12-15-2021, 08:46 AM
Hi Folks,
Yes, almost any AA5 I have ever worked on has the power switch on the "chassis" (B-) side of the line. My best guess is that reason for doing so was to keep the switch, usually mounted on the volume control at a low potential to cut down on hum. Voltage-wise it makes sense, current-wise, not so much. Safety-wise, it makes absolutely no sense.
Most "hot chassis" sets connect the chassis to the power line through a cap or cap with a high value resistor in parallel. However, some (my Emerson 330 for example) has the chassis wired directly to B-, through the power switch. The chassis mounting bolts are accessible from the bottom of the radio, and the back, of course, is missing. The plug in one direction could make the chassis hot with the switch closed. the plug the other way has chassis at ground potential with the switch closed but hot, through the heater string, with the switch open.
My RCA 8X71 AM-FM is the same way, but at least there is an interlock and no chassis screws protruding.
I always modify any AA5 set that I work on to put the switch on the hot side and the neutral (Wide prong of plug) to the B- side. On a true hot chassis, it may be a good idea to isolate neutral - B- from the Chassis using a 0.1uF cap with a 250K resistor in parallel from Neutral-B- to the chassis. This provides a path for RF (the Tuning Cap frame is usually grounded to the chassis) and prevents hum from the chassis floating.
In the rare case when placing the switch on the hot side instead of the neutral side produces too much hum and lead dress or shielding does not resolve, leave the power switch out of the circuit and install a switch in the hot side of the Line Cord. Yeah, not factory, not authentic, but safer.
On transformer powered sets, use safety caps, one across the transformer after the switch, the other between chassis and neutral (wide prong on plug)
Yes, almost any AA5 I have ever worked on has the power switch on the "chassis" (B-) side of the line. My best guess is that reason for doing so was to keep the switch, usually mounted on the volume control at a low potential to cut down on hum. Voltage-wise it makes sense, current-wise, not so much. Safety-wise, it makes absolutely no sense.
Most "hot chassis" sets connect the chassis to the power line through a cap or cap with a high value resistor in parallel. However, some (my Emerson 330 for example) has the chassis wired directly to B-, through the power switch. The chassis mounting bolts are accessible from the bottom of the radio, and the back, of course, is missing. The plug in one direction could make the chassis hot with the switch closed. the plug the other way has chassis at ground potential with the switch closed but hot, through the heater string, with the switch open.
My RCA 8X71 AM-FM is the same way, but at least there is an interlock and no chassis screws protruding.
I always modify any AA5 set that I work on to put the switch on the hot side and the neutral (Wide prong of plug) to the B- side. On a true hot chassis, it may be a good idea to isolate neutral - B- from the Chassis using a 0.1uF cap with a 250K resistor in parallel from Neutral-B- to the chassis. This provides a path for RF (the Tuning Cap frame is usually grounded to the chassis) and prevents hum from the chassis floating.
In the rare case when placing the switch on the hot side instead of the neutral side produces too much hum and lead dress or shielding does not resolve, leave the power switch out of the circuit and install a switch in the hot side of the Line Cord. Yeah, not factory, not authentic, but safer.
On transformer powered sets, use safety caps, one across the transformer after the switch, the other between chassis and neutral (wide prong on plug)
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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