G'day all,
I am currently working on a Yamaha EM-1600 powered mixer.

The problem is across all channels (6), it is a hum, like the unshielded mic. input hum. The problem is in the pre-amp stages of all the channel cards, the problem is not after these stages, it is in them all. I feel there may be some form of power suply radiated hum.

Looking on the CRO, there is no discernable ripple from either the +ve or -ve rails.

The problem can be shut down by bringing the fader to zero, but this is not too good when trying to amplify anything .

IC202 seems to be where the problem is coming from, the output of this chip feeds the fader pot.

Hope someone knows of this problem, or any modification that might overcome it.

Regards,
Rod vk3kbr

G'day all again,
well, no takers.... I have finally found the fault. It is magnetic field radiation from the power transformer. Why it has taken this long to become a pest, is anybody's guess.

I have proved this problem by first removing the transformer, connecting it to the set by flying leads, and silence was the order of the day.

Replacing the transformer into the case, wrapped in aluminium "flashing", has also quitened the thing to zero. Unfortunately the space in the case will not allow me to use this as a cure, but at least I now know what the problem is.

Regards,
Rod vk3kbr