white point after set is turned off

[delaitt]

Hello,

I have a very old set (90 degree CRT). The CRT is a AW43-80. The problem is that there is a white point in the center on the screen for 2/3 seconds when the set if turned off. I don't think there is a dot suppression functionality on these old crts but i'm not sure. The CRT has an ION trap which i had to move slightly to reduce the brightness of the image.

Is the white point meant to be a feature of these old CRTs ? Is there a way to do a dot supression system for these old sets ? Could anyone sched some light on this

[Danny]

Hi
Firstly we need to know the make and model of set .
Secondly you shouldn't adjust the ion trap to reduce the brightness of the picture, so you may have another fault.
Is the picture too bright when the set is working?
Danny

[radiogammon]

I think this is quite normal. I remember my old sets in the 60's always had a bright white dot for a few seconds in the center of the tube when turned off.

[delaitt]

There is no make for the TV! My uncle who was an electronic engineer made it for my dad in early 60s i think. It is a french TV who was functioning in 819 line and which was then converted later on to both 819 and 625.

It includes a PL36, 2 x PCF80, PCL82 (vertical osc), PL83, AW43-80. I've made the schematic. I've already changed all high voltage capacitors, some resistors in the horizontal and vertical oscillators which has changed values and also a number of decoupling capacitors in the vertical oscillator unit. I'm inputting a video signal to the PL83. The voltage on the cathode is 170v and the voltage on the wehnelt is at around 150v when the crt lights (after having changed the positioning of the ion trap) on and those voltage are stable. i know i should not change the positioning of the ion trap but the white point when the sets if off is really bright and i don't want to damage the crt. The TV is not too bright when the TV is on even when the ion trap in correct position. I don't recall this white point when i had switched it on few times at the beginning.

The voltage for G1 comes from the brightness potentiometer which is connected to the on/off button, so when the set if turned off, then G1 gets connected to the earth through a resistor.

If the white point is a feature on these 90 degree CRT, isn't it possible to create a dot suppression system to cut off the spot maybe by putting a negative voltage to the wehnelt ? Has anybody done this ? I'm quite worried that this strong white point will burn the screen ?

[Zelandeth]

I'd not worry about this. It's quite common (at least in my experience) for older sets to do this. The Kolster Brandes KV024 I have here has a visible dot in the centre of the screen which remains visible for quite a long while (if you turn the lights off over half a minute). The old colour sets (presumably those with a delta gun) used to show a little triad of coloured dots...which used to fascinate me no end!

Worth trying to just turn the brightness down - on my set that will actually have an effect on the dot.

[delaitt]

humm, the white point is really really bright. The spot cuts off for 2 seconds when the set is turned off and then the white point appears after this for 3 seconds. The brightness potentiometer has not effect when this happens. The brightness potentiometer is connected to G1. I have checked it including its resistors which connect to the earth and HT and they are all ok. I also tried to connect G1 to the earth when the tv is turned off but i've not seen any improvement.

Thierry.

[Steve_P]

We need the circuit diagram - or at least make and model, to see if there is any Spot Suppression and how it works. If there isn't any, then there's no fault!

If there is, it should be (ha ha!) pretty simple to sort.

Cheers,

Steve P

[FERNSEH]

By 1960 various initiatives were tried to discharge the EHT capacitor rapidly when the set was switched off.
The introduction of electrostatic CRTs and high EHT potentials made it necessary to solve the white spot problem.
The most common method was to connect the "earthy" side of the brightness control to the mains neutral side of the on-off switch. The theory behind this system was that the CRT control grid voltage will rise after the mains was disconnected, the CRT beam current will increase and the tube will be discharged rapidly.

I know that this rapid discharge system works well in my 23" 405 only Pye TV set.
If the mains plug is withdrawn to remove the power an intense white spot will remain on the screen for a long time. Switch the set off by the front control and the spot disappears very quickly.

I assume that your set has a conventional brightness control, that is, a potential divider between HT and chassis.

DFWB.

[Zelandeth]

Okay, let's try that again...was halfway through a reply, and my graphics drivers decided to fall over, and make my screen into a magenta and green mess.

I just remembered something from when I was first investigating this whole vintage TV thing. I recall in an article of the restoration of a very old pre war set (think it was an Ecko...but I wouldn't swear to it, as I can't remember for the life of me where I read it), one of the old moifications that was discovered on the set was a manual control to turn on and off the EHT section, independent of the rest of the chassis. Quite what the purpose of this would be, I'm not certain, nor how it would be wired up. Though one possiblity that springs to mind would be a simple switched supply to the heater in the EHT rectifier valve - don't quote me on this though, as there are far, far more knowledgeable people out there - I'm just thinking out loud. I guess the easiest way to get rid of your dot would be to shut off the power to the CRT heater. Could be a bit complicated though if that's supplied as part of the heater chain in the rest of the set though.

What my set does when it's shut down, is that the image actually shrinks rapidly down to the dot - which once it starts to fade due to the dropping EHT voltage, you can actually see is a mirror image of the CRT cathodes shape.

