Quoted from DennisT:
I hope this is a simple fix. My Flash has always had what could be described as a loud ac hum at idle. The board is a type 1 version 2 and has been recapped. There’s no hum when connected to dc power on the bench. I also have another board I tried that also has the same humming issue.
I searched the forum and didn’t see anything about this problem.
Anyone else ever have this problem?
These boards will have some hum but it should be inaudible unless you get your ear up to the speaker or turn the volume way up. If two borads have identical type hum its probably not the sound board.
I had a time warp where the power board square 6 pin (only 2 wires used), where ground comes in, burn up and cause the nasty hum out of the sound board. Even tho the sound board has its own transformer tap and power supply, a ground problem on the main power supply must be able get noise into the sound board. Try reseating that plug with the power off and see if the hum changes.
Check the flash manual page 21 and 22, these pages are reversed in the IPDB scan pdf file. It should not apply to a type 2 sound board but they have some notes about noise you may want to read.