01-05-2014, 07:24 PM
The meter operation seems normal if it starts narrow and then widens as the tubes warm up.
The shadowmeter operates by indicating the plate current of the AVC controlled tubes. As the signals get stronger, the negative AVC bias voltage increases which decreases the plate current, causing the shadow to narrow.
If the AVC voltage is too low, there will be little deflection. Can you measure the AVC voltage with a DVM when tuning in a strong station? You should have at least 10 V of AVC voltage with a strong local signal. If you are not getting that much then there will be too little change in plate current to narrow the shadow significantly.
You can measure the AVC voltage by connecting the negative probe of the DVM to chassis and the positive probe to the top of the volume control ( detector output).
The shadowmeter operates by indicating the plate current of the AVC controlled tubes. As the signals get stronger, the negative AVC bias voltage increases which decreases the plate current, causing the shadow to narrow.
If the AVC voltage is too low, there will be little deflection. Can you measure the AVC voltage with a DVM when tuning in a strong station? You should have at least 10 V of AVC voltage with a strong local signal. If you are not getting that much then there will be too little change in plate current to narrow the shadow significantly.
You can measure the AVC voltage by connecting the negative probe of the DVM to chassis and the positive probe to the top of the volume control ( detector output).
