01-06-2017, 11:07 AM
(This post was last modified: 01-06-2017, 11:08 AM by Geordie McBoyne.)
Corrosion of the copper inner conductor of the co-ax cable was the problem. Now sorted and the sound is coming through, although sound vision spacing of the IF frequencies is way out. Nevertheless, things are starting to happen.
The audio developed across the 33Kohm demodulator load resistor is positive going, one could question why this was done because if the diode was reversed to give a negative going audio and DC component a convenient source of AVC voltage would be available and it would be unnecessary to employ a separate AVC rectifier diode
My take of this circuit is because a high slope straight pentode was employed as the sound IF amplifier it's possible that high signal levels will result in a high negative AVC voltage which in turn will cause some distortion of the negative going part of the modulated IF signals. The positive side of the modulated signal waveform will be unaffected. Once again this attention to circuit design confirms that the modifications were the work of a very competent engineer.
Geordie McBoyne.
The audio developed across the 33Kohm demodulator load resistor is positive going, one could question why this was done because if the diode was reversed to give a negative going audio and DC component a convenient source of AVC voltage would be available and it would be unnecessary to employ a separate AVC rectifier diode
My take of this circuit is because a high slope straight pentode was employed as the sound IF amplifier it's possible that high signal levels will result in a high negative AVC voltage which in turn will cause some distortion of the negative going part of the modulated IF signals. The positive side of the modulated signal waveform will be unaffected. Once again this attention to circuit design confirms that the modifications were the work of a very competent engineer.
Geordie McBoyne.
