[Hallicrafters] Hallicrafters SX-111 ( very high pitched sound )

[Mike]

I have good working SX-111 mk1  in every way except after its been on for a while for no reason at all sometimes you will hear a very loud high pitch sound thru the speaker. This sounds overides receiving anything over the air. I can sometimes cut the power off and then back on and it goes away and sometimes it don't until you leave the receiver off for a little while. Does anyone know what is causing this ? 


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