Preparing a TV set for November the 2nd. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 30th Oct 2016, 2:41 pm

In a few days time we'll be celebrating eighty years of high definition television in the UK.
For the event I've decided to revive a TV which might have been in use on the 2nd of November 1936. In my collection there is the Cossor 137T and an HMV 900.
So for starters the mighty Cossor 137T of 1936? Just take a look at that scary cathode ray tube! I'm not too keen on handling that thing. The bulb is a thin as a wine glass!

DFWB.

alynG1TBL · 30th Oct 2016, 3:12 pm

It would be rude not too!!! Best 73's Alyn G1TBL

FERNSEH · 30th Oct 2016, 4:11 pm

Last Friday the set was connected to mains and in no time at all a bright raster appeared on the screen but there was no picture or sound. This fault proved to be an easy repair. The receiver unit HT supply was found to absent and that was the result of the rectifier valve having an open circuit filament.
The original HT rectifier valve was a Cossor 4/100BU, the nearest equivalent is the Mullard FW4-500, I have neither of these valves in stock and for a temporary substitution an Osram U14 will be used until the correct valve can be found.

The set now displays a rather dim picture, but what else can be expected from an eighty year old CRT! Note the compression at the left of the picture. Frame and line holds are excellent, the picture remains locked over a wide range of rotation of the hold controls.

Other details: the horizontal picture shift has two controls. One shift control for the vertical. The sound output valve is a Cossor type 2P, a two volt filament directly heated triode. The TV receiver is a superhet, the IFs have not been determined yet. Like the contemporary GEC TVs the Cossor will have low vision intermediate frequency and that's the reason a full wave video demodulator is employed. A Cossor DDL4 low capacitance double diode is used as the vision demodulator.

The attached picture was taken yesterday.

DFWB.

G4YVM David · 30th Oct 2016, 4:18 pm

Impressive. Excuse my ignorance but is the set receiving broadcast TV or are you generating it?
David

FERNSEH · 30th Oct 2016, 4:28 pm

Hi David,
The Aurora standards converter generates a Test Card C pattern when no 625 line video is inputted.

DFWB.

dave walsh · 30th Oct 2016, 4:37 pm

Gosh that CRT IS painfully thin. Do you have to work behind a protective "screen"? Why not show "The Shape of Things To Come"-I think that's 1936

Dave W

FERNSEH · 30th Oct 2016, 4:46 pm

Hi Dave, The film "Shape of Things to Come" was indeed 1936. Absolutely, a film perfect to watch on a pre-war TV set.

The film can be found on YouTube.

I just hope that the CRT doesn't need removal for service to the upper chassis unit. I'm generally OK handling CRTs but that tube is a mighty scary looking thing!

DFWB.

FERNSEH · 30th Oct 2016, 8:13 pm

Later during Saturday evening the Cossor TV developed a timebase fault. It's likely one of the triode CRT horizontal deflection plate drivers has failed.
The EHT supply is "upside down" and follows the practise found in many oscilloscopes inasmuch being negative with respect to chassis. This method can have many advantages over a positive supply. Mainly the final anode and deflector plates are at near ground potential but does mean that the cathode and control grid of the CRT are at the 5000volts negative EHT potential. Other pre-war TV receivers which have a negative EHT systems are the Murphy A42V and early RGD models.
The line timebase has two shift controls, under this fault condition only one works. I might be lucky and find one of the 5megohm stand off resistors has gone high value or OC.
The first attached picture shows the fault condition. The second picture shows the components associated with the CRT grid and cathode. Note the selenium rectifiers used for picture DC restoration and the glass spark gaps.

DFWB.

FERNSEH · 31st Oct 2016, 2:07 pm

Scope connected to the anodes of the Cossor 41MTA and 41MTB line timebase triodes.
600 volts peak to peak sawtooth waveform is present across anode load resistors of each valve. Triode MTA is the blocking oscillator, there is a third winding on the transformer for sync injection. Triode MTB serves as a the phase inverter.
So we now know the timebase is working correctly so the DC conditions at the base of the CRT will have to checked.

DFWB.

Welsh Anorak · 31st Oct 2016, 4:02 pm

Hi
Forgive my ignorance but does the negative WRT chassis EHT mean that there is 5kV of lethal potential on the CRT base?

FERNSEH · 31st Oct 2016, 4:12 pm

That's right, the full EHT potential is present at the cathode and grid (wehnelt electrode as it was known) The focus electrode A2 will be about 1000 volts. The deflector plates are nearer ground potential, depending on the setting of the shift controls, possibly about 400 volts.
Negative EHT was common practise in oscilloscopes.

DFWB.

HamishBoxer · 31st Oct 2016, 4:14 pm

I believe so. Hairy or what!

ukcol · 31st Oct 2016, 4:29 pm

So, presumably the video signal had to be coupled to the CRT grid or cathode via a capacitor rated at EHT voltage. Is this correct?

