Radio help please

[601man]

I have wired up my com radio and everything works except when I connect the mic socket to the metal dash I get a hum( grounding feedback issue) ?

 

If I hold the socket in the air, nothing I hum, If it touches the metal fuse then the hum. Somone told me to isolate the socket by heat shrinking the threaded part and fiber washers on each side of the nuts..

 

Never heard of this before.. Is that the normal installation or will it be just hiding an issue.

 

Chris

 

[quote="Dick Gower, post: 586082, member: 13612

 

The other common problem area of vulnerability to RF feedback is the MIC input circuit and, in particular, the outer shield running from the headset to the inside of the radio via the MIC ground input (which is not the same as the other grounds such as power and should not be connected to any other grounds). In extreme cases it is therefore necessary to insulate the MIC jack mounting to achieve this and stepped plastic washers are available for the purpose.

 

Plastic and rag aeroplanes are particularly vulnerable to RF feedback.

 

Good luck!

 

 

[bstrachan]

I have wired up my com radio and everything works except when I connect the mic socket to the metal dash I get a hum( grounding feedback issue) ?If I hold the socket in the air, nothing I hum, If it touches the metal fuse then the hum. Somone told me to isolate the socket by heat shrinking the threaded part and fiber washers on each side of the nuts..

 

Never heard of this before.. Is that the normal installation or will it be just hiding an issue.

 

Chris

 

----------------------

 

Not only is isolating the "sleeve" of the mic and phone jacks normal, it is MANDATORY. The sleeve is NOT a ground, it is the signal return line for the mic or phone. There are various ways of isolating these jacks, just make sure they are not connected in any way to the metal fuselage. If your mic and phone lines have a braided shield, that shield should be grounded at the radio end and left floating (unconnected to anything) at the jack end. Standard procedure.

[jetboy]

Insulating the headset jacks from airframe metal is normal best practice.

 

The other best practice is to wire the -ve supply (return) wires from all the avionics to one ground stud or bus bar, attached directly to the battery / charging system common ground point.

 

problems arise with VHF radios typically the antenna end of the cable is grounded and the radio end is also grounded at a different place. This one is hard to avoid, but you can avoid adding any more paths by isolating all other connections from airframe.