Decca Bradford Teletext mod

[dominicbeesley]

Hello all,

Does anybody have a copy of the mod for getting rid of the teletext flyback lines on a Bradford. I hear tell that there is something in Television from 1990 but I don't have access to these. I'm hoping its fairly simple

Dom

[KeithsTV]

I do recall an article in Television which included mods to eliminate teletext flyback lines but can't remember exactly which issue it was in. I've looked through my issues around 1990 but can't find it there. I suspect it's in an earlier issue. I'll have another look tonight.

I've also found the issues which cover the Bradford chassis which I can scan and send to you if you wish. There's also a mod to the flywheel sync time constant to make it video compatible.

Keith

[dominicbeesley]

I've just found the other thread... "TELEVISION, September '90 p866" is the reference given.

Any info greatly appreciated. Dave (the previous owner) has already done the flywheel mod and it does seem fairly stable on any source.

I think he was less fussed about the Teletext lines due to local conditions meaning he didn't get the problem

Dom

[KeithsTV]

Hi Dom

I'll check, scan and post it up here tonight.

Keith

[KeithsTV]

Hi Dom

Here's the relevant page. It was in the letters page. No wonder I didn't spot it.

Keith

[Andy Green]

Sorry if this is a little off topic, (mod move it to a more apporpriate place if you want) but I just could let this point go!
In in the same letter answering the teletext blanking mod, there is a bit about the reception of Sky channel on VHF - I must admit I don't ever remember any TV licence beibg given out to Sky for tereestrial rebroadcast - even if it were, it would have been on UHF, not VHF - the VHF band II channels were full with two channels being broadcast from high sites! (300 ft typical) There would have been no diplexer or room for another high power transmitter just for sky to sell sets! There were 'deflector' transmitters for rebroadcasting BBC and ITV, at the time they were illegal, and often would add a satellite channel to the line up, but again, these were always on UHF.
(Also, referring to the UK as 'The mainland' is a big no-no from an Irish perspective! - Imagine how well France doing the same thing to the Britain would go down!)

[Hybrid tellies]

Interesting mod that, have to admit though that the Decca Bradford caused us very few problems with teletext flyback lines.
I had a slight problem with ITV and CH4 on my own 10 chassis which was cleared by moving one of the fly lead sensor adjustments on the convergance board whilst setting up the convergance. To my delight this also cleared the teletext lines on both effected channels.

[KeithsTV]

Andy

Just for completeness here's the rest of that letters page.

Keith

(perhaps this should be moved to another section)

[Andy Green]

Yes, I see now, Keith - the cable distribution explanation is far more likely!
Just noticed several typos in my earlier reply - the main RTE Sites are typically at 3000 feet, not 300!, and are band III TV, not band II !

[dominicbeesley]

Thanks Keith,

If you professionals were doing such a mod for real how would you have done it? A flying lead to an extra board made up and shoved on the chasis or would you have bodged the extra components onto the decoder board?

Cheers

Dom

[KeithsTV]

As there are only 4 components to be added it should be possible to add the components onto the decoder board. Alternatively a small piece of board with the 4 components fitted across R287 with a flying lead to the convergence panel. Just make sure the board/components can't touch anything else on the decoder board. I'm sure the professional approach would be the extra board.

Keith

[dominicbeesley]

Evening all,

I've just got round to making up the teletext mod on a littel daughter board hanging off the decoder panel and .... well it almost works. Most of the two fly back lines are now hidden except for the very start i.e. the bit in the middle of the picture!

This may be due to the fact that I didn't have the right bits to hand so changed some of the components - the input cap is two 0.022uf's in parallel and the 820R is a 1K the diode is a 1N4005

Now I thought that this change of R (if it caused an effect) would be to blank out _too much_ not too little have I misunderstood the circuit?

The C and D changes shouldn't be critical or am I missing something.

Cheers

Dom

probably time to go sleep now....

[dominicbeesley]

Further looking at this both lines are still present but only for a small part of the left hand part of the screen: crappy diagram attached (grey shows original teletext lines, black shows what is left).

Any ideas? is this some sort of negative going spike coming from the line input to the circuit upsetting things?

Cheers

Dom

[oldeurope]

Do you know this one?
http://bs.cyty.com/menschen/e-etzold.../uvb/uvb_e.htm

Kind regards,
Darius

[Simon.P]

This was my mod. I had these lines on a CVC9 any my Decca 30 at the same time.

I did have a problem with spurious blanking which showed up about a year later, due probably to line pulse pickup. Try connecting a capacitor from pin 6 of the MC1327 to the earth line. Try about 0.01uF.

I ty-rapped the DC blocking cap (1000V mixed dielectric) to the back of the convergence panel, and tacked the remainder to the back of the PCB round the MC1327.

[dominicbeesley]

Wow - what a coincidence I tried playing with this again last night and came to a similar conclusion, not tried it yet.

It seems the mod is pretty sensitive to the "speed" of the flyback of the frame, messing with the height and linearity controls got the lines to disappear but at the expense of a very distorted picture!

I'll give it a go this week and get back with the results.

Cheers and thanks for the info (and the original mod!)

Dom

[dominicbeesley]

Hi simon,

I tried a capacitor in parallel with the 10k resistor:

0.01uF - this caused the left 1/5 of the screen to blank out and teletext lines to move on by the same amount
0.1uF - worked a treat! All teletext now expunged!

Have to admit to being baffled as to what has happened here but it works!

Schematic attached

Cheers

Dom

[Simon.P]

Originally Posted by dominicbeesley

Hi simon,

I tried a capacitor in parallel with the 10k resistor:

0.01uF - this caused the left 1/5 of the screen to blank out and teletext lines to move on by the same amount
0.1uF - worked a treat! All teletext now expunged!

Have to admit to being baffled as to what has happened here but it works!

Schematic attached

Cheers

Dom

Hi Dom,

Thanks for the feedback.

I had to replace the MC1327 with a TDA1327, due to failure of the original chip. It might be that there is some difference between the two, although the TDA works fine as a substitute. The original mod needed some experimentation to make it work.

Simon