FERNSEH · 31st Oct 2016, 4:37 pm

That is indeed correct. However, by employing a negative EHT supply the designer can get away with only one capacitor rated at EHT voltage. The deflector plates being near ground potential require coupling capacitors rated at less than 1000 volts or so.
Use a positive EHT supply and four EHT rating coupling capacitors are needed to isolate the timebases.

DFWB.

FERNSEH · 31st Oct 2016, 5:49 pm

Using the Simpson 260 multimeter set to the 1000 volt range I find that the CRT X1 plate measures between 700 to 1000 volts depending on the setting of the shift control. The X2 plate measures only 400 volts and the shift control has no effect.

DFWB.

FERNSEH · 31st Oct 2016, 9:19 pm

Later this evening I will return to the workshop to remove the CRT from the upper chassis unit. Great care will be exercised removing the tube, note the cushions placed each side of the cabinet to provide protection from any shocks to the fragile glass bulb.

DFWB.

Aerodyne · 31st Oct 2016, 9:30 pm

Absolutely fascinating stuff. What an amazing piece of engineering this TV is. To think it dates right back to 1936... they really were 'pushing the envelope' as the trite modern saying goes, designing and making devices for the first time.
Rather you than me, handling the CRT though. Isn't it great that there are folks around with the skill, knowledge and enthusiasm to keep such amazing technology in existence?
Very, very well done.
Tony

FERNSEH · 31st Oct 2016, 9:54 pm

We believe that design work started on the 137T late 1935. A. C. Cossor were well experienced making CRTs. Company started as a scientific glassware maker.
Link to the repair of the cabinet:
http://www.forum.radios-tv.co.uk/vie...php?f=5&t=7575

DFWB.

FERNSEH · 1st Nov 2016, 6:34 pm

Got more problems with the Cossor 137T so let's take a look at the HMV 900, another set that was available November 1936.

DFWB.

Heatercathodeshort · 1st Nov 2016, 6:59 pm

Great pity about the Cossor David. There can't be a lot wrong and much rarer than the common as muck 900.
I will run my 900 tomorrow and post a few pics. John.

30th Oct 2016, 2:41 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Preparing a TV set for November the 2nd. In a few days time we'll be celebrating eighty years of high definition television in the UK. For the event I've decided to revive a TV which might have been in use on the 2nd of November 1936. In my collection there is the Cossor 137T and an HMV 900. So for starters the mighty Cossor 137T of 1936? Just take a look at that scary cathode ray tube! I'm not too keen on handling that thing. The bulb is a thin as a wine glass! DFWB. Attached Thumbnails

30th Oct 2016, 3:12 pm	#2
alynG1TBL Tetrode Join Date: Oct 2006 Location: Severn Valley,Worcestershire, West Midlands, UK. Posts: 56	Re: Preparing a TV set for November the 2nd. It would be rude not too!!! Best 73's Alyn G1TBL

30th Oct 2016, 4:11 pm	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Last Friday the set was connected to mains and in no time at all a bright raster appeared on the screen but there was no picture or sound. This fault proved to be an easy repair. The receiver unit HT supply was found to absent and that was the result of the rectifier valve having an open circuit filament. The original HT rectifier valve was a Cossor 4/100BU, the nearest equivalent is the Mullard FW4-500, I have neither of these valves in stock and for a temporary substitution an Osram U14 will be used until the correct valve can be found. The set now displays a rather dim picture, but what else can be expected from an eighty year old CRT! Note the compression at the left of the picture. Frame and line holds are excellent, the picture remains locked over a wide range of rotation of the hold controls. Other details: the horizontal picture shift has two controls. One shift control for the vertical. The sound output valve is a Cossor type 2P, a two volt filament directly heated triode. The TV receiver is a superhet, the IFs have not been determined yet. Like the contemporary GEC TVs the Cossor will have low vision intermediate frequency and that's the reason a full wave video demodulator is employed. A Cossor DDL4 low capacitance double diode is used as the vision demodulator. The attached picture was taken yesterday. DFWB. Attached Thumbnails

30th Oct 2016, 4:18 pm	#4
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	Re: Preparing a TV set for November the 2nd. Impressive. Excuse my ignorance but is the set receiving broadcast TV or are you generating it? David

30th Oct 2016, 4:28 pm	#5
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Hi David, The Aurora standards converter generates a Test Card C pattern when no 625 line video is inputted. DFWB.

30th Oct 2016, 4:37 pm	#6
dave walsh Dekatron Join Date: Jan 2006 Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex. Posts: 5,817	Re: Preparing a TV set for November the 2nd. Gosh that CRT IS painfully thin. Do you have to work behind a protective "screen"? Why not show "The Shape of Things To Come"-I think that's 1936 Dave W

30th Oct 2016, 4:46 pm	#7
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Hi Dave, The film "Shape of Things to Come" was indeed 1936. Absolutely, a film perfect to watch on a pre-war TV set. The film can be found on YouTube. I just hope that the CRT doesn't need removal for service to the upper chassis unit. I'm generally OK handling CRTs but that tube is a mighty scary looking thing! DFWB.

