UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

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

Reply
 
Thread Tools
Old 27th Oct 2017, 6:17 pm   #141
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 5,956
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.
Heatercathodeshort is offline   Reply With Quote
Old 27th Oct 2017, 7:35 pm   #142
Ted Kendall
Octode
 
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 1,035
Default Re: HMV 1804 Television.

Home stretch, now! Thread of the year?
Ted Kendall is offline   Reply With Quote
Old 27th Oct 2017, 9:26 pm   #143
rambo1152
Hexode
 
Join Date: May 2010
Location: Manchester, UK.
Posts: 376
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.
__________________
--
Graham.
G3ZVT
rambo1152 is offline   Reply With Quote
Old 27th Oct 2017, 10:20 pm   #144
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 6,354
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 .....
__________________
Julie {formerly known AJS_Derby}
julie_m is offline   Reply With Quote
Old 28th Oct 2017, 1:17 am   #145
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 131
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
dtvmcdonald is offline   Reply With Quote
Old 28th Oct 2017, 9:05 am   #146
peter_scott
Octode
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 1,657
Default Re: HMV 1804 Television.

Quote:
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
peter_scott is offline   Reply With Quote
Old 28th Oct 2017, 9:27 am   #147
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,718
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.

Last edited by FERNSEH; 28th Oct 2017 at 9:36 am.
FERNSEH is offline   Reply With Quote
Old 28th Oct 2017, 8:06 pm   #148
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,718
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.
Attached Thumbnails
Click image for larger version

Name:	HMV1804_105342.jpg
Views:	136
Size:	126.0 KB
ID:	151546   Click image for larger version

Name:	HMV1804_105343.jpg
Views:	146
Size:	125.0 KB
ID:	151547  
FERNSEH is offline   Reply With Quote
Old 18th Feb 2018, 8:41 pm   #149
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,718
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.
Attached Thumbnails
Click image for larger version

Name:	HMV1804_180218.jpg
Views:	107
Size:	28.8 KB
ID:	157705  
FERNSEH is offline   Reply With Quote
Old 19th Feb 2018, 8:06 am   #150
Boater Sam
Nonode
 
Boater Sam's Avatar
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 2,781
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.
__________________
Boater Sam.
BVWS Member
Boater Sam is offline   Reply With Quote
Old 19th Feb 2018, 11:05 am   #151
ukcol
Dekatron
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,435
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?
__________________
Best Regards, Colin McLaughlin
BVWS Member

Last edited by ukcol; 19th Feb 2018 at 11:08 am. Reason: spelling
ukcol is offline   Reply With Quote
Old 19th Feb 2018, 12:11 pm   #152
Argus25
Octode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,107
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.
Argus25 is offline   Reply With Quote
Old 19th Feb 2018, 12:57 pm   #153
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,718
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.
FERNSEH is offline   Reply With Quote
Old 19th Feb 2018, 2:07 pm   #154
mark_in_manc
Heptode
 
Join Date: Nov 2011
Location: Manchester, UK.
Posts: 769
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.
mark_in_manc is offline   Reply With Quote
Old 19th Feb 2018, 2:56 pm   #155
Boater Sam
Nonode
 
Boater Sam's Avatar
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 2,781
Default Re: HMV 1804 Television.

Coca Cola, cheaper. I'd take the cone out completely first, easy enough to do.
__________________
Boater Sam.
BVWS Member
Boater Sam is offline   Reply With Quote
Old 19th Feb 2018, 5:10 pm   #156
ukcol
Dekatron
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,435
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.
__________________
Best Regards, Colin McLaughlin
BVWS Member
ukcol is offline   Reply With Quote
Old 19th Feb 2018, 6:21 pm   #157
line sync
Hexode
 
Join Date: Sep 2009
Location: Kidderminster, Worcestershire, UK.
Posts: 312
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
line sync is offline   Reply With Quote
Old 19th Feb 2018, 8:02 pm   #158
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,718
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.
FERNSEH is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 2:28 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.