MC275 Hum Conundrum

[rkic]

When I move a pair of needle nose pliers towards capacitors C1 and C12 (input caps) the existing hum gets louder. As I approach the caps from a different angle the hum gets louder with the closer cap. I tried an aluminum framed magnifying glass - this also increased the hum. A plastic probe did not increase the hum. The hum would increase by probably 6 - 9 db.
Gentlemen - your thoughts?

and a happy fourth to all of you.

 

[dewickt]

Sounds right to me, this is the input stage with gain, you and the pliers are an antenna picking up 60 hz from local sources, line cord, lights etc., and transfering it to the input stage, common occurence. Try putting your other hand on the chassis and see if the hum lessens, also turn the gain control to 0 and see if this helps, an open input loves to pick up humm.