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Old 16th Feb 2022, 11:34 am   #1
Ian - G4JQT
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Default Bush TV22/TV24 flyback lines

This has come up before regarding some modification to reduce the flyback lines on this set, but I have not found any clear or specific info on what can be done and how effective it is.

All I've found is that when the contrast and brightness are correctly set, they shouldn't be generally noticeable. Well sort of...

Unless I've misunderstood something, it seems real off-air transmissions were such that the black level didn't wander so much on these simple sets and flyback lines were not really noticeable. But with standards converters this can be a problem with some video source material.

I've read on another forum that the mod is simple, but again no clear details for the Bush TV22 - unless I've just missed them. I did ask that if the solution was so simple why didn't Bush do it. The reason seemed to be that the extra (small cost) probably wouldn't have made much difference on real signals, despite apparently Bush's later sets having these additional components.

So, does anyone have details of this modification or modifications and whether it does make a noticeable difference to signals coming from say, the Aurora on the simple TV22 or TV24?

Thank you.

Ian
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Old 16th Feb 2022, 1:38 pm   #2
stevehertz
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Default Re: Bush TV22/TV24 flyback lines

Good call, I'd be interested to know this stuff too as I have a 'mint' TV22a awaiting the soldering iron, AKA a full restoration.
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Old 16th Feb 2022, 5:08 pm   #3
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Default Re: Bush TV22/TV24 flyback lines

Worth noting the contrast control VR1 varies the cathode bias on the RF amplifier and the sound and vision IF amplifier. Across the control VR1 is a 50mfd electrolytic capacitor. We know that the vision RF and IF signals consist of the carrier frequency and sidebands. Those sidebands will have modulation frequencies from 50Hz to to the upper video frequencies. So it follows the cathode bias voltage should be free of any video signal content.
Connect an oscilloscope with a DC input across vision detector load resistor R14. There should be a true sync tip and black level.
Alternatively, monitor the video at the control grid of the video amplifier.
Some early TV22 and TV24 receivers have direct DC coupling between the anode of the video amplifier and the CRT cathode.

DFWB.
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Old 16th Feb 2022, 6:42 pm   #4
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Default Re: Bush TV22/TV24 flyback lines

That's the theory - what's the solution?
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Old 16th Feb 2022, 11:11 pm   #5
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Default Re: Bush TV22/TV24 flyback lines

I don't believe there is any noticeable difference between an off-air signal and that from an Aurora.
I am sure signal in an Aurora is properly clamped. I can see no reason for it being any other ways.

The link below is to a clip that I took some years ago that highlights the lack of clamping in a TV22.
It appears in the clip that the greyscale in the centre of the picture is changing.
But in fact only the background level is changing.
The effect seen on the greyscale is due to the lack of black level clamping.
https://youtu.be/wGo8vR9KWGs

I have no solution though

Frank
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Old 17th Feb 2022, 1:22 am   #6
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Default Re: Bush TV22/TV24 flyback lines

My theory about why back "in the day" people didn't notice these black
level problems is that producers had monitors like a typical TV and simply
did not design lighting or sets (etc) that generated problems. It's sort of
like the early Technicolor films, where the inventor's wife Natalie ruled with a stern hand as "color consultant".
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Old 17th Feb 2022, 9:22 am   #7
peter_scott
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Default Re: Bush TV22/TV24 flyback lines

It wasn't only TV22s that had bad blanking. Here's a photo my father took in the 1950s of a total eclipse taken off the screen of our Philips 1400A projection set that lacked decent black level control.

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Old 18th Feb 2022, 1:33 pm   #8
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Default Re: Bush TV22/TV24 flyback lines

Quote:
Originally Posted by stevehertz View Post
That's the theory - what's the solution?
Introduce proper frame and line blanking circuits. Such circuits could be included in any set but who's going to bother doing such modifications to a simple receiver like the TV22?

Quote dtvmcdonald: "My theory about why back "in the day" people didn't notice these black level problems"
That's right, back in the times when the TV22 was in use I doubt if any TV viewers ever complained about such matters as faint flyback lines.

DFWB.

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Old 18th Feb 2022, 2:38 pm   #9
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Default Re: Bush TV22/TV24 flyback lines

It was good for what it was. As long as it was bright with good linearity, focus and sound, customers would have been delighted.
It might only have cost little money to fit it as original equipment but having sold many hundreds of new receivers [not TV22s, I'm not that old!] I can state that as little as £5 difference in the price of a TV can sway a customer.

You can eliminate flyback lines on the 20 series Bush. It's quite simple. Disconnect the A1 from the Metrosil and place a 1mohm resistor in series with the A1 lead. Connect a capacitor typically .001-.002 from the junction of capacitor and the A1 lead to the frame oscillator anode. You can also try it on the anode of the frame output valve. Simple, give it a try. John.
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Old 18th Feb 2022, 4:17 pm   #10
Andy - G8MNM
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Default Re: Bush TV22/TV24 flyback lines

You can fit the blanking components as per the Bush TV53/62 etc.
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Old 18th Feb 2022, 4:38 pm   #11
Andy - G8MNM
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Default Re: Bush TV22/TV24 flyback lines

This is the setup on the TV53. I have done the mod to my TV22. I guess the value of the coupling capacitor (820pF) isn't too critical. Good luck with the restoration.

There is a good write up of an excellent restoration on here somewhere! No doubt someone can post a link.

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Old 18th Feb 2022, 4:44 pm   #12
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Default Re: Bush TV22/TV24 flyback lines

Heatercathodeshort wrote: "You can eliminate flyback lines on the 20 series Bush. It's quite simple. Disconnect the A1 from the Metrosil and place a 1mohm resistor in series with the A1 lead. Connect a capacitor typically .001-.002 from the junction of capacitor and the A1 lead to the frame oscillator anode. You can also try it on the anode of the frame output valve. Simple, give it a try. John."
Later Bush TVs such as the TV33, TC36C and TV43 did incorporate frame flyback blanking. The negative going flyback pulse present in the frame blocking oscillator was supplied to the grid of the CRT.
In the TV24A and I guess it is the same for the five channel TV22A, the sawtooth waveform from the frame multi-vibrator is differentiated by a 0.005uF capacitor and the negative part of the waveform used for frame flyback blanking. Pulse supplied to CRT grid.
But in reality to do the frame blanking properly a pulse of at least 1mS is required to blank out any above black level activity during the 1.4mS frame sync and blanking period.

DFWB.
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