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Does the squeaking happen only when stations are tuned in?
I presume it only happens with Shortwave... if you can get other bands, does it happen with other bands' stations?
And if so, does it squeak 'with' the sounds, or independent of them or continuously?
Please describe the problem a little more.
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Try turning up the gain, then cup your hand over the microphone, and key the microphone.
Does the squeal sound, at this point?