05-16-2014, 08:01 PM
The operation of the volume control seems normal to me. As Jerry mentioned, this is not your usual audio volume control. One section attenuates the antenna signal, while the second section adjusts the voltage on the screen grids of the RF amp tubes which determines the amplification.
Together, the resulting effect of the dual control is to set the sensitivity of the receiver. There is no automatic volume control, so audio level will depend on the signal strength and the setting of the control itself. Takes a bit of getting used to if you are more familiar with later radios equipped with AVC and an audio volume control.
When you touch the grid cap of one of the 24 tubes, you are detuning the resonant circuit associated with that grid with the extra capacitance of your body. So the tuning cap needs to be adjusted slightly to compensate for the added capacitance to maintain resonance of that stage. Again this is normal and not indicative of a problem.
Be very careful if you decide to open up the large tar filled filter cap can. It also contains the audio choke which drives the output tube grids. You may be better off adding caps external to the can rather than risk damaging the delicate windings of the choke inside. Some things are better left alone....
The Nostalgia Air schematic shows the contents of the can:
http://www.nostalgiaair.org/PagesByModel...040334.pdf
Together, the resulting effect of the dual control is to set the sensitivity of the receiver. There is no automatic volume control, so audio level will depend on the signal strength and the setting of the control itself. Takes a bit of getting used to if you are more familiar with later radios equipped with AVC and an audio volume control.
When you touch the grid cap of one of the 24 tubes, you are detuning the resonant circuit associated with that grid with the extra capacitance of your body. So the tuning cap needs to be adjusted slightly to compensate for the added capacitance to maintain resonance of that stage. Again this is normal and not indicative of a problem.
Be very careful if you decide to open up the large tar filled filter cap can. It also contains the audio choke which drives the output tube grids. You may be better off adding caps external to the can rather than risk damaging the delicate windings of the choke inside. Some things are better left alone....
The Nostalgia Air schematic shows the contents of the can:
http://www.nostalgiaair.org/PagesByModel...040334.pdf