30th Oct 2016, 8:13 pm	#8
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Later during Saturday evening the Cossor TV developed a timebase fault. It's likely one of the triode CRT horizontal deflection plate drivers has failed. The EHT supply is "upside down" and follows the practise found in many oscilloscopes inasmuch being negative with respect to chassis. This method can have many advantages over a positive supply. Mainly the final anode and deflector plates are at near ground potential but does mean that the cathode and control grid of the CRT are at the 5000volts negative EHT potential. Other pre-war TV receivers which have a negative EHT systems are the Murphy A42V and early RGD models. The line timebase has two shift controls, under this fault condition only one works. I might be lucky and find one of the 5megohm stand off resistors has gone high value or OC. The first attached picture shows the fault condition. The second picture shows the components associated with the CRT grid and cathode. Note the selenium rectifiers used for picture DC restoration and the glass spark gaps. DFWB. Attached Thumbnails

31st Oct 2016, 2:07 pm	#9
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Scope connected to the anodes of the Cossor 41MTA and 41MTB line timebase triodes. 600 volts peak to peak sawtooth waveform is present across anode load resistors of each valve. Triode MTA is the blocking oscillator, there is a third winding on the transformer for sync injection. Triode MTB serves as a the phase inverter. So we now know the timebase is working correctly so the DC conditions at the base of the CRT will have to checked. DFWB. Attached Thumbnails

31st Oct 2016, 4:02 pm	#10
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Preparing a TV set for November the 2nd. Hi Forgive my ignorance but does the negative WRT chassis EHT mean that there is 5kV of lethal potential on the CRT base?

31st Oct 2016, 4:12 pm	#11
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. That's right, the full EHT potential is present at the cathode and grid (wehnelt electrode as it was known) The focus electrode A2 will be about 1000 volts. The deflector plates are nearer ground potential, depending on the setting of the shift controls, possibly about 400 volts. Negative EHT was common practise in oscilloscopes. DFWB.

31st Oct 2016, 4:14 pm	#12
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,932	Re: Preparing a TV set for November the 2nd. I believe so. Hairy or what! __________________ G8JET BVWS Archivist and Member V.M.A.R.S

31st Oct 2016, 4:29 pm	#13
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Preparing a TV set for November the 2nd. So, presumably the video signal had to be coupled to the CRT grid or cathode via a capacitor rated at EHT voltage. Is this correct?

31st Oct 2016, 4:37 pm	#14
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. That is indeed correct. However, by employing a negative EHT supply the designer can get away with only one capacitor rated at EHT voltage. The deflector plates being near ground potential require coupling capacitors rated at less than 1000 volts or so. Use a positive EHT supply and four EHT rating coupling capacitors are needed to isolate the timebases. DFWB.

31st Oct 2016, 5:49 pm	#15
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Using the Simpson 260 multimeter set to the 1000 volt range I find that the CRT X1 plate measures between 700 to 1000 volts depending on the setting of the shift control. The X2 plate measures only 400 volts and the shift control has no effect. DFWB.

31st Oct 2016, 9:19 pm	#16
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Later this evening I will return to the workshop to remove the CRT from the upper chassis unit. Great care will be exercised removing the tube, note the cushions placed each side of the cabinet to provide protection from any shocks to the fragile glass bulb. DFWB. Attached Thumbnails

31st Oct 2016, 9:30 pm	#17
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Preparing a TV set for November the 2nd. Absolutely fascinating stuff. What an amazing piece of engineering this TV is. To think it dates right back to 1936... they really were 'pushing the envelope' as the trite modern saying goes, designing and making devices for the first time. Rather you than me, handling the CRT though. Isn't it great that there are folks around with the skill, knowledge and enthusiasm to keep such amazing technology in existence? Very, very well done. Tony

31st Oct 2016, 9:54 pm	#18
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. We believe that design work started on the 137T late 1935. A. C. Cossor were well experienced making CRTs. Company started as a scientific glassware maker. Link to the repair of the cabinet: http://www.forum.radios-tv.co.uk/vie...php?f=5&t=7575 DFWB.

1st Nov 2016, 6:34 pm	#19
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Preparing a TV set for November the 2nd. Got more problems with the Cossor 137T so let's take a look at the HMV 900, another set that was available November 1936. DFWB. Attached Thumbnails

1st Nov 2016, 6:59 pm	#20
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Preparing a TV set for November the 2nd. Great pity about the Cossor David. There can't be a lot wrong and much rarer than the common as muck 900. I will run my 900 tomorrow and post a few pics. John.