TV22 flyback lines

[Argus25]

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.

I replaced the one in my TV22 with a mil spec 1kv rated 470pF dipped silver mica type and have had no issues with that over some years , a 600V rated one would probably also be fine. I can't recall what voltage rating the original one had.

[Argus25]

Originally Posted by mark pirate

I have now fitted another new EY51 which has made no difference whatsoever.

Does the filament temperature of the EY51 look normal ?

If there is a transformer turns ratio error or winding placement on the core and coil coupling issue, and the EY51 filament was running under temperature, the emission would be low, raising the internal resistance of the EHT supply and allowing it to drop under load, thereby emulating the effect of a low emission EY51.

[Freya]

Simplest way would be to fit an EHT diode as a temporary measure to eliminate the EY51.

[mark pirate]

Does the filament temperature of the EY51 look normal ?

Yes, the heater looks normal. Both EY51's are new old stock.

Simplest way would be to fit an EHT diode as a temporary measure to eliminate the EY51.

I could try an HV diode, I have one in the workshop somewhere. I will also add an EHT smoothing cap to see if that helps.

Mark

[Heatercathodeshort]

A simple circuit that will clear flyback lines from receivers employing tetrode tubes. The pulse is applied to the A1.

Disconnect the wire from pin 10 A1 [duodecal base] and connect it to a spare tag. Connect a 270K resistor from the spare tag to pin 10.
Connect a resistor around 2.2M from pin 10 to earth.
Connect a .001uf capacitor from the anode of the frame oscillator to pin 10.

The value of the 2.2m can be varied to obtain a slightly lower A1 voltage than specified. This will increase the contrast range of an elderly tube. Try a few values until the brightness control if about right when 3/4 advanced.

This costs nothing and is usually very effective. I have just this week carried out this small mod on a Ferguson 968T with a Mullard MW31-18 tube. John.

[Argus25]

Originally Posted by Heatercathodeshort

A simple circuit that will clear flyback lines from receivers employing tetrode tubes. John.

That looks good.

When I was experimenting with RC networks to eliminate flybyack lines I found that the time constant of the RC network was important, in that if it is too long, it results in a vertical tilt in picture brightness from the top towards the bottom of the scan , dimming the scan off towards the top. If the time constant is too short there is insufficient blanking of the lines. Sometimes a two stage RC network is needed for a good result.

(In good professional monitors & TV's there is a blanking amplifier, more parts of course and the beam also gets blanked during horizontal retrace. There is a beautiful blanking amplifier in the Conrac avionics video monitor I posted about recently)

[Argus25]

Originally Posted by mark pirate

I could try an HV diode, I have one in the workshop somewhere. I will also add an EHT smoothing cap to see if that helps.

Mark

Sounds like a good idea.

I assume you have been measuring the EHT with a standard TV EHT probe ?

I guess also you are running the set on an isolating tranny with a grounded chassis for test purposes.

One safe method that might help diagnose the EHT drop would be to wrap a couple of turns of eht wire around the transformer body and grounding one end and then off off to a scope to look at the waveform and see if the peak amplitude drops down a similar % as the EHT drop with an increase in brightness. If it did drop the same %, that would help exclude anything to do with the EHT rectification process, the EY51 or the total EHT filter capacitance. It would suggest that energy was being lost from the primary circuit.

In the TV22 the metrosil (or the circuit components associated with it) as noted by Boom, could be the cause of the trouble, so if you could get another metrosil to try, that would be good.

[mark pirate]

A simple circuit that will clear flyback lines from receivers employing tetrode tubes. The pulse is applied to the A1.

Thanks for posting the circuit John, I will give it a go on the TV22, I think you may remember this set, you had a look at it a couple of years ago after being fitted with the rewound LOPT.

I assume you have been measuring the EHT with a standard TV EHT probe ?

No, my meter is a very accurate RBM unit (see photo).
I will try another metrosil, as I have a spare somewhere....

Mark

[Boom]

If you do change the metrosil remember that its associated wax cap' in on the other side of the tag board and easily missed.

[mark pirate]

Yes Dave, I know about the hidden one!
Apart from the mica caps, the whole lot are new, I have run the set for a couple of evenings for a couple of hours or so and it is very stable.
the picture is quite good apart from the flyback lines

Mark

[Argus25]

Mark,

The vertical output tube is conducting during scan time and switched off during vertical flyback. So the anode voltage on the vertical output tube goes rapidly positive during flyback.

