TV22 flyback lines

[mark pirate]

I am having problems with flyback lines on this set, it had an excellent picture prior to the LOPT dying.
I managed to get a rough TV24 and harvested the LOPT, luckily it is fine and got fitted to the set last week.
I fitted a new EY51 just for good measure and EHT is fine and plenty width available, but after setting it up and adjusting to the Bush manual I find the flyback lines are very prominent when adjusted for a normal picture.

It also does seem that the picture is not as sharp and bright as it was, I have readjusted the ion trap for maximum brightness.
Any ideas?

Mark

[Boom]

I've never seen a TV22 without flyback line. They don't have supression.

See here and particularly follow up posts #9 and #14.

https://www.vintage-radio.net/forum/...ad.php?t=34767

[Ian - G4JQT]

I believe it's normal. Maybe you have the brightness up too high?

Do you have a Test Card on DVD? Of course there's one in the Aurora.

If so:

Adjusting contrast and brightness

This may seem too trivial to need explaining, but these two controls are somewhat interdependent and there is a correct way of setting them. So, for those like me who have not seen a procedure clearly described, and to minimise any random twiddling, I summarise here two descriptions of how to do this using Test Card C or the Tuning Signal (or if necessary, presumably any stationary scene that contains all ranges from peak white to black). It applies to any television set once it has warmed up:

“Television Engineers’ Pocket Book” method:
• Turn contrast and brightness to minimum
• Increase brightness until faint glow is seen, then gently reduce brightness until glow only just disappears
• Increase contrast until peak whites show clearly and light greys are correct shades
• Adjust brightness until blacks and dark greys are correct shades
• Readjust contrast for best contrast between whites and light greys

“Correcting Television Picture Faults” method:
• Turn contrast and brightness to minimum
• Remove aerial plug from receiver
• Increase brightness for a blank raster, then reduce brightness until raster just disappears
• Reconnect aerial plug and slowly increase contrast control until an evenly illuminated picture is obtained
• If necessary, slightly readjust brightness control for evenly illuminated picture

But to be honest, this is fine for a broadcast signal from the BBC when everything was carefully adjusted, but from a standards converter you still need to juggle the controls sometimes depending upon the programme source. Not all DVDs are the same for a start!

[Boom]

Originally Posted by Ian - G4JQT

Do you have a Test Card on DVD? Of course there's one in the Aurora.

I don't know the technicalities or whys but for some reason a test card will not give flyback lines (Well my TV22 doesn't anyway or any other that I have fired up here).

[Ian - G4JQT]

The Test Card is designed to have 'average' picture brightness. Of course in reality, this is quite rare - especially if you're not using a well adjusted off-air source.

Maybe the source you're using needs the brightness and/or contrast turned down a bit.

[Freya]

Try increasing the signal from the aurora, which gives more contrast; meaning you turn the brightness down and the flyback lines go.

Sorry if this has been tried already.

[mark pirate]

I don't know the technicalities or whys but for some reason a test card will not give flyback lines (Well my TV22 doesn't anyway or any other that I have fired up here).

The test card via the Aurora does not show flyback lines on my set either!
I have tried several DVD's & vintage programs via my media player, some are better than others, but the flyback lines are still visible at normal brightness

I am sure that there is a fault with the set, as turning up either contrast or brightness increases the picture size vertically and reduces the width and goes out of focus.
The set has been totally recapped and any out of tolerance resistors replaced.
I have swapped PL38, PZ30 & fitted a new EY51.

line and frame hold are rock steady.

Mark

[Boom]

Originally Posted by mark pirate

turning up either contrast or brightness increases the picture size vertically and reduces the width and goes out of focus.

Classic symptoms of low EHT. Does the HT line stay up (238VDC iirc) or vary with the brightness and picture stretching?

I had some strange goings on with my TV22 which were traced to the metrosil. A VDR worked fine as a replacement. I can't remember what the voltage out of the metrosil should be but I'm sure someone will know. (I think it is pushing 400VDC)

[Peter.N.]

I can't now remember the circuit details of the TV22 now but excess A1 volts will cause flyback lines on many TV's also I seem to remember will the A1 decoupling capacitor being o/c.

Peter

[Freya]

There are several non specific articles that describe how to add the circuitry to remove lines, BOOM (Dave) tried this back in 2003 here https://www.vintage-radio.net/forum/...d.php?p=608732 with limited success. I think it can be done with a minimal component count.
One article I have below,

[Argus25]

Originally Posted by mark pirate

I have tried several DVD's & vintage programs via my media player, some are better than others, but the flyback lines are still visible at normal brightness

Ok, well this raises some issues that I have mentioned in the past relating to video RF modulators for vintage TV's.

