A 5" Multi-Purpose Monitor

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Postby AncientBrit » Mon Apr 07, 2014 10:15 pm

Yes, that's a good point Steve about heater currents.

Regards,

Graham
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Postby Steve Anderson » Tue Apr 08, 2014 3:48 pm

I had considered using something like a 555 with the appropriate duty-cycle to drive a MOSFET such that the average power was the same as if the CRT had it's standard voltage applied. This raises efficiency and reduces the supply current. This obviously requires use of a DC supply which the 1" monitor had - a 12V/7Ah lead-acid battery.

My concern was when semis fail they can go open or short whereas resistors invariably go open or high...safer even if wasteful of power. Or should the 555 stop for some reason in the 'wrong' state.

Steve A.
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Fri Apr 11, 2014 3:08 pm

Back onto the matter of the CRT video grid drive...it's nearing the final version and it has been vastly simplified. If you need the 10MHz of bandwidth you're looking at six transistors on the high-voltage side, if 2MHz is adequate then only four are required.

But I'm having a problem trying to Eurpean-ise the semis, particuarly RF transistors in TO92 packages in both polarities (NPN-PNP).

Examples that seemed promising were BF199 (NPN), BF450 (PNP), checking suppliers websites showed little or no stock, discontinued and so on. The transistors I use here are Far-Eastern, from the 2SA/2SC series, but unobtainable in Europe. Doing a broad sweep of Farnell UK and RS UK didn't really help.

So gents, any suggestions? Ft of 500MHz+.

Steve A.

As a side-note, one of the biggest problems with this has been AM break-in. We have a 5kW AM transmitter on 675lHz jsut 2km to the south of us - it gets in everywhere. With audio/NBTV frequencies, no problem, you just clobber the response of the circuit. If only we could get Eddystone die-cast boxes here....
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Mon Apr 14, 2014 5:13 pm

This home-brew opto-isolator has been further simplified as shown below in the circuit diagram.
5ADP1v2 Defn & Grid Mod 3-Model.gif
5ADP1v2 Defn & Grid Mod 3-Model.gif (28.05 KiB) Viewed 16077 times

The input/low-voltage side is a straight-forward current driver for two very bright red LEDs. There is a bleed current of about 3mA through the LEDs at all times which also serves to ensure no semis are ever cut-off. (R107) More on this soon. The input voltage is 4.0V p-p provided by preceeding circuits, the plug-in boards mentioned previously.

The output/high-voltage side consists of a moderate speed photo-diode feeding a grounded-base stage, about 3V p-p appears across R121. This is boosted to a suitable voltage by TR122-3, as shown around 35V p-p.

R122 along with the bleed current through the LEDs ensures that at 0V input TR122-3 are just slightly conducting such that there's only a few standing volts appear across R126, again this is to ensure nothing gets cut-off. The items shown in blue colour will need to be adjusted for the particular CRT type as well as each individual specimen. Until I get the CRT run-up in it's final configuration and take some voltage measurements these could well change for myself. The optical arrangements and component types play a large part in the gain of this, this will have to be adjusted in each case.

Viewing the waveforms below and the rise/fall times gives a bandwidth af about 2.5MHz. (Rise-Time waveform was the same.) In this case I used the scope probe with its 15pF of input capacitance to simulate the CRT and strays in the final application.

I have changed the semis to European-friendly versions, TR101-3 will be changed to 2N3904s in the final version just to ensure they're OK, I don't anticipate any problems though. These are not 500MHz (Ft) devices, 300MHz, but they seem OK.

R107 serves to provide the bleed current through the LEDs, varying the voltage appied will shift the black-level at the gid, currently it's simply tied to -12V.

Photo shows rough-and-ready nature of this many times rebuilt prototype.

More to come for sure...

Steve A.
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Fall-Time 1.gif
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HB Opto 5.jpg
HB Opto 5.jpg (65.58 KiB) Viewed 16077 times
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Re: A 5" Multi-Purpose Monitor

Postby AncientBrit » Mon Apr 14, 2014 5:31 pm

That's very elegant Steve.

Keep up the development

Kind regards,

Graham
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Mon Apr 14, 2014 6:34 pm

Thanks Graham,

The changes/additions required for a 10MHz bandwidth version are shown in the beefed-up version of the output below. In addition the photo-diode would need replacing, 5-6MHz is about its limit - even that's better than its specification.

The circuit below isolates the CRT grid and stray capacitances from the collector circuit of TR123, TR124-5 provide the charge current for those capacitances. The additional current from the -ve supply is negligable.

But I wll add...I haven't tried this yet - not until I find a faster photo-diode.

Steve A.
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5ADP1v2 Defn & Grid Mod 4-Model.gif
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Fri Apr 18, 2014 4:38 pm

Well at this stage there's not much more I can do with the grid drive until I get the CRT fitted to the non-existant chassis - yes, I'm still waiting for the metal-bashers.

So onto the deflection amplifiers. My proposal is below, I'll build up the first one and see how it goes. The items in red are not set in stone and some may not be (the AOTs) until I can attach the active CRT. The BF459/259s will need small heatsinks as they'll be dissipating around 0.6W each.

