HMV 902. Replace EHT transformer.

[bobhowe]

Hi David a proper engineered set hope to see some light soon kind regards Bob

[Geordie McBoyne]

Result of today's activities, a very bright raster. Will now turn my attention to the TRF vision unit.

Geordie McBoyne.

[peter scott]

Wow!

[Mike Watterson]

Brilliant indeed!

[ppppenguin]

Excellent. Not many CRTs in pre-war sets are this bright.

[Geordie McBoyne]

Starting to make sense of the video fault. 'Scope connected to the grid of the CRT. After adjustment a perfect positive going video waveform. Probe transferred to the CRT grid, a sawtooth waveform of 20volts P - P. Where's that coming from? The timebase shows the frame blanking capacitor C5 is connected via R14 to the slider of the brightness control but no line blanking. The timebase chassis will have to come out so that more tests can be carried out.

Geordie McBoyne.

[Murphyv310]

Oh dear that video waveform looks rather sick David.
I'm sure you have it fixed in a jiffy.

[LENINGRAD T2]

The diagram does not show a typical final vision amplifier. Was this standard for early HMV/Marconi receivers? Does 6 levels of amplification of the video signal allow you to obtain good image parameters in practice?

[ppppenguin]

It was very difficult to get useful gain for baseband video with pre-war valves. Hence the use of a high slope CRT driven directly from the detector in some sets.

[peter scott]

The diagram does not show a typical final vision amplifier. Was this standard for early HMV/Marconi receivers? Does 6 levels of amplification of the video signal allow you to obtain good image parameters in practice?

http://www.nostalgiatech.co.uk/new_page_8.htm