Playing with crt's

[dr pepper]

I've wanted to connect a pic to a crt for a while, to draw silly shapes, just for fun.
Finally got round to trying something, I connected a flyback & driver circuit to a crt using the eht cap and the 8 pin connector, put 6.3v ac to the heater and powered the flyback driver.
I expected connecting a 10k resistor between a colour cathode and ground would produce that colour dot in the centre of the screen, being able to then focus it using the controls on the flyback.
Doesnt quite work that way, as soon as I connect the 10k to a colour cathode I get a white dot, all 3 guns on, and its tricky to focus.
I managed to burn the screen, but thats ok.
What kind of drive do the cathodes need?, presumably near ground for fully on and maybe 150v for dark, so I need another ht supply.
I'm going to borrow the xray machine from work and see how much this tube generates, blowing a hole in the screen must have taken plenty electrons.

 

[Nigel Goodwin]

It shouldn't produce any x-rays (or at least well below permitted levels) - it's designed not to - however, if you increase the EHT too far TV tubes do produce x-rays, and TV's generally have circuits to prevent that happening (and on old Japanese service manuals it was even labelled 'xray protection').

It's not 'blown a hole' in the front of the tube, all it's done is worn the phosphor out - bear in mind you've got all the energy for the entirely screen area in one spot - it VERY rapidly destroys the phosphor. Even a single line (across or down) rapidly permanently damages the phosphor - bearing in mind (for a horizontal line) you have about 600 times the energy that it normally receives.

Cathodes are high for dark, low for bright - it's usual to use a simple cascode circuit to drive it, usually with a feedback resistor from the top collector to bottom base, and an input resistor feeding the bottom base (VERY crude opamp type thing).

 

[tvtech]

^^^^Pro advise

What more can I say.

LOL Nigel, I am making you scratch your head with the whole CRT thing...so nice though that you have not forgotten

Happy tvtech

 

[dr pepper]

I did some checks before pulling the tube, but I didnt take a voltage measurement for the cathodes, I have a 120v bench supply so I'll see if that can vary a colour from off to on, i think why they are all comming on at onceis that I'm using the pcb that plugs onto the tube, still with all the comps on, they are probably causing all the cathodes to fire.
I dont want video inpit, just a on off control, so the pic can select a colour.
There might be a tapping on the flyback suitable to power the cathode drive voltage.
Yes I burned up the phosphor at the centre, if I'd blown a hole it would have eventually burn through the glass!, for testing I'll keep the focus wide to lessen burn.
I could put together a circuit that switches bewteen 2 voltages for the cathodes, and have a trimmer for bright and dark settings.

 

[Nigel Goodwin]

The brightness is set by the difference between cathodes and grid - but in 'recent' TV's the grid is normally kept around chassis, and the cathodes high - in a colour TV you have to feed the cathodes, as you only have a single grid.

Back in the B&W days some sets fed the cathode, some fed the grid - but with colour you don't have the option.

It would make sense to use the existing drivers, which are normally on the CRT base PCB.

 

[dr pepper]

Yes I get that, I think, raise the voltage of all 3 cathodes to reduce brightness, pretty much as the video signal would.

I also just realised why focus and accel voltage is tricky to adjust, its not connected, I made an incorrect assumption the eht ground is the same pin as focus and accel, but doing a little research shows its not, ground on the aux windings is focus/screen ground, eht ground side is a different pin.
I guess the focus ground pin isnt meterable as it probably has silly value resistors, I'll see if I can figure out the drive winding and heater winding by looking at the main board, that just leaves the aux winding, I might be lucky and eht/focus grounds will be linked togther on the board.

I'll trace out the pcb on the base.

 

[dr pepper]

OK, so I traced out some of the pins on the flyback, I now have the crt heater powered by the flyback.
In search of a suitable 200v or so power supply for the cathodes I found that the drive voltage input winding used as an o/p (I'm driving the flyback with my own wound on primary so the original primary I was thinking could be used as a ht o/p) is up over 400v, even though the heater winding is 6.5v, I expected a primary voltage of 150v or so

 

[dr pepper]

Ok then.
I tried to use the drive circuitry on the crt connector pcb, there is a 3 pin and a 5 pin connector, the 3 pin I sussed was gnd, heater ac and ht, for the ht I tried 150v aquired from one of the flybacks o/p's.
And the 5 pin I sussed as r,g,b and 2 supplys, looks like 1 in and 1 out, its less that 20v as its divided by 2 and applied to a follower then a 16v rated cap, then its used as a reference for the colour amps on board, so I applied +20v to this, 20v is marked on the main board so its a guesstimate.
Couldnt get anything to work though.
So I isolated all 3 rgb cathodes, putting a 100k pot to ground from each cathode mixes in an amount of colour like I wanted, should I apply a voltage to each cathode, or just a resistance to ground?
Convergence isnt great but it'll do for what I want to do.