There are a series of interactions which occur, depending on the design of a TV set and the design of the video modulator feeding it, which can result in retrace lines occurring at times and not others. I'll go through it.

Firstly, retrace lines will be visible if the CRT beam is not cut off (black level or below) during vertical flyback. As most know you can force the CRT beam to cut off with active blanking from a differentiated signal from the vertical output stage, but many TV sets, like Bush TV22 don't have this.

So the system relies on the correct setting of the TV's contrast and brightness controls for the video signal's black level to provide correct retrace blanking. This also relies on the DC axis or DC position of the video signal driving the CRT's grid or cathode being stable for its sync tip voltage (or black level which has a fixed relation to the sync tip) and independent of the scene's picture content or video signal voltage.

Generally there are two methods in vintage TV's by which the DC axis or black level of the video is stabilized for the CRT drive. One is to AC couple the video from the video detector stages and video amps to the CRT and use a DC restorer diode (which clamps on sync tips) and sets the black level. This system is relatively immune to the problem I'm about to describe.

The other method is to simply have the video detector direct coupled through the video amp stages directly to the CRT, the video signal DC axis or black level is maintained this way. The Bush TV22 is like this, with the DC coupling maintained from the video detector & video amp to the CRT cathode......So what could go wrong here ?

What is not realized is that this method only works properly when the video signal source (at the TV station or RF modulator unit) modulating the video carrier, has its DC axis or black level stabilized at the actual RF modulator.

The video signal in many RF modulator units is "lazily AC coupled in without clamping". Therefore as the scene brightness changes the black level shifts and this unstable black level appears at the TV sets video detector and ends up at the CRT.

In the 405 system where the syncs modulate the carrier to a lower level, higher white scene video levels result in decreased black levels, or a lower carrier level on black if the signal is not clamped at the modulator and only AC coupled to it. So if you are viewing a signal coming via a video modulator with AC coupling (without a clamping) and you set the brightness and contrast controls to the correct setting, so the retrace lines are just blacked out, when the scene goes to a low contrast one, the black level lifts and the retrace lines appear.

This is the inevitable consequence of designers of some RF modulators not thinking about or bothering to clamp the DC axis of the video signal at the RF modulator. Of course in the original BBC TV station transmissions, the sync tip or black level always corresponded to a specific video RF carrier level and was not allowed to wander around with changes in video signal level.

So my advice is, before concluding there is a defect in the TV22, make sure the video RF signal source is coming via a modulator with video signal DC axis clamping. Clamping on the video sync tip is just fine. A circuit that does this is on page 4 of this article:

http://worldphaco.com/uploads/RF_MOD...TELEVISION.pdf

[mark pirate]

Thanks Stephen, while the set is on the bench I will experiment with that flyback reduction mod.

Classic symptoms of low EHT. Does the HT line stay up (238VDC iirc) or vary with the brightness and picture stretching?

When the set was originally restored the set had a really outstanding picture, the CRT (MW22-16) tests good as new, then after a couple of hours the LOPT chucked in the towel

I had it rewound, but it gave only 6kv max of EHT and even with the width on full would not fill the screen.
The present LOPT gives close to 8kv and the width is fine with the control in the centre position.

The testcard looks good, but when watching a programme the picture lacks punch, it is not possible to get the 'soot & whitewash' when turning up the contrast control.

The HT is 204v (both main caps are new).
With my luck, it is probably the LOPT, although it has been run for a few hours and stays cool in operation, I will try another EY51 later.

Mark

[Freya]

I think it would pay to run the overwind on the bench psu for a few hours to get any residual moisture out, my Ekco T164 had your exact symptoms and the dry out sorted it right out. I sealed it with anti corona spray afterwards.
The resistor in the picture that I uploaded could be 2.2k but its a bit unclear.

[mark pirate]

The TV24 the LOPT came from had been stored in a garage for decades, in was in pretty poor condition with surface rust & corrosion to the aluminium chassis.
I removed the LOPT and cooked it in the oven to remove any residual damp, this melted off most of the pitch and showed that the lams were in good shape.

It has been stored indoors in the warm for at least 6 months before fitting to my set, the EHT does not drop during use, but does lose close to 2kv when brightness is increased from min to max.

Mark

[Boom]

Originally Posted by mark pirate

I had it rewound, but it gave only 6kv max of EHT

I have heard from several people that when a LOPT is rewound that the wax tuning capacitor across the LOPT has had to be varied to get it all resonating properly.

