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Removal of Teletext lines on A823

 
BRC 3000
(@brc_3000)
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Posts: 48

Does anyone know a fix for removing teletext breakthrough on the A823AV chassis. I wondered if there was a mod from back in the day to speed up the flyback time or slightly increasing the length of the field blanking.  I see to recall Television magazine doing an article on mods on common sets for this problem but can't remember which issue. The problem shows mainly when I play back vhs cassettes recorded years ago when text was transmitted.

 Dave 

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Topic starter Posted : 20/01/2018 6:32 pm
crustytv
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There's a whole "service dept" available up top, servicing the sets, stock faults, CTV tips, B&W sets,VCR stock faults, knowledge base,  service data and  .......

The A823 is covered  here at the bottom of the page https://www.radios-tv.co.uk/remove-teletext-lines/

 

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Posted : 20/01/2018 6:34 pm
Till Eulenspiegel
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Many years ago I modified a few of the remaining rental A823 CTVs with a proper field blanking circuit.  I do remember that a  "one shot" monostable was triggered by the leading edge of the frame flyback pulse, or it could have been the frame sync pulse.   The monostable consisted of a two transistor circuit. These days our old friend the 555 timer could be employed.   The stretched 1 to 1.4millisecond blanking pulse was introduced into the base circuit of 3VT5, the luminance emitter follower. Again, it was a long time ago but I would think an NPN transistor was connected between the base of 3VT5 and ground.  The positive going blanking pulse turned the transistor on thus shorting out the base bias of 3VT5.

I can report the modification was fully effective and the teletext line were completely blanked out.   The last set to be modified in this manner was a Bush CTV1122.   The set had an unfortunate fate.

Till Eulenspiegel.

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Posted : 20/01/2018 11:16 pm
Cathovisor
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From memory, there were two mods to the A823: the official RRI one tried to speed up the field flyback, which didn't work; later, a mod was published in Television which lengthened the field blanking. That did work - as I recall no additional components were needed, just changes in values.

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Posted : 20/01/2018 11:38 pm
BRC 3000
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Posted by: Chris

There's a whole "service dept" available up top, servicing the sets, stock faults, CTV tips, B&W sets,VCR stock faults, knowledge base,  service data and  .......

The A823 is covered  here at the bottom of the page https://www.radios-tv.co.uk/remove-teletext-lines/

 

Thanks for the pointer Chris. I have taken a look but must admit the mod for text breakthrough thats on that link is a bit confusing. it is as follows.

  • Replace 4R3 with 2 120K resistors.  ( in parallel i take it? )
  • Replace 4VT2 with a BF337 or equivalent.
  • Disconnect 4VT2 Collector and leave the leg on the component side. ( so cut the leg and leave it soldered in the board, then do what with it?)
  • Solder in the 3 additional components 33K resistor, .03uF 250V capacitor, and BA148 diode ( solder these in where?)
  • Replace 4R4 with another BA148 diode. ( cathode towards the grid or collector of 4VT1)  If anyone remembers the the issue of Television that published the mods I could take a look and see if there are more details in there. Unless anybody an throw more light on the instructions above.

Till, thanks for the ideas around constructing a pulse lengthening circuit using the 555. i might look in to that if i cant get this Tv magazine mod sussed out.

 

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Topic starter Posted : 21/01/2018 8:15 pm
crustytv
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Hi,

Sorry that was not of much help.

Posted by BRC 3000

If anyone remembers the the issue of Television that published the mods I could take a look and see if there are more details in there.

I've now added the 1982 article you reference from Television, to those pages, though its no clearer on detail it might be less confusing. Also a whole raft of more sets now covered.

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Posted : 21/01/2018 8:43 pm
Nuvistor
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Found this on UKVRRR, gives a circuit with the mod.

https://www.vintage-radio.net/forum/showthread.php?t=19569

 

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Posted : 21/01/2018 9:33 pm
BRC 3000
(@brc_3000)
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Thanks Chris and Frank, that little schematic clarify s whats required for the mod. I wonder how critical it is to use a Bf337 and BA148 diode as I dont have either so will need to order them specially. 

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Topic starter Posted : 22/01/2018 10:22 pm
Nuvistor
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I cannot remember doing any mods for this on the A823, I must have done but it’s long lost. I would have used what was suggested, BF337 and BA148 were staple items in tbe spares store so cannot suggest replacements, if you have the specifications you may be able to match them with items you have.

 

 

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Posted : 22/01/2018 10:42 pm
TVJON74
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The following could be used instead of BF337 according to the transistor data on here.
BF258, BF259, BF658, BF659, 2N5058, 2N5059

Jon

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Posted : 22/01/2018 11:23 pm
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TVJON74
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A quick search on the web suggests these could be used instead of BA148
BA157...159, BA245, BA248, BY 206...207

Jon

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Posted : 22/01/2018 11:29 pm
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