30-05-2017, 09:50 AM
Testing the sound circuits of the HMV 1804. With 100millivolts P - P of 10.5Mc/s modulated signal supplied to the input IF transformer an audio signal of 2.5 volts is developed across the demodulator load resistor. The signal across the load resistor rises slightly when the AVC is shorted out.
When you consider the input voltage is 100mV P-P and half of the modulated signal waveform is removed by diode action of the demodulator the gain of the IF amplifier is actually 50 times. I'd reckon that's a very satisfactory figure. The IF amplifier valve is a W61, a valve that resembles the 6K7G.
But this still doesn't answer the question, why isn't the sound coming through from signals supplied to the aerial socket?
Geordie McBoyne.
When you consider the input voltage is 100mV P-P and half of the modulated signal waveform is removed by diode action of the demodulator the gain of the IF amplifier is actually 50 times. I'd reckon that's a very satisfactory figure. The IF amplifier valve is a W61, a valve that resembles the 6K7G.
But this still doesn't answer the question, why isn't the sound coming through from signals supplied to the aerial socket?
Geordie McBoyne.