[Argus25]

Originally Posted by mark pirate

the EHT does not drop during use, but does lose close to 2kv when brightness is increased from min to max.

If the EHT drops by this much, with an increase in brightness it indicates two things. Firstly the CRT is probably in good order and able to reach a good beam current. Secondly, the internal resistance of the EHT voltage, as a power source, is too high and dropping under the small load of the beam current. Or the filtering/smoothing capacitance is inadequate.

Since the bulk of the source resistance is in the EHT rectifier and not the transformer or EHT overwind, I would replace or check that rectifier first.

The EHT rectifier charges the capacitance of the CRT anode between peaks at a 10125 Hz rate. This capacitance is increased by the CRT's external aquadag. But some CRT variants don't have external aquadag, so the capacitance of the final anode can be low enough that the EHT voltage sags down between peaks, lowering its average value, when the CRT's beam current increases.

So assuming the EHT rectifier is good, the 2kV voltage drop could be due to insufficient capacitance. The solution there is to add extra capacitance such as a 300 to 750pF 15kv doorknob capacitor on the CRT's anode connection. I added one to my TV22 because it got fitted with a non aquadag CRT. There was plenty of room for it in the EHT cage.

[mark pirate]

Since the bulk of the source resistance is in the EHT rectifier and not the transformer or EHT overwind, I would replace or check that rectifier first.

I have now fitted another new EY51 which has made no difference whatsoever.
A check on the HT shows that no drop occurs when the brightness is advanced, in fact it increases by a couple of volts.

I found a 270k resistor between chassis & cathode that had gone high, replacement has made a slight improvement in picture quality.

I am sure that the problem is the LOPT, I will replace the tuning cap to see if that helps, although trying this with the rewound LOPT did not help.

Mark

[Boom]

Originally Posted by mark pirate

I will replace the tuning cap to see if that helps, although trying this with the rewound LOPT did not help.

You don't want to try just changing the cap' Mark. You will need to vary the value (if that is what is causing it).

[FERNSEH]

Frame flyback lines will be always a problem with any set which doesn't have a proper DC restored video waveform applied to the modulating electrode of the CRT. The TV22 is one of those TV receivers in which the negative going video waveform is AC coupled to the CRT cathode. In practise it is partial AC coupling because the coupling capacitor is part of a DC potential divider, it is necessary to reduce the heater to cathode voltage so that the ratings of the CRT are not exceeded. Mullard recommend that the h-k voltage should not exceed 125volts.
One method to improve matters is to speed up the frame flyback time so that any evidence of any disturbances during the frame sync and blanking interval will appear at the top of the screen. Alternatively, a frame blanking pulse should be supplied to the control grid of the CRT, (assuming cathode modulation) The blanking pulse can be up to 1400microseconds wide. Obviously you are not going to find such refinements in a cheapo domestic receiver.
Returning to the loss of the video DC component, the well known result of this is the variation of the reference black level during scene changes. The picture background brightens up when the contrast is low.

DFWB.

[Argus25]

Originally Posted by FERNSEH

Frame flyback lines will be always a problem with any set which doesn't have a proper DC restored video waveform applied to the modulating electrode of the CRT. The TV22 is one of those TV receivers in which the negative going video waveform is AC coupled to the CRT cathode.

It is DC coupled, the 47k in the top leg of the divider passes the DC component and the 0.1uF the AC component. Because the divider resistor is 270k then 270/( 270 +47) or 85% of the DC component is retained.

I have found with my TV22, that when is fed by a correctly modulated video carrier (with clamping as noted in my post above) and the contrast and brightness set to the correct value, the retrace lines are never visible regardless of the scene content or contrast. But if the RF signal comes from a modern modulator, especially one that was intended for the reverse modulation scheme where the video sync increases the carrier level, but its being fed with a reversed video signal, its hopeless and the DC component is lost and retrace lines appear.

I often wondered why many professional early studio video monitors (like Marconi) had no vertical retrace blanking. It took me a while to realize it was so the operator-technician could accurately set the contrast and brightness controls.

With domestic television, the Americans were early adopters of internal retrace blanking in most TV sets because they figured that many customers & users were hopeless at setting the controls correctly and it was cheap usually with a single R-C coupling circuit. Of course when color came and more controls got added to the TV, it became glaringly obvious that a large % of people didn't know to correctly set their TV controls. Those who did would rush to the sets to re-adjust them on entering the room, irritating the set's owners.