Remove Hum from AA5

[jr_tech]

How do you handle those radios that are wired in such a way that the chassis becomes "hot" when the power switch is off, due to back-feeding of the hot side of the line through the heater string?

jr

[bob91343]

You don't. Or you unplug them first.

[Reece]

It's possible to make a hot chassis or "semi-hot" chassis radio safe once it's in the case. Ones from the late fifties and the sixties were made this way with no hot exposed metal. The safety of older sets can be improved with some ingenuity. For example, steel underchassis screws can be replaced with nylon ones which are plenty strong for most table radios. Backs can be provided that ventilate adequately yet keep fingers out.

[maxhifi]

This one is all on one PC board, and the knobs can't come off. I'm not that worried about safety.

I noticed something interesting last night; the hum continues when the radio is off. The clock motor is mechanically transmitting vibrations through the cabinet to the speaker cone, which is adding to the 60Hz noise.

Aside from that didn't get any bench time last night. I may modify a radio with no clock in it instead, to get the clock noise out of the picture.

[wa2ise]

Originally Posted by maxhifi

This one is all on one PC board, and the knobs can't come off. I'm not that worried about safety.

I noticed something interesting last night; the hum continues when the radio is off. The clock motor is mechanically transmitting vibrations through the cabinet to the speaker cone, which is adding to the 60Hz noise.

To improve safety here, you can rewire the clock radio power switch to switch the line that goes to the rectifier tube, and hard wire the line that goes to the radio's ground. And the line that goes to the radio's ground should go to the power plug's neutral side (on a modern 2 prong power cord, the wider of the two prongs).