems or the special replacement parts needed to fix them.

2. Sep 6, 2009

### BBerson

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Getting the General Radiotelephone License (GROL) looks pretty easy with a $50 lesson book and test fee. The repair station would not be easy. I have A&P-IA, would that be sufficient? Or is a repair station required to get factory assistance and repair manuals? My brother signed up for the Cleveland Institute of Electronics home study course about 1970. I was 15, and went through about the first half. The course was quite good, I thought. They have an option now for$200 here:http://www.ciebookstore.com/bookstore/catNo/5/productid/131/addtocart.asp

3. Sep 6, 2009

### djschwartz

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4. Sep 6, 2009

### BBerson

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That is what I was thinking. My only interest would be in basic troubleshooting. I already know that earning a reasonable income is impossible. I may try to find a book about simple testing and maintenance.
BB

5. Sep 8, 2009

### BBerson

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dj,

I found my mic jack is installed without any insulated washers. I found the Sigtronics installation here http://www.sigtronics.com/pdf/air_pdf/spa-400.pdf and it calls for two insulated washers per mic jack, just as you advised.

The repair station did not use the insulated washers. Probably because it was easier to just install directly in the aluminum panel.
The Grob G109 is all fiberglass. So maybe it will not make any difference.

Also, I was finally able to read the intercom operating instructions and noted that the co-pilot mic doesn't work when the intercom is off.

Still wondering what a switch on the side of the intercom does that says: "keep alive". Nothing seems to matter if on "keep alive" or "off". So I leave it on keep alive for good luck.

thanks dj

6. Sep 8, 2009

### djschwartz

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It depends on what else might be grounded to that aluminum panel. If the ignition is grounded to that panel along with the mic jacks then you almost certainly will have a ground loop that couples ignition noise into the mic circuit. The panel will need to be connected back to the negative of the battery somehow and all currents coming to that panel will flow through that same ground wire together to get back to the battery. And I'll bet that ground wire isn't very big since it doesn't carry the full battery charge or engine start currents. That's a classic setup for a ground loop. The insulated washers should help and that will be a lot simpler than trying to reroute grounds with heavier wire.

One advantage of a non-conductive structure is that it forces one to think about the ground return side of electrical circuits. In an aircraft with a metal structure people tend to assume they can just attach a ground to the airframe where ever it is convenient. That may work OK for simple DC power to lights, gyros, an other insensitive accessories; but, it can wreak havoc with radios.

Dave

7. Sep 9, 2009

### BBerson

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I do not really understand what a ground loop is exactly. But I will install ground wires to the mic jack and see what happens. Right now, the mic jack is grounded in the aluminum panel by the mic jack itself.

Is a ground loop caused by two ground paths?

If I understand correctly, I should eliminate the existing mic ground in the panel by installing insulated washers. Then I will solder ground wires to the mic jack solder lugs and connect these ground wires to the intercom ground. So the electrons will return on the ground wires only and not the instrument panel. Is that right?

8. Sep 9, 2009

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