 

[Nigel Goodwin]

Ok then.
I tried to use the drive circuitry on the crt connector pcb, there is a 3 pin and a 5 pin connector, the 3 pin I sussed was gnd, heater ac and ht, for the ht I tried 150v aquired from one of the flybacks o/p's.

The HT for the CRT base is normally about 200V, with the cathode running at about 150V.

And the 5 pin I sussed as r,g,b and 2 supplys, looks like 1 in and 1 out, its less that 20v as its divided by 2 and applied to a follower then a 16v rated cap, then its used as a reference for the colour amps on board, so I applied +20v to this, 20v is marked on the main board so its a guesstimate.

Anything remotely 'modern' will have a feedback pin for setting the auto-greyscale

Couldnt get anything to work though.
So I isolated all 3 rgb cathodes, putting a 100k pot to ground from each cathode mixes in an amount of colour like I wanted, should I apply a voltage to each cathode, or just a resistance to ground?

You're not used to valves are you (which is what a CRT is)

Putting a resistor in the cathode is called 'self-biassing', the current through the resistor causes the cathode to be positive with respect top the grid.

 

[tvtech]

A clean Workshop today. For the first time in Seven Years......absolutely everything here is under control and accounted for...

And I mean EVERY single job.

Waiting spares....1 set. Spare ordered today.

And that is it. Nothing else to do. We had a beer this afternoon @ around 4.30 . And celebrated. Reminds me of my days with a Service Company here in South Africa when we were all awesome.....Boss phones: "What are you Guys doing?". Answer "We are having a beer". Silence from him. Than he would chirp..."but it's too early"......our chirp " check tomorrow first thing.....we are going to party now". Shop is clean.

And so it was. All technician work up to date..no moaning Customers....happy times for all

Special day today for me....hard slog but worth it again.

Happy memories of good times

For customers, service is EVERYTHING....

Regards,
tvtech

 

[tvtech]

I may as well carry on here....

Instead of starting a new thread. Anyway, anyone who has ever been a freelance Tech will know that you are called to sort dwang out...like things that permanent hired tech's are unable to do.......stuff piles up behind them.....they leave the piled up stuff...for you.

And then they call you and give you a pile of poo to fix.....and then laugh at you while you battle with the **** they give you...

LOL....I have seen it all....I know the tricks....seen the lot...

Being doing this since 1998. And it just gets better. My legs and stuff are not the same anymore....balance a bit stuffed. But, hey my memory is fine.

And the Workshop I run here is the best I ever had....tidy as a pin...I always speak about 7 Years...thing is I pulled it right about 6 Years ago...

The problem was the Guy that got himself fired......for fighting.....and wasting.

Now, before I say anymore: I am soooo serious about wastage. Easiest way to kill a Company is to waste....

Like.......I am maybe like a Hitler when it comes to that...if a part is stuffed...throw it away. It must never enter the system again..........
On the other hand....(this only works with competent Techs......are you sure...yes)....not sure...and there are five other new ones lying there. And the Guy says they all don't work

Same fault, changed the IC 5 times over....and then my Blood Pressure goes ballistic.

Logic is a Mother....if you cannot understand it, you are lost.

Non Political tvtech

Sigh...I am happy. Why such a battle to get people to THINK though???? FFS, logic is not hard...or is it ???

Sorry for the rant Guys. I feel better now. Out of my system.

I really don't know why you all tolerate me. For me a place where I can shout and scream and chirp...and post my frustrations.
I really do not know what to say. Maybe I have some friends here. Maybe I speak about stuff that others want to but don't.
All I know is that I unconditionally love ETO and all Members here.

And I am not finished yet....

