09-11-2016, 03:21 PM
The cause of the instability was traced to an open circuit cathode bypass capacitor which is shared between the IF amplifier and frequency changer valves. The value of the capacitor is 500,000cm, work that out in microfarads.
It was discovered when the volume control was turned down the instability disappeared. I was expecting the usual method of volume control, that is a 500Kohm potentiometer across the detector load resistor tapping off the desired audio to the AF triode amplifier.
Not so, in this receiver the volume control adjusts the cathode bias of the FC and IF amplifier valves, it's not a nice arrangement but that's how it is.
It's all very strange because the AVC loop operates within the gain controlled valves, so it happens when the volume control is turned up the negative AVC voltage goes up and reduces the gain of the FC and IF stages.
The set operates on a 100 volt HT supply. The screen grids of the type 6A7 frequency changer and type 78 IF amplifier are supplied 37 volts from a common potential divider across the HT supply. The bottom end of the potential divider is returned to the cathode of the valves. 37volts might be a bit low.
Because the set has a low IF a three gang tuning capacitor is employed for bandpass tuning to reduce image interference.
Other valve types used in this set are a type 75 double-diode-triode as the detector and audio amplifier and a type 43 output pentode.
Geordie McBoyne.
It was discovered when the volume control was turned down the instability disappeared. I was expecting the usual method of volume control, that is a 500Kohm potentiometer across the detector load resistor tapping off the desired audio to the AF triode amplifier.
Not so, in this receiver the volume control adjusts the cathode bias of the FC and IF amplifier valves, it's not a nice arrangement but that's how it is.
It's all very strange because the AVC loop operates within the gain controlled valves, so it happens when the volume control is turned up the negative AVC voltage goes up and reduces the gain of the FC and IF stages.
The set operates on a 100 volt HT supply. The screen grids of the type 6A7 frequency changer and type 78 IF amplifier are supplied 37 volts from a common potential divider across the HT supply. The bottom end of the potential divider is returned to the cathode of the valves. 37volts might be a bit low.
Because the set has a low IF a three gang tuning capacitor is employed for bandpass tuning to reduce image interference.
Other valve types used in this set are a type 75 double-diode-triode as the detector and audio amplifier and a type 43 output pentode.
Geordie McBoyne.