More to come...

In fact the next major undertaking will be the power supply, currently running at six transformers! These will be placed in a separate box well away from the CRT - on the floor. It's also one way of reducing the footprint of this thing on the work-bench.

Some time back I was given a dead (very dead) Cossor oscilloscope. Its mains thansformer has eight (8) secondary widings. The EHT for the CRT was derived from a tube-based oscillator much like the overwinding on a LOPT in a TV. So I guess six non-custom transformers isn't that bad after all?

Steve A.
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5ADP1v2 Defn & Grid Mod 6-Model.gif
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Tue Apr 22, 2014 6:48 pm

A few hours of peace and quiet and the deflection amplifiers are starting to be assembled. Photo is at about 60% complete, checkout tomorrow...

Steve A.
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Mon Apr 28, 2014 5:58 pm

Well, all of this has come to a rapid halt. I'm now in the Philppines until - who knows when...end of year I suspect. But I'll be around...

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Re: A 5" Multi-Purpose Monitor

Postby gary » Mon Apr 28, 2014 10:08 pm

Well any Filipino's a Palomino... oh wait!

(that was for people who are familiar with F-Troop* - don't bring the PC police down on me).

* or Roy Rogers if you want to go back further...
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Tue Apr 29, 2014 5:26 pm

OT - yes, but...almost got dismissed from a contract in the UK a few years ago r.e. non-PC comments. Ridiculous. I was working for the BBC, one of the most PC companies there could ever be. I made a crack and the whole place broke up in laughter - but I was later taken aside and admonished...ho hum...

PC doesn't exist in Asia...I even watched a couple of episodes of "It Ain't Half Hot Mum" last night...one of the biggest earners in DVD sales for said organization...even though they cannot broadcast them any more in the UK...the demand is still there...

Steve A.
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Re: A 5" Multi-Purpose Monitor

Postby gary » Tue Apr 29, 2014 8:33 pm

Steve Anderson wrote:
I even watched a couple of episodes of "It Ain't Half Hot Mum" last night...one of the biggest earners in DVD sales for said organization...even though they cannot broadcast them any more in the UK...the demand is still there...

Steve A.


Are you serious???? That's ludicrous.

Well now, I must go on to YouTube and watch the lot in protest!
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Wed Jul 30, 2014 1:36 am

Steve Anderson wrote:Well at this stage there's not much more I can do with the grid drive until I get the CRT fitted to the non-existant chassis - yes, I'm still waiting for the metal-bashers. Steve A.


And you know something...I'm still waiting. Never again. Bollocks. I'll pay the full price and get it delivered when I want it. Three months...you're having a laugh...

Steve A.
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Re: A 5" Multi-Purpose Monitor

Postby Klaas Robers » Sat Aug 23, 2014 9:13 pm

Steve, I am surprised about the circuit diagram of your deflection amplifiers. It is almost identical to mine, although I used a type of transistors that are intrinsic high voltage and high power. First the circuit diagram:
Image
Yes, vacuum tubes. High voltages are needed any way, the EF80 was the tube for TV mf-amplification. It was the successor of the famous EF50. Enormeous amounts of them are lying around, so it is easy to select pairs of them that have almost the same characteristics.
And then: is this bulky? Here you see the board with one of the amplifiers:
Image
Of this "module" two are made, one for X, one for Y. So, a hybrid circuit. Use the components for what they are good in.
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Sun Aug 24, 2014 12:25 pm

Klaas,

As ever, very neat and tidy. I have used pentodes in the past for deflection amplifiers but many times I found they had too much gain for my applications. So I generally have strapped them as triodes. For use in NBTV and other low frequency applications the Miller effect does not pose any great problem. A selction of what I have used before appears below. (I can't be bothered working out how to get the images/drawings in-line).

As for EF80s. I have a number of NOS tubes of this type, eight or so I think. These too I would probably end up using as triodes. To that end a chap called Tom Schlangen has spent some time producing a set of plate curves for a triode-mode EF80. It looks quite linear with a useful mu of around 40. pdf attached.

In general I'm still thinking about this display when I have some spare time, but other things are more pressing at the moment.

Steve A.
Attachments
EF91 Deflection 1.gif
EF91 Deflection 1.gif (9.2 KiB) Viewed 15616 times
EF91 Deflection 1.jpg
EF91 Deflection 1.jpg (76.1 KiB) Viewed 15616 times
ECC91 Deflection 1.gif
ECC91 Deflection 1.gif (11.03 KiB) Viewed 15616 times
ECC91 Deflection 1.jpg
ECC91 Deflection 1.jpg (42.86 KiB) Viewed 15616 times
9002 Deflection 1.gif
9002 Deflection 1.gif (8.05 KiB) Viewed 15616 times
9002 Deflection 1.jpg
9002 Deflection 1.jpg (45.16 KiB) Viewed 15616 times
EF80 Triode.pdf
(15.11 KiB) Downloaded 795 times
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