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Old 24th Sep 2020, 10:31 pm   #15
Ambientnoise
Heptode
 
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 915
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

A few random thoughts, tracing hum can be a pain. The following might give some clues.

If that’s a neon, then remove it as they can induce hum/buzz. I presume the chassis is connected to mains earth and to 0v at one point ? If the hum goes with shorted inputs then it will be coming in at the very front end. Double check for clean breaks in the veroboard tracks, minute whiskers can give all sorts of problems. Try powering from a couple of 9v batteries and see what happens to the hum. Try unbolting the transformer and insulating from the chassis, but link it to earth. Ensure the metal cases of the pots are connected to chassis. Ensure the board fixings are not causing shorts to chassis.
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