Feedback problems with astatic echo boards

This is a classic indication of a lost shield or ground connection in the mic circuitry. Double check these connections for good contact in the mic *and* the plug at the other end of the cable.

Check the grounding and SWR or "match"of the antenna system, too. A poor SWR (greater than 1.5:1 or 2.0:1) can cause the reflection of RF back to the radio. Modifications can increase sensitivity of the mic to this unwanted RF.

hi,

A) wiring the RK56 mic direct to the Cobra 29 LTD Classic seems to

be no problem about it.

B) using EB-1 Astatic echo board in between the two might produce

hum if :

1) the unit ( Astatic ) is not properly shielded , meaning it should

be mounted on a metal casing, and have a proper grounding

on the case.

2) there is unregulated supply on the echo board.

3) the echo board is mounted near the large transformer of the

power supply

4) there is no proper matching between the echo board out

and the Cobra mic in.

5) the output of the echo board is not properly shielded.


here is a simple matching between the output of the echo board

and the input of the mic in Cobra 29

the in = to be connected to the echo out

the out = to be connected to Cobra mic in


the variable resistor or sometimes called the potentiometer

will match the impedance of the echo out to the mic in.

it can be any from 5 K to 10 K ohms





hope this help you, thanks for using Fixya!

also try a 100 ohm 1 watt resistor for your talkback