Fernseh - that's probably by my KB shows the dot for so long then - as I have to pull the plug to shut it off, thanks to a broken shaft on the power/volume control! No idea how it would behave if turned off normally.

[FERNSEH]

Hi Zel,
Your ITT-KB does indeed have the earthy end of the brightness control returned to mains neutral side of the off-on switch.

Here is a circuit diagram to clarify a few ideas that could be tried to discharge the EHT rapidly. A capacitor could be connected to the slider of the brightness control, this will maintain the CRT G1 (Wehnelt) voltage for a considerable time after switching off. The first anode voltage could also be maintained longer by adding an additional resistor and capacitor.

DFWB.

[FERNSEH]

Here's another idea. Insert a silicon diode and a resistor of say, 100kohms in series between the slider of the brightness control and the control grid of the CRT. The anode of the diode is connected to the brightness control slider. The 0.1mfd capacitor must remain connected between the CRT and chassis.

when the set set is switched off the voltage supply to brightness control will fall off rapidly, the diode will cease to conduct and a voltage will remain across the capactor and in turn maintaining a voltage at G1 of the CRT.

The CRT will draw sufficient current to discharge the EHT capcitor.

DFWB.

[delaitt]

thanks for the various responses! the diode solution with the capacitor looks interesting. If i was implementing this solution, do you know what is meant to happen when the set of turned off and G1 remains active to draw enough current from the EHT ? I'm asking as when the set of turned off, the cathode voltage will decrease to 0 and G1 will become positive. Does this mean that the screen will suddenly be very bright or it can't because there is not enough EHT to sustain this scenario or just for few ms ?

Here is a schematic of the relevant area of the tv set. It includes the schematic of how the crt is connected and the brightness control and on/off switch.

[delaitt]

would a 1N4148 be suitable as a silicon diode ?

[FERNSEH]

I tried out series diode circuit this evening, the chosen diode was a 1N4004. I don't think the 1N4148 will be up to the job as the PIV is only 50V.

At the first attempt the circuit did not operate at all, the spot remained visible after disconnecting the mains supply. The series resistor was 120Kohms and the capacitor was 0.1mfd.

After changing the values to 1Mohm and 0.33mfd the spot quickly disappeared after withdrawing the main plug.

The set used for the experiment was a 1960 Pye model V600A. This set employs an early American twin panel 23" CRT, type 23SP4.
As mentioned before the V600A employs the arrangement where one side of the brightness control is returned to the mains neutral side of the power switch.
Does your set use a series ballast resistor in the valve heater chain or has it a mains transformer?
The insertion of a voltage dependant resistor in the brightness control circuit was common in some 1960s TV sets. I notice that a contributor on the Radiofil forum has posted a circuit diagram which uses a VDR in the brightness control.
DFWB.

[delaitt]

After changing the values to 1Mohm and 0.33mfd the spot quickly disappeared after withdrawing the main plug.

I will try this.

The insertion of a voltage dependant resistor in the brightness control circuit was common in some 1960s TV sets. I notice that a contributor on the Radiofil forum has posted a circuit >diagram which uses a VDR in the brightness control.


yep. I've opened up that thread on radiofil and one member posted an article yesterday which discusses about spot suppression using VDR. The idea is similar which is to discharge the EHT in the tube quickly by drawing enough current. The diode solution might be easier as sourcing the right VDR maybe a bit more tricky (at least i don't have one readily available in my components store!).

Does your set use a series ballast resistor in the valve heater chain or has it a mains transformer?

It has its own transformer. There is a dedicated 6.3V for the CRT heater, IF unit (EF80 heaters) and valve sound amplifier. The heaters for the PL36, PY88, PCF80, PL82 are in series (still using the transformer). There is also another set of heaters put in series for the tuners, PL83 (still using transformer).

[delaitt]

I tried the diode. I don't have anymore white point when the set turns off but the brightness potentiometre does not work well anymore. I reduced the resistor to 10K otherwise the screen would not light-on. But even with a 10K resistor, the problem with the brightness potentiometre is that the screen goes black when the brightness control is increased too much and that the brightness level remains the same when the screen lights on if i decrease the brightness control.

The circuit used for brightness control is as follows:

+ HT ---- R (150K in // with 200K) ----- Pot 500K ----- 150K ---- ground.

[FERNSEH]

I don't think the additional diode and capacitor in the CRT grid circuit will work to well in your set. However, I notice the switch which is in the HT feed to the brightness control switches off the grid voltage to the CRT when the set is switched off. If the switch is connected to the chassis end of the brightness control, that is to the 150kohm resistor, I'm certain that the spot extinguish system will operate in a similar manner as the 1960s sets that were mentioned earlier in this discussion.


DFWB.

[delaitt]

Just reconfigured the switch to the chassis end instead of +HT and it works great!

thank you very much for your replies and expertise. I'm now working on the next problem with this TV set