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FERNSEH · 1st Mar 2015, 4:04 pm

Hi Mark,
don't mess about checking frame timebase components in turn. Change all those capacitors marked in red by John. That action should solve the frame linearity problem. C34 (0.022mfd) is the sawtooth forming capacitor, a leak in this component will result cramping at the bottom of the picture.
C36 (0.1mfd) is the coupling capacitor will actually cause cramping at the top of the picture if it is leaky. There is a negative DC component across C34. So it follows that the KT33C will be over biased when C36 is leaky. Change it anyway. C37 (0.082mfd) and C38 (0.015mfd) must be replaced. If the correct values are not to hand you can make up the right values by mixed value capacitors in parallel. Or, 0.082mfd can be made up by two capacitors in series, 0.1mfd and 0.47mfd. Works out to be 0.084mfd, near enough I reckon.
You're on the home straight now.

DFWB.

1st Mar 2015, 4:04 pm	#112
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Marconiphone VRC74DA Hi Mark, don't mess about checking frame timebase components in turn. Change all those capacitors marked in red by John. That action should solve the frame linearity problem. C34 (0.022mfd) is the sawtooth forming capacitor, a leak in this component will result cramping at the bottom of the picture. C36 (0.1mfd) is the coupling capacitor will actually cause cramping at the top of the picture if it is leaky. There is a negative DC component across C34. So it follows that the KT33C will be over biased when C36 is leaky. Change it anyway. C37 (0.082mfd) and C38 (0.015mfd) must be replaced. If the correct values are not to hand you can make up the right values by mixed value capacitors in parallel. Or, 0.082mfd can be made up by two capacitors in series, 0.1mfd and 0.47mfd. Works out to be 0.084mfd, near enough I reckon. You're on the home straight now. DFWB.