Therefore, if you place a series resistor & capacitor from the CRT cathode to the plate of the vertical output tube, the capacitor will differentiate the anode waveform to create a pulse and put a positive going spike on the cathode during flyback, which will blank the beam during vertical flyback.

I would recommend, start with a high value resistor and low value cap such as 470k and a 50pF capacitor. Work up in capacitor value and lowering the resistor value until the flyback lines just disappear.

If the capacitor value is too large, the low frequency response of the RC network will be too long and there will be a grey tilt on the raster brightness from the top downwards. If the time constant too short, there will be insufficient retrace blanking.

[Argus25]

Another way that works is to couple a negative going pulse to the CRT control grid 1. (this method might be less likely to cause other unwanted effects as it doesn't couple a pulse back into the video output stage's anode circuit).

This would be done by placing a 47k resistor in series with the grid (this won't affect the operation as the CRT's grid current is negligible) But it provides a way or impedance between the brightness control circuit and the grid to introduce a negative going blanking pulse. Then again a series RC network to couple it in, but this time from the blocking oscillator output (top leg of height control) that drives the vertical output tube, because it has an opposite phase signal to the plate signal and it goes rapidly negative during flyback.

If you are playing around adding blanking circuits, make sure not to take the CRT's cathode voltage too high, or to take the CRT grid voltage positive with respect to the cathode by mistake.

One insurance policy to prevent the latter during experimentation is to place three 1N4148 diodes in series (to make a low capacitance high reverse voltage breakdown diode) and connect them across the grid & cathode of the CRT (so they are reversed biased) meaning the diode cathode connects to the CRT cathode and the diode anode connects to the CRT grid. This way if the CRT's grid attempts to go more positive than the CRT's cathode the diodes forward conduct, preventing it.

I put safety diodes like this on nearly all my vintage TV's during repairs or modifications. You will often see these in the grid-cathode circuits of CRT scopes.

[FERNSEH]

Returned my Bush TV22 to the workbench with the intention of doing some frame flyback suppression experiments. However, before any attempt to do any work of that nature can be started the vertical instability fault must be solved. Picture is stable at very low brightness settings. Fault condition might be caused by something simple like the CRT aquadag not properly connected to the chassis.
Second attachment shows a stable picture with the brightness control turned well down.
DFWB.

[Boom]

I had a very similar problem on a TV32 and never did find out what was causing it so sold the set on after changing everything including the LOPT and re-aquadagging the CRT. I came to the conclusion that despite a really good picture it could well have been the final anode having a dodgy connection inside the CRT.

[Argus25]

Originally Posted by FERNSEH

Fault condition might be caused by something simple like the CRT aquadag not properly connected to the chassis.

If that was happening, it could also explain why the EHT drops 2kV with increase in brightness. The aquadag and CRT glass creates the EHT filter capacitance with the crt's internal anode.

However, I notice that the picture image shows some line tearing. There could be something amiss with the video signal at the plate of the video driver tube where the signal for the sync is taken from V7a, R28,R29,C26 in my trader circuit. If the syncs were compressed as the contrast increased, it would likely give both this effect and poor retrace blanking too. You need to put the scope on the output of the video detector, output of the video driver and video output stage to check for distortion or compression of the sync part of the video signal.

[Argus25]

Sorry,
The remark about the eht drop related to Mark's TV 22, not the one posted by Fernseh.

Also on the issue of coupling a negative going pulse to the CRT grid, I notice in the circuit that the vertical yoke connections (transformer secondary that connects to the yoke) are such that the flyback voltage will be going negative on one of them, connections C or D (check with the scope first) , so the series RC network could connect here to the correct one and to the CRT grid with the added series grid resistor. There should be plenty of voltage available there, that by the time the signal is differentiated with the series RC network that a good negative going pulse can be applied to the grid during flyback.

[Ian - G4JQT]

A question from a non-TV person:

If suppressing (or at least reducing) the flyback lines on the TV22/TV24 was so simple with only a few parts, why didn't Bush ever get around to doing that do that? The model was in production for some considerable time.

Ian

[Argus25]

This is a question that I hoped to had answered in previous posts. When all is "normal" and the TV was receiving a correctly modulated carrier signal from the TV station, and the TV's brightness and contrast controls correctly set, then no flyback lines are visible, so there was nothing for Bush to fix.