Now I am....messed beer over myself by mistake LOL

Time to sleep

 

[tvtech]

Good night all

 

[dr pepper]

I'm kinda familiar with valves, I have over a dozen valve radios, but not picture tubes I only have a couple of tube telly's.
Makes sense the cathode being biased, (cathode biasing is used in most radios, 'cept fot those really odd designs that use grid leak resistors), pulling it down externally unbiases it and it spits off electrons and creates the dot on the phosphor.
A 100k to +200 and a mpsa42 switching to ground via a 10k so far seems to do the job well.
I was wondering if that other pin was a reference feedback, it only takes a few v to vary the brightness of each colour, thats why the cap is only a 16v one.
While the mrs wasnt looking I wizzed the back off the telly in the kitchen, didnt need to measure the supply to the connector board, its marked 200v.
Thanks Nige.
My 'shop is allways in a state of nearly tidy.

 

[dr pepper]

OK its sort of sinking in.
Going back to college I remember heater,cathode, g1,g2,g3,anode on a crt.
g1 brightness control, g2 screen (acceleration), g3 electrostatic focus.
Seems that on more modern sets G1 has been dropped, or grounded (as you implied), and g2 and g3 still provide the same function.
Brightness presumably is done by shifting the bias on the rgb lines.
I took apart the wifes tv, it has heater, 200vht, rgb, 12v and feedback marked on the neck board, scoping these I found that the 12v connection had nothing on it, disconnecting it made no change, there was plenty of signal on the feedback pin, the rgb signals to the cathodes were around 120v with 80v or so drive to them with horizontal blanks, still not sure what the 12v does.
So a variation of my first idea of something to drive this would be a potential divider from 200v, with a transistor/resistor in series across the lower leg, and instead of connecting them to ground, connect them to a variable dc voltage which would suffice as a brightness control.

 

[Nigel Goodwin]

OK its sort of sinking in.
Going back to college I remember heater,cathode, g1,g2,g3,anode on a crt.
g1 brightness control, g2 screen (acceleration), g3 electrostatic focus.
Seems that on more modern sets G1 has been dropped, or grounded (as you implied), and g2 and g3 still provide the same function.

Because it's colour, and you only have one grid, but three cathodes - if all you're wanting to do is vary the brightness, then you could vary the grid if you wanted.

Brightness presumably is done by shifting the bias on the rgb lines.

Yes, because they are the only separate colour connections, so all settings are done via those.

I took apart the wifes tv, it has heater, 200vht, rgb, 12v and feedback marked on the neck board, scoping these I found that the 12v connection had nothing on it, disconnecting it made no change, there was plenty of signal on the feedback pin, the rgb signals to the cathodes were around 120v with 80v or so drive to them with horizontal blanks, still not sure what the 12v does.
So a variation of my first idea of something to drive this would be a potential divider from 200v, with a transistor/resistor in series across the lower leg, and instead of connecting them to ground, connect them to a variable dc voltage which would suffice as a brightness control.

The 12V would normally be feeding electronics (usually an IC) on the CRT base - yours might not have it.

 

[tvtech]

^^^Crikey...you are fit...have an amazing memory....kick ass daughter....lovely places to visit etc...

I am missing something as far as TV repairs go.....but wait.........Africa here...
Nothing much to see here where I am....far away from Cape Town and all the beauty..

Deepest, thickest, ones are dealt with here. Money is good but there is nowhere to go. Beauty lies far, far away in Cape Town.
I suppose I should save for a Week...and then party hard there...

And I could also take some nice photo's of interesting things....

Thinking here at last....

Regards Nigel,
tvtech

 

[dr pepper]

Thanks, kinda makes sense now.
There are 3 ic's on the neck board, but it still works fine with the 12v disconnected, maybe they are powered from the ht and the neck board is a revision that doesnt need 12v, I didnt trace out the main board, might not be connected.
I'll have one last go with the original neck board as is, and then I'll look into my other ideas, I only want to vary brightness with a manual pot.

 

[tvtech]

Sorry for taking things Off Topic dr pepper

I never mean it..it just happens when I am happy

Regards,
tvteh

 

[dr pepper]

No probs, I'm used to holding a conversation with the mrs that contains several subjects at the same time.

 

[Nigel Goodwin]

^^^Crikey...you are fit...have an amazing memory....kick ass daughter....lovely places to visit etc...

There's many seriously beautiful places in Africa, and I'm sure pretty close to you