Teletext

[Roy]

A question for those who restore dual-standard or early 625-only equipment – how do you approach the problem of teletext “interference”? Is there a magic list of modifications to work from, or do you experiment with the flyback and blanking, or just put up with the twinkling lines?

Now that practically everything is broadcast in widescreen format with dark areas at the top and bottom, the data-twinkle effect is more of a distraction than ever.

Thanks

Roy

This receiver has twinkly lines, experimenting may take a long time to cure! or alleviate the problem, but if you could say what model/chassis you are working on perhap's some might have had this problem before and can comment.

[ppppenguin]

A lot of earlier 625 and dual standard sets had poor vertical blanking. It didn't matter before teletext so manufacturers didn't bother.

[oldeurope]

Hi Roy
It is better to suspress the TXT in the video signal.
Than you don't need to change anything on your
telly.

An example how this can be made, you can see in
my 405 converter Blatt 3

http://www.your-normwandler-page.de.vu/

look at Q36 and D33.
The emitter followes the DC-level of the sync tips.
The monostable triggered with the V- puls gives a
high level during V and the video text (TXT) .
So during V and TXT Q36 pulls down the katode of D33
to the sync tip level and the TXT will be blanked.
The sync pulses survieve because of the Ua-k voltage
drop (don't know the english word).

Hope you undersand what I've explaind.

Here you can see the new V-sync with suspressed TXT:
http://www.pixum.de/viewalbum/?id=1303905



Kind regards
Darius

[Roy]

A possible workaround is to use a Freeview digital TV adapter with UHF output. The adapter does not transmit any teletext data in the field flyback blanking period, so if the receiver has slow flyback and/or poor blanking, it no longer matters.

However I would still like to have the option of applying a modification to the receiver, so that it can be used without the adapter. If anyone has details of modifications for the RBM A823 series or the Baird M620 series, please let me know!

Thanks again

Roy

[rigsby27]

Hi Roy,

Have you a specific set or chassis in mind?.There were cobbled together mods to overcome this problem,but varied from set to set.
I may still have the more common one's somewhere in the black hole I call my workshop.

Cheers,
Tony.

[oldeurope]

Hi all,
Jeffrey toled about the poor vertical blanking of some
sets.
Did you notice the reduced level V- sync in my oscilloscope picture? You get this when you use
a schottky for D33.
Flyback lines are well suspressed this way. I have
no trouble with this sync.

Darius

The rank bush murphy set's A823... i seem to recall text lines were a problem that disappeared when a signal diode on the r.h.s vertically mounted? ftb was snipped out of cct, the linearity may have suffered but the customer was happy.

I got around this with some early B&O colour stuff by adding a BC108 transistor in series with the cathode of the luminance output valve (as some sets have allready) driven by a monostable circuit with an adjustable delay (a few more BC108's). The input came (via a big capacitor) from the frame transformer. By these means it is possible to cut the tube off for a pre-set time after the frame flyback is complete, thus removing the offending lines. It was very satisfing to turn the control and watch them disappear!

Be wary of modifications that speed up the frame flyback, they can put the output stage under too much strain.

The same setup should work for other sets too, depending upon from where the sync is extracted.

[Roy]

Thanks for the suggestions.

Be wary of modifications that speed up the frame flyback, they can put the output stage under too much strain.

Good point, I will concentrate my experiments on blanking rather than flyback speed. In any case, a mod to increase flyback speed (even a manufacturer approved one) might not be fully effective nowadays due to the increased number of data lines being broadcast.

Time to search for that postage-stamp sized piece of stripboard I was saving …

Roy

[Hybrid tellies]

Most of the mods that we did back in the 80's were those obtained from the manafacteurs and involved both speeding up flyback and mods to the blanking stage. All these mods involved the early transistorized colour chassis such as the BRC3K, Philips G8 and the RBM A823.
There were never any mods required for Hybrid tellies I suppose due to the higher flyback spped employed. Any teletext interference on such sets were down to either frame stage or video blanking faults.
One major problem that did effect many duel standard mono sets was teletext interference to the sound which resulted in a very unpleasant continuous buzzing sound on 625. Some sets like the BRC1400 were particulary bad with many mods offered by the backroom boys of BRC but often these were not entirely succesfull.

[Roy]

Quite by chance, I found the answer to the problem in an old issue of “Television” magazine, while searching for something else. The January 1989 issue contains a brilliantly simple modification for the A823 which increases the amplitude of the field flyback blanking pulse sufficiently to suppress the lines. It works perfectly – not a trace of a teletext line remains, even on the darkest scenes. The flyback speed is unchanged, so reliability should not suffer.

The modification was published in the readers’ letters section of the magazine, so doesn’t appear in the index - perhaps that explains why it took me 15 years to discover it.

I have scanned the circuit from the page and can provide a copy if anyone else is interested.

[Ed_Dinning]

Hi Gents, this was a topic that appeared frequently in the letters section of " Television " over the years. A spot of research may yield the mod details if you have the back numbers.

Ed