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Old 27th Oct 2017, 7:17 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, 8:35 pm   #142
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Default Re: HMV 1804 Television.

Home stretch, now! Thread of the year?
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Old 27th Oct 2017, 10:26 pm   #143
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Default Re: HMV 1804 Television.

Quote:
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, 11:20 pm   #144
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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, 2:17 am   #145
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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, 10:05 am   #146
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Default Re: HMV 1804 Television.

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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, 10:27 am   #147
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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, 9:06 pm   #148
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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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Old 18th Feb 2018, 9:41 pm   #149
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Default Re: HMV 1804 Television.

The HMV 1804 hasn't been switched on since October last year. Today the set was switched on, after a short warm up time a rock steady appeared without needing any adjustments.
Picture is slightly out of focus because the KT61 sound output valve isn't plugged in.

DFWB.
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Old 19th Feb 2018, 9:06 am   #150
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Default Re: HMV 1804 Television.

Looks like it will continue to be a reliable runner. It should after all the work you have put in.
How did you do with the speaker, or is that why the KT61 is out?
I've re-coned a few round ones using the central part with speech coil but can think of no way to make elliptical cones.
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Old 19th Feb 2018, 12:05 pm   #151
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by FERNSEH View Post
Picture is slightly out of focus because the KT61 sound output valve isn't plugged in.
By the time I started in the radio & TV serving trade c1963 the HMV 1804 and its contemporaries were generally no longer in service. I did see a lot of sets from the 50s though that used magnetic focusing but this was achieved with mechanically adjustable permanent magnets.

I don't remember ever seeing a set that used an electro-magnet in the focusing system. A quick look at the Trader sheet for the 1804 shows that the coil for the focus magnet is in series with the HT supply and the focus control is a variable resistor connected in parallel with the coil. This arrangement explains why (what at first sight seemed odd to me) removing the sound output valve changes the focus.

Was adjusting the focus by electromagnetic means common in early sets David?

Last edited by ukcol; 19th Feb 2018 at 12:08 pm. Reason: spelling
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Old 19th Feb 2018, 1:11 pm   #152
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by ukcol View Post
Was adjusting the focus by electromagnetic means common in early sets
Practically all early American sets pre war and post war sets, with magnetic deflection, used magnetic focus, with a focus coil. As did the 1939 English HMV904/5.

But in the post war period, most UK made sets used a permanent focus magnet, yet in the USA, it was practically always a focus coil.

As time went by, in the USA at least, many CRT's moved to electrostatic focus in the post war period. It was cheaper and saved on the focus coil assembly.

However, it was in fact never quite as good as magnetic focus, the reason being that the focus was more dependent on the control grid potential and the beam current. So with high beam intensity a tad more focus voltage is required on the focus electrode to keep the beam in focus. Where magnetic focus it tends to stay in focus with increasing beam intensity.

In short, a better focused image across a wide range of beam intensities is obtained with magnetic focus. Unless, dynamic focus (modulation of the focus voltage with beam intensity) is applied to the electrostatically focused CRT, as is done in some oscilloscopes and then the issue is circumvented.
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Old 19th Feb 2018, 1:57 pm   #153
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Default Re: HMV 1804 Television.

UKCol wrote: "Was adjusting the focus by electromagnetic means common in early sets David?"
The first generation EMI TVs employed an electrostatic focus CRT, the gun assembly was complex in it's design and construction. The later models such as the HMV 904 and Marconi 706 of 1938 employed a simple triode gun CRT with a focus coil.
The infamous HMV 1807 of 1949 used a permanent magnet focus magnet and final adjustment of the focus was achieved by altering the EHT voltage.
Whereas the HMV 1804 was an over engineered monster the 1807 chassis was made as cheaply as possible. The cabinet wasn't though, it was made to the usual EMI high standard of construction quality.

DFWB.
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Old 19th Feb 2018, 3:07 pm   #154
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Default Re: HMV 1804 Television.

Is the speaker cone beyond saving, or could you glue on a foam surround? I bought some from China on ebay - very cheap, available in lots of diameters, turned up eventually!

Maybe 'naval jelly' (gloopy phosphoric acid rust treatment) might be a good way to clean up the speaker chassis without getting it all over the cone / in the magnet gap, and without abrading it and producing lots of unhelpful ferrous dust.
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Old 19th Feb 2018, 3:56 pm   #155
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Default Re: HMV 1804 Television.

Coca Cola, cheaper. I'd take the cone out completely first, easy enough to do.
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Old 19th Feb 2018, 6:10 pm   #156
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Default Re: HMV 1804 Television.

Thank you for your replies David and Hugo.

Leaving electrostatic focusing to one side, I expect the reason later sets used permanent magnets was that cheaper and better magnetic materials were becoming available. The field energized loudspeaker disappeared from new designs for the same reason.
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Old 19th Feb 2018, 7:21 pm   #157
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Default Re: HMV 1804 Television.

David have you forgotten I offered you a loudspeaker for your 1804 last year when you were in hospital ?
You said you would get back in touch but must have forgot.
It`s still here if you want it.

Robin
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Old 19th Feb 2018, 9:02 pm   #158
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Default Re: HMV 1804 Television.

Hi Robin,
Thanks for the offer of the loudspeaker. I'll send you a PM. The loudspeaker in the 1804 is unrepairable.

"Is the speaker cone beyond saving, or could you glue on a foam surround? I bought some from China on ebay - very cheap, available in lots of diameters, turned up eventually!"

Hi Marc,
That's well worth bearing in mind. I've a few loudspeakers that need the cone outer surround replacing.

DFWB.
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Old 6th Jul 2018, 12:44 pm   #159
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Default Re: HMV 1804 Television.

Robin's loudspeaker has been fitted and is excellent but I must take care and avoid destroying it in same manner as the original was.
The cone of the original loudspeaker was blown out as a result of the severe motorboating effect in the audio amplifier. Replacing various decoupling capacitors has reduced the very low frequency oscillations but the problem still persists at high volume control settings.

The attachment shows the sound stages in the highly modified HMV 1804.
The obviously clever engineer who modified the set to operate on a different transmitter had also designed circuits for optimum sensitivity and picture quality. The original sound IF amplifier was changed to a Mazda SP61, we can't have that sort of thing in an EMI set so I've changed the valveholder to an international octal type so that an MO Z66 or W62 vari-mu pentode can be fitted.

But I still haven't found a solution to the audio motorboating problem. Short circuit the grid of the DH63 amplifier to ground and the oscillations stop, Even shorting the pins 3 and 4 of the D63 series noise limiter to ground stop the low frequency oscillations. The 1meg grid resistor of the double-diode-triode is OK. The volume control is between the amplifier and the grid of the output valve. The cathode bias resistor of the DDT is unbypassed as it is it is in the EMI circuit, in fact there have been no alterations to the audio amplifier circuit either by me or the mystery engineer.
The modifications were done to the IF amplifier, demodulator and noise limiter circuits.
Sound problems apart the set works very well indeed, just a few minor jobs to do now such as the double-diode interlace filter and an RF gain control.

DFWB.
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Old 6th Jul 2018, 1:50 pm   #160
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Default Re: HMV 1804 Television.

Are there any shared screen resistors and decouplers on the audio? I try to make each screen have its own resistor and cap decoupler.
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