A scary Baird T23. - Page 5 - UK Vintage Radio Repair and Restoration Discussion Forum

brianc · 8th Feb 2017, 8:33 am

Well done, David. A splendid result.
You won't be needing my spare CRT then - or we could swop!!

beery · 8th Feb 2017, 9:17 am

That's a great result David

I do remember seeing your collection of scary Cossor tubes. They were definitely worth saving...

Cheers
Andy

Brigham · 8th Feb 2017, 10:15 am

Is it worth investigating the possibility of re-gunning the poor examples? I heard once that the glass itself changes in character over the years, becoming more brittle and liable to surface cracking.

FERNSEH · 8th Feb 2017, 4:17 pm

That's why removing and refitting the CRT into this set is a well rehearsed procedure. Removal is easy, you simply disconnect the three leads on the CRT base, unplug the line scanning coils and EHT connector, then by using the metal adaptor plate the CRT rests on use the plate as a device to lift the tube straight up and away from the cabinet.
Refitting the CRT is a bit more tricky but essentially the reversal of removing it. First tape the adaptor plate to the CRT bowl and in a similar manner as removing the tube use the plate to lower the CRT neck through the focus coil.
Reconnect all leads. The difficult part during the CRT installation procedure is guiding the CRT adaptor plate on to the four 2BA fixing studs.

DFWB.

FERNSEH · 8th Feb 2017, 4:21 pm

Quote:

Originally Posted by Brigham

Is it worth investigating the possibility of re-gunning the poor examples?.

Well, there is a spare tube available here if any known CRT rebuild firm can be found to regun it.

DFWB.

Heatercathodeshort · 8th Feb 2017, 5:55 pm

Don't play those bagpipes too loudly David. That huge bulb may object to the cacophony.

If you look at the phosphor under a microscope David it is made up of millions of tiny prisms. They arrange them during manufacture for the electron beam to hit them at the correct angle depending if the tube is mounted for direct view or via a mirror.

After all these years some of the prisms fall off giving a 'ghost' effect to the picture.
Difficult to explain but I'm sure you understand. John.

FERNSEH · 8th Feb 2017, 6:02 pm

Quote:

Originally Posted by Heatercathodeshort

Don't play those bagpipes too loudly David. That huge bulb may object to the cacophony.

Hi John
I prefer the Northumbrian small pipes. Much kinder to fragile CRTs.

DFWB.

FERNSEH · 8th Feb 2017, 7:45 pm

Today. I've found another Cossor 65K CRT and is no evidence that it was ever fitted in a Baird T23. When a Cossor 65K CRT is installed in the Baird T23 a wire link is connected between the cathode socket to one of the heater sockets. This CRT does not have the heater to cathode wire link.
The reason for doing this is because the original Cathodevisor CRT has a directly heated cathode. The tube cathode voltage is set to a fixed potential which is supplied from the power supply unit. The brightness is adjusted by varying the CRT grid voltage. Positive going video from the RF unit is fed to the EHT power supply unit. A DC restorer diode in the EHT PSU clamps the sync tips to the voltage adjusted by the brightness control.
All known Baird T23 sets are fitted with the Cossor 65K CRT. However, there is no reason why this CRT cannot be fitted in the Baird T5 or other 15" Baird pre-war TV sets.

DFWB.

FERNSEH · 10th Feb 2017, 10:25 pm

Before I become brave again and remove and refit the CRT it's time to take a look at the frame timebase. The picture is off-set to the bottom of the screen.
The single valve timebase employs an oscillator transformer which also performs the vertical scanning by means of those extended pole pieces.
The DC current through the transformer primary is cancelled out by a third winding, see the attached circuit diagram. Earlier this evening the timebase unit was removed in order to find the picture shift fault. Well it turns out that the picture shift winding has been disconnected. Later on I'll reconnect the winding and report back the results.

DFWB.

beery · 11th Feb 2017, 8:03 am

Hi David,

It is worth checking your coils against the T5C (single standard) arrangement. It had an extra feedback winding.
See the attached picture and note the unconventional pin numbering.
According to the numbering shown there is a coil between pins 7 and 1 (conventionally pins 6 and 10).

Cheers
Andy

FERNSEH · 11th Feb 2017, 11:34 am

Hi Andy,
The extra outer winding is used for frame sync pulse injection. I'll check to see if that winding is present in the frame transformer in the T23. The later set employs an interlace diode and the sync pulse is connected direct to the grid of the 41MP valve.

DFWB.

FERNSEH · 24th Mar 2017, 10:32 pm

The frame sync injection winding is present in the oscillator transformer but it is not connected to anything.
This week just one small job done to the set and not a very exciting one either. The original mains connector at the rear of the cabinet has missing pins. Made by Bulgin it's of a type I have never been able to find. The first attachment shows the original connector sans it's screw in pins. A special bracket has been made to mount a standard mains connector, second picture.

