Grounding A Leslie when powered by separate AC cord

[dannicdem]

My Leslie 710 has a separate AC cord and is switched. It looks like the 2 prong AC cord was installed aftermarket maybe 30 years ago as the insulation looks old. My question to the group is should the AC power be rewired with a ground? There is a slight hum coming from this Leslie 710 which is audible when powered up either with the Hammond A-100 on or off. Is this a needed upgrade?

In reading through the posts here, I'm a speculating that the power caps (I think "caps" means capacitors) might need to be replaced to eliminate the hum.

Best!

Dan D.

[geoelectro]

"Three wire" power cords with the earth ground usually cause hum rather than fix it. That's because they create a ground loop situation. The third wire for ground is more a safety thing, not a noise or hum thing.

You might try powering up the Leslie and with the Leslie cable unplugged from the organ and see if the hum is still there.

You are right that power supply capacitors are suspect when dealing with hum. Also, a power amp problem can cause it like an open output transistor which puts DC across a speaker.

I would pull the power supply out and flip it over. The large filter caps have screw terminals that you should tighten. A loose screw there will cause hum. I would measure the two main supply rails right on the cap terminals looking at the DC and AC levels. AC found on a DC supply line indicates a bad filter. I would also measure the amplifier outputs for DC as well.

Geo

[dannicdem]

Have just acquired a flawless Leslie 720 that was connected to a Thomas organ. The Leslie 720 has a separate AC power cord which is a three prong plug. The grounding wire (green) is bolted to the power supply metal casing box.

If I am understanding the advice correctly, it sounds like the grounding wire should be disconnected.

Thanks in advance for the advice and experience of the forum members.

Best!

Dan D.

[tucsondave]

No.
The chassis ground and circuit (audio) ground are separate. As Geo said the chassis ground is a safety thing.

td