At 02:23 PM 7/6/2018, you wrote:
Heath SB101. When I turn audio gain up over one quarter of range, radio takes off into a loud squeal. RF gain or mode settings make no difference. Suggestions? Bad PCB ground maybe?
Maybe a microphonic audio tube? Turn down the volume under a quarter of range and gently tap on the audio tubes one at a time (might as well do it to all of them) and see if you can hear a thumping noise in the speaker. That would indicate the tube is sensitive to vibration, so when the audio is turned up the sound will make the tube vibrate and cause feedback, just as it was an open microphone. Hope that helps.
Alex
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