Archive for the ‘Transmit Squeals or hums’ Category

There are two distinct types of squeals. An audio feedback and an RF squeal or hum.

An audio feedback (squeal) is caused when the speaker is located to close to the mike and the talkback function is set to loud. The more sensitive the mike the worse the audio feedback. The less distance from the mike to the speaker the worse the squeal.

Audio feedback defined: A phenomenon that occurs when the sound waves produced by speakers interact with an input transducer such as a microphone or phonographic cartridge. If the feedback exceeds a certain amount, any of various undesired effects may occur, such as howling, whistling, motor-boating, or excessive cone movement. Also, the sound heard as a consequence of this. Also known as acoustic feedback, acoustic regeneration, acoustic howl, audio frequency feedback, audio feedback, or howl.

An RF squeal or growl has a much harsher sound as compared to an audio feedback. It is normally caused from the radios own signal re-entering the radio and effecting the mike amp circuit or the transmitter circuit. This phenomenon normally occurs when there is a ground problem at the antenna, radio chassis, or inadequate voltage supply.

The first step is to figure out which type of squeal you have in order to fix The problem.

Turn the mike gain all the way to the right. It should squeal badly when keyed.

Now cup your hand over the mike and key the mike. Does it squeal?

If it does you probably have an RF squeal, if not you probably have an audio squeal.

Do you have the talkback function? This is a function that allows the user to monitor there audio transmission.

If so try turning it down or off. Turning it off should eliminate the squeal if the problem is an audio feedback or squeal. Turn it off and verify the condition by getting a radio check from another radio operator.

To fix an audio squeal: Turn the external speaker around facing away from you. If the speaker has a noise filter switch turn it on. Did this resolve the squeal?

If you still have the squeal try moving the speaker farther away from you. If your talkback has a variable volume control reducing the volume and turning the speaker facing away from you may be the trick. If neither resolves the problem try replacing the mike with either an Astatic 636L or a Turner RK56 noise canceling mike.

The RF squeal can be a bit more complicated.

The RF squeal can be caused from any of the following:

1) Inadequate ground on the antenna or case of the radio.

2) The radios ground wire location or to long of a ground wire. Never connect the ground wire to the battery.

3) High antenna SWR normally caused from lack of ground.

4) Inadequate shielding in the mike cord.


Unfortunately there is no scientific testing that will tell us exactly which option is causing the problem.

The fix involves process of elimination.

To fix an RF squeal:

1) Attach short braided straps from the case of the radio to the dash. The screw must go through the dash material into metal. The strap must be a basket weave type material and must be short. No longer than 18 inches. Two straps are better than one. Coax shielding works well for this application.

2) Check your SWR. Set the SWR and verify the SWR is lower than a 2.0. Move the coax while you are checking the SWR. Does the SWR fluctuate? If the SWR fluctuates read up on troubleshooting antenna system problems in our blog.

3) Wrap the mike cord around your hand about 4 wraps half way down the cord and key the mike. Does it squeal now? If this stopped the squeal purchase a .01 pf 50v ceramic disc capacitor. Cut the legs short, take the mike plug apart and solder the capacitor from the shield pin (bare wire) to the modulation pin (the wire the shield wire wraps around). This will be pin 1 to pin 2 if you have a 4 pin mike plug and a Cobra, Uniden, President, Galaxy, or Midland radio.

4) If the DC ground wire feeding the radio is attached to the battery remove it, cut it short and connect it to a ground close to the radio. A screw can go through plastic dash providing it screws into metal behind the plastic.

 

 

 

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