05-13-2013, 08:10 PM
The cathode to filament voltages of the 78 RF and 6A7 tubes are a bit puzzling. According to the schematic, the cathodes of both of these tubes are connected together, so their voltages should be the same. They are also higher than they should be. There should be a 200 ohm resistor from the cathodes to ground which should develop the 2.8V bias. Your voltages are much higher which is producing more negative bias, reducing the gain of these tubes and therefore the sensitivity of the set. You might check the cathode circuit wiring of these tubes for any errors if you did any work in this area.
The cathode voltage of the 78 IF tube is also high which also reduces your sensitivity. Check the value of the 2500 ohm resistor from cathode to ground. If it has increased in value, it will cause the cathode voltage to be higher than normal.
You might try measuring the cathode voltage of these tubes with the negative probe of the voltmeter connected to the chassis instead of the filament (cathode to chassis ground). The superimposed AC filament voltage may be distorting the DC voltage readings.
The cathode voltage of the 78 IF tube is also high which also reduces your sensitivity. Check the value of the 2500 ohm resistor from cathode to ground. If it has increased in value, it will cause the cathode voltage to be higher than normal.
You might try measuring the cathode voltage of these tubes with the negative probe of the voltmeter connected to the chassis instead of the filament (cathode to chassis ground). The superimposed AC filament voltage may be distorting the DC voltage readings.
