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FERNSEH · 2nd Mar 2015, 4:08 pm

Hi Mark,
have you access to an oscilloscope? If you have then the waveform across R24, the vision detector load resistor, should be examined.
A common condition exists with positive modulation signals where the peak white is crushed when the signal levels are too high. When the contrast control is advanced the sync pulse height increases but the active video part becomes compressed. As mentioned earlier the contrast control is in wrong position where it is in the first RF stage. it should be the RF gain control and the contrast control should be further on in the vision RF stages.
Reason for the crushing of the video signal is caused by the use of a straight RF pentode in the controlled stage. Vari-mu valves are better.

DFWB.

2nd Mar 2015, 4:08 pm	#130
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Marconiphone VRC74DA Hi Mark, have you access to an oscilloscope? If you have then the waveform across R24, the vision detector load resistor, should be examined. A common condition exists with positive modulation signals where the peak white is crushed when the signal levels are too high. When the contrast control is advanced the sync pulse height increases but the active video part becomes compressed. As mentioned earlier the contrast control is in wrong position where it is in the first RF stage. it should be the RF gain control and the contrast control should be further on in the vision RF stages. Reason for the crushing of the video signal is caused by the use of a straight RF pentode in the controlled stage. Vari-mu valves are better. DFWB.