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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 27th Oct 2017, 6:17 pm   #141
Heatercathodeshort
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Default Re: HMV 1804 Television.

It's called 'Circular Modulation'. Rather complicated involving the use of 6 X Z66 valves, 2 X KT44s and a specially wound teroidal line output transformer and scanning yoke.

Either that or David has trimmed up a bit of fine gauze and stuck it in the middle of the tube. I suspect the latter.. John.
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Old 27th Oct 2017, 7:35 pm   #142
Ted Kendall
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Default Re: HMV 1804 Television.

Home stretch, now! Thread of the year?
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Old 27th Oct 2017, 9:26 pm   #143
rambo1152
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Default Re: HMV 1804 Television.

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Originally Posted by Heatercathodeshort View Post
It's called 'Circular Modulation'. Rather complicated involving the use of 6 X Z66 valves, 2 X KT44s and a specially wound teroidal line output transformer and scanning yoke.

Either that or David has trimmed up a bit of fine gauze and stuck it in the middle of the tube. I suspect the latter.. John.

Better yet, Darryl could design a ion burn corrector into the Aurora FPGA.

I typed that as a joke, but the more I think about it the more sense it makes.
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Old 27th Oct 2017, 10:20 pm   #144
julie_m
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Default Re: HMV 1804 Television.

Correcting for ion burn by boosting the signal within a central circular area should not actually be too difficult a task, especially compared to resampling between line standards and colour encodings. It would need one output frame's worth of memory containing a table of values from 0 - 255 (or pairs of values 0 - 15, for half the memory requirement but then needing a bit more processing; the eternal battle of time versus space, and all that .....) representing the gain to be applied to that pixel.

The awkward bit is going to be programming it to the exact area of the damage on the picture tube, and the amount of gain required ..... Because if you got that wrong, it would look even worse! Anywhere the gain was too high, the image on the tube would be excessively bright, and vice-versa. There might also be a bright ring (if the boosted area was oversized, so increasing the signal where the phosphors already produced plenty of light) or a dark ring (if undersized, so leaving some of the dim phosphor with an un-boosted signal) in the middle of the picture.

I suppose it's also still technically over-running the tube, just using the set's video amplifier as an accomplice in the crime. Star that shines twice as bright, and all that .....
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Old 28th Oct 2017, 1:17 am   #145
dtvmcdonald
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by FERNSEH View Post
"You haven't added a circuit to boost the video gain in mid screen have you?"
Hi Peter, Can you imagine the circuits needed to do that? And with valve technology!
Two words: iconoscope camera
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Old 28th Oct 2017, 9:05 am   #146
peter_scott
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Default Re: HMV 1804 Television.

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Originally Posted by julie_m View Post
Correcting for ion burn by boosting the signal within a central circular area should not actually be too difficult a task,
Ah, but David is limiting us to valve technology.

However, if we didn't need to deal with concentrated switch off spots then using dtvmcdonald's suggestion of tilt and bend controls might help with this set.

Peter
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Old 28th Oct 2017, 9:27 am   #147
FERNSEH
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Default Re: HMV 1804 Television.

"Ah, but David is limiting us to valve technology." Hi Peter,
that's right, one has to abide by the rules which means only using the technology of those times and as the HMV 1804 was made in 1946 it has to be valves. In 1946 big octal valves were still the norm and certain miniature all glass valves were beginning to appear in equipment but not in the HMV because the 1804 is essentially a pre-war set. The pre-war model was the 1800.
From the ETF, the HMV 1800: http://www.earlytelevision.org/hmv_1800.html

DFWB.

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Old 28th Oct 2017, 8:06 pm   #148
FERNSEH
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Default Re: HMV 1804 Television.

"Home stretch, now! Thread of the year? "

Hi Ted, All the picture problems have been sorted out but there's the loudspeaker to attend to. It's in a sorry state. The loudspeaker is attached to the chassis and is a special EMI type, note the square magnet. A new cone and suspension required.

DFWB.
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