What's happening when the volume knob seems to work ok but won't turn down the radio all the way?

With some radios this is normal--turns down to a very low level, but not gone all the way. With Zenith portables, this is built into the radio. It serves both a biasing purpose and also to alert the owner of the radio that it is still on and is consuming the batteries. With AC sets, this can just have to do with how the resistance material is constructed and/or how the low end of the volume control is connected to the common supply in the radio (B-). If the volume control will not turn the volume down to an almost silent level--the volume stays somewhat high, though lower than full on, then you may have a faulty volume control. Either the low end is not connected properly, or it is broken internally. To test a volume control, disconnect all wires leading to it and measure the resistance across the high and low taps. Then measure the resistance from the center to each tap while operating the control. If the control has any other taps (usually found alone about 180 degrees off from the three main terminals), it is usually customary to measure the resistance of the tap to the low end of the control.

In case you are confused as to which terminal is the high and which is the low, with the shaft facing you and the terminals facing down, the low terminal will be at left, and is fully contacted internally when the control slider is rotated fully counter-clockwise. The right terminal is the high terminal, and is in full contact with the control slider when the control is rotated fully clockwise. To further aid in visualization if you have never taken one of these controls apart, there is a 300 degree (of a circle) circular resistance material of which each end is connected to one of the end terminals. The center terminal connects to the rotor, which is connected to the control shaft, which has a slider that makes contact with the resistance material at different positions as the control is rotated.

Hope I did not go into too much detail for you. Some people in here can't handle detail.

Thomas

:With some radios this is normal--turns down to a very low level, but not gone all the way. With Zenith portables, this is built into the radio. It serves both a biasing purpose and also to alert the owner of the radio that it is still on and is consuming the batteries. With AC sets, this can just have to do with how the resistance material is constructed and/or how the low end of the volume control is connected to the common supply in the radio (B-). If the volume control will not turn the volume down to an almost silent level--the volume stays somewhat high, though lower than full on, then you may have a faulty volume control. Either the low end is not connected properly, or it is broken internally. To test a volume control, disconnect all wires leading to it and measure the resistance across the high and low taps. Then measure the resistance from the center to each tap while operating the control. If the control has any other taps (usually found alone about 180 degrees off from the three main terminals), it is usually customary to measure the resistance of the tap to the low end of the control.
:
:In case you are confused as to which terminal is the high and which is the low, with the shaft facing you and the terminals facing down, the low terminal will be at left, and is fully contacted internally when the control slider is rotated fully counter-clockwise. The right terminal is the high terminal, and is in full contact with the control slider when the control is rotated fully clockwise. To further aid in visualization if you have never taken one of these controls apart, there is a 300 degree (of a circle) circular resistance material of which each end is connected to one of the end terminals. The center terminal connects to the rotor, which is connected to the control shaft, which has a slider that makes contact with the resistance material at different positions as the control is rotated.
:
:Hope I did not go into too much detail for you. Some people in here can't handle detail.
:
:Thomas

Thanks so much, Thomas. I appreciate all the detail you care to supply.

David