The problem is that now, these TV's are being driven by other signal sources from RF modulators where there is no clamping of the video black level at the actual modulator. Therefore the black level wanders around with average scene contrast or video signal level. I have mentioned this many times and in my modulator articles, though few seem to pay any attention to it. Also RF modulators designed for 625 line opposite modulation systems never have the clamping on the negative going part of the signal where its required for a 405 modulation scheme and most home project RF modulators AC couple the video to modulate the RF. In fact I haven't seen an RF modulator for 405 line work organised to do it properly, to replicate the controls of the BBC transmissions yet, except my own one (not meaning to sound judgmental). It has just been overlooked.

Because the design of the Bush TV22 relies on DC coupling of the video signal from the video detector through to the CRT cathode, the black level (or sync tip level too) is unstable and affected by the video signal level when receiving a signal from a typical RF modulator. So no matter how hard you try to adjust the contrast & brightness controls on the set, the retrace lines will appear sooner or later.

I don't experience this issue because I made my RF modulators to have video signal clamping, the black level is stable (as it would have been from the BBC's original transmitters) so I never see the retrace lines on my TV22.

A lot of other design sets that use AC coupling to the video output stages and have a DC restorer diode at the CRT, never get this issue even if the black level is wandering around at the RF modulator and therefore at the video detector, because the correct DC component is inserted last thing at the CRT drive. So using a RF modulator with no clamping won't be a visible problem in these TV sets.

Also, as I mentioned the Americans were very early adopters of internal retrace blanking, because many customers couldn't adjust their sets properly and complained of retrace lines. Also, in TV sets that were budget models, that just had AC coupling of the video to the CRT (many early transistor versions) all had to have retrace blanking as there was no DC coupling of the signal and no DC restorer either.

[Nuvistor]

Has Argus rightly points out with the system working correctly flyback blanking is not required, i.e the DC content and picture /sync ratio preserved from studio to TV screen.

However there are instances were this was not achievable, fringe area reception, reduced gain of the signal circuits as the set ages etc. Anything that can degrade the 70:30 picture sync ratio. The link is to a PT article in 1953 for modifications to sets to include frame flyback blanking so the need was there. Interestingly a circuit of the HMV pre war flyback blanking circuit is included.

Whether the set designers at Bush didn’t include the circuit for technical reasons or from a cost reason I don’t think we will ever know. The cynic in me is towards the cost, the sets were expensive enough and any reduction in price of the set would have been welcome.

Now with the ability to have good signals and have the TV in fully working condition a proper modulator is probably worth aspiring to.

Page 315

http://www.americanradiohistory.com/...on-1953-12.pdf

Does anyone know if the popular (cheap) mean level AGC system made the flyback lines more of a problem, the sets I repaired all had flyback blanking from new? If faulty I fixed the circuit but never gave a lot of thought to it.

[Heatercathodeshort]

Originally Posted by Ian - G4JQT

If suppressing (or at least reducing) the flyback lines on the TV22/TV24 was so simple with only a few parts, why didn't Bush ever get around to doing that do that? The model was in production for some considerable time. Ian

Very good question Ian. I have often wondered that also considering the extra cost of components being around 5p in today's money.

Practically, I think it all came down partly to very close costings, the general lack of public spending money in 1950, and the intense price battles between the manufacturers fueled by the great British public that always purchased the cheapest available unless recommended otherwise by a good friend or someone that had been a radio operator in the war and 'knew all about television'.

The Bush TV22 cost about the same as a good radio or middle range radiogram and was just about affordable, especially if you had money earned in war time factories, and service pay.

In the USA things were different. I am playing with my first receiver from our friends over the water. It is a TRUETONE 10" table model from 1948.
It is a superhet with a 12 channel tuner, 4 vision IF amplifiers, vision AGC flywheel sync flyback suppression and a massive mains transformer to supply heater and HT current.[120v]
Only 3 octal valves, the rest B7G and B9A. Even the EHT rectifier is a B9A.
So far the results are stunning. So much gain that the screen displays the familiar snow with hiss on sound.

The big difference is that it cost a great deal of 1948 dollars compared with our little AC/DC TV22. It Britain, cost was everything. Technical advances improved as the 50's moved on but think how much better viewers in fringe areas would have been if the circuits of the Pye V4 [1953]had been fine tuned and adopted universally and a little more RF/IF gain had been available.

The TV22 and the whole television industry did a very good job and I doubt if many service engineers had many complaints about the flyback lines on certain pictures. Just the good old accepting British public and maybe we should be thankful for that ! Regards,John.