Quote:
Originally Posted by FERNSEH
The CRT in the Marconi 702 is good but the contrast is bad. The video drive to the CRT grid is only four volts. Remember these first generation EMI TV receivers do not employ a video amplifier. High level video is supplied to the CRT and sync separators direct from the video detector load resistor.
The HMV 904 which is being discussed in another thread does have a video amplifier valve, an MS4B RF tetrode. The valve also works as an anode bend detector.
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Yes, sorry I forgot to mention that the detector in the HMV 904 was an anode bend detector so you get detection and some amplification for free.
I've read that trying to extract sync information straight from the vision detector was notoriously problematic due to the low level of the signal at that point. But sync extraction after any AF amplification was difficult due to the loss of the DC component?
The use of an anode bend detector seems to be a rather clever way of solving that problem. I don't know if it was used in later sets, but it seems unlikely?
The resurrection of a technique from the early days of radio seems odd even for 1938. In my years of radio collecting I've ever once (many years ago) come across a radio using anode bend detection, that was a Brandes radio from 1927/1928 (they became Kolster Brandes in 1929). Being stumped as to how the circuit worked, I finally found a description of it in the "Manual of Modern Radio" 1933, and even then it was described as obsolete.
The reason for its obsolescence at least in radio work was because it introduced distortion, but in the days of horn and moving iron loudspeakers that was the least of their worries. The distortion issue evidently wasn't an issue for the vision circuit in the HMV 904.