DFWB.

8th Feb 2017, 8:33 am	#81
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: A scary Baird T23. Well done, David. A splendid result. You won't be needing my spare CRT then - or we could swop!!

8th Feb 2017, 9:17 am	#82
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Re: A scary Baird T23. That's a great result David I do remember seeing your collection of scary Cossor tubes. They were definitely worth saving... Cheers Andy __________________ www.youtube.com/user/andyvalve100 Last edited by beery; 8th Feb 2017 at 9:17 am. Reason: added name

8th Feb 2017, 10:15 am	#83
Brigham Octode Join Date: Mar 2006 Location: Co. Durham, UK. Posts: 1,115	Re: A scary Baird T23. Is it worth investigating the possibility of re-gunning the poor examples? I heard once that the glass itself changes in character over the years, becoming more brittle and liable to surface cracking.

8th Feb 2017, 4:17 pm	#84
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: A scary Baird T23. That's why removing and refitting the CRT into this set is a well rehearsed procedure. Removal is easy, you simply disconnect the three leads on the CRT base, unplug the line scanning coils and EHT connector, then by using the metal adaptor plate the CRT rests on use the plate as a device to lift the tube straight up and away from the cabinet. Refitting the CRT is a bit more tricky but essentially the reversal of removing it. First tape the adaptor plate to the CRT bowl and in a similar manner as removing the tube use the plate to lower the CRT neck through the focus coil. Reconnect all leads. The difficult part during the CRT installation procedure is guiding the CRT adaptor plate on to the four 2BA fixing studs. DFWB.

8th Feb 2017, 5:55 pm	#86
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: A scary Baird T23. Don't play those bagpipes too loudly David. That huge bulb may object to the cacophony. If you look at the phosphor under a microscope David it is made up of millions of tiny prisms. They arrange them during manufacture for the electron beam to hit them at the correct angle depending if the tube is mounted for direct view or via a mirror. After all these years some of the prisms fall off giving a 'ghost' effect to the picture. Difficult to explain but I'm sure you understand. John.

8th Feb 2017, 7:45 pm	#88
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: A scary Baird T23. Today. I've found another Cossor 65K CRT and is no evidence that it was ever fitted in a Baird T23. When a Cossor 65K CRT is installed in the Baird T23 a wire link is connected between the cathode socket to one of the heater sockets. This CRT does not have the heater to cathode wire link. The reason for doing this is because the original Cathodevisor CRT has a directly heated cathode. The tube cathode voltage is set to a fixed potential which is supplied from the power supply unit. The brightness is adjusted by varying the CRT grid voltage. Positive going video from the RF unit is fed to the EHT power supply unit. A DC restorer diode in the EHT PSU clamps the sync tips to the voltage adjusted by the brightness control. All known Baird T23 sets are fitted with the Cossor 65K CRT. However, there is no reason why this CRT cannot be fitted in the Baird T5 or other 15" Baird pre-war TV sets. DFWB. Attached Thumbnails

10th Feb 2017, 10:25 pm	#89
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: A scary Baird T23. Before I become brave again and remove and refit the CRT it's time to take a look at the frame timebase. The picture is off-set to the bottom of the screen. The single valve timebase employs an oscillator transformer which also performs the vertical scanning by means of those extended pole pieces. The DC current through the transformer primary is cancelled out by a third winding, see the attached circuit diagram. Earlier this evening the timebase unit was removed in order to find the picture shift fault. Well it turns out that the picture shift winding has been disconnected. Later on I'll reconnect the winding and report back the results. DFWB. Attached Thumbnails

11th Feb 2017, 8:03 am	#90
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Re: A scary Baird T23. Hi David, It is worth checking your coils against the T5C (single standard) arrangement. It had an extra feedback winding. See the attached picture and note the unconventional pin numbering. According to the numbering shown there is a coil between pins 7 and 1 (conventionally pins 6 and 10). Cheers Andy Attached Thumbnails __________________ www.youtube.com/user/andyvalve100

11th Feb 2017, 11:34 am	#91
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: A scary Baird T23. Hi Andy, The extra outer winding is used for frame sync pulse injection. I'll check to see if that winding is present in the frame transformer in the T23. The later set employs an interlace diode and the sync pulse is connected direct to the grid of the 41MP valve. DFWB.

24th Mar 2017, 10:32 pm	#92
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: A scary Baird T23. The frame sync injection winding is present in the oscillator transformer but it is not connected to anything. This week just one small job done to the set and not a very exciting one either. The original mains connector at the rear of the cabinet has missing pins. Made by Bulgin it's of a type I have never been able to find. The first attachment shows the original connector sans it's screw in pins. A special bracket has been made to mount a standard mains connector, second picture. DFWB. Attached Thumbnails