A 5" Multi-Purpose Monitor

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

A 5" Multi-Purpose Monitor

Postby Steve Anderson » Sun Mar 23, 2014 7:33 pm

This is a break-off thread from what started in 'Video to NBTV converter' within the 'NBTV' section of this forum.

This is a 5" CRT monitor which with use of plug-in boards it is able to display a number of TV formats. Of prime concern here will be NBTV, but also SSTV (with and without frame-store), 120, 180 and Baird's 240-line standards will also be possible.

Of the NBTV projects planned are the standard 32/12.5 mode, a 32/50Hz mode (as per Graham Lewis's 'Frame Rate Convertor) and line doubling/interpolation. Others may appear.

The biggest problem is to modulate the CRT grid-cathode circuit with signals that run from DC up to some 2MHz whilst the grid-cathode circuit sits at around -1400V. Opto-isolators at first seemed the answer, but all those I am able to obtain here are too slow, even using some tricks.

So what do you do when you can't buy something? DIY...so here's my home-brew opto-isolator.

It uses a standard but fast red LED and a BPW34 as the optical elements, a few high-speed transistors and careful attention to reducing stray capacitance. To do this a lot of the circuit was 'air-flown'.

Below is a few photos of the prototype (messy but it works) and a couple of 'scope screen-scrapes showing the bandwidth. Judging from the PAL burst I would but the -3db point at around 4MHz. The old '0.35' Rule Of Thumb supports this, 0.35/71ns = 4.9MHz - a bit optomistic perhaps.

Pre-emphasis may improve this, but at this stage it's good enough.

Steve A.
Attachments
HB Opto 1.jpg
HB Opto 1.jpg (49.48 KiB) Viewed 19538 times
HB Opto 2.jpg
HB Opto 2.jpg (57.4 KiB) Viewed 19538 times
HB Opto 3.jpg
HB Opto 3.jpg (67.59 KiB) Viewed 19538 times
Square Wave 1.gif
Square Wave 1.gif (5.84 KiB) Viewed 19538 times
PAL Video 1.gif
PAL Video 1.gif (7.51 KiB) Viewed 19538 times
Last edited by Steve Anderson on Sun Mar 23, 2014 8:30 pm, edited 2 times in total.
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Postby Harry Dalek » Sun Mar 23, 2014 7:47 pm

I like your way of thinking and do it your self circuit on the optocoupler idea !

I am also happy to see old slow scan tv has not been over looked only one problem you will have nothing left to make when you get this monitor finished . :wink:
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Postby Steve Anderson » Mon Mar 24, 2014 2:07 pm

Had a little fiddle-around with this home-brew opto-isolator this morning in an attempt to increase the bandwidth a bit.

First was to reduce the collector load resistance in the grounded-base stage by a factor of two. To restore the output voltage two LEDs where to illuminate the photo-diode.

The reduction of the collector load resistor shaved off 10ns in the logic switching application and the two LEDs, now in series halved their effective junction capacitance.

It still could be better but it is now 'good enough', switching speed is around 55ns. As a result the PAL sub-carrier is slightly improved. 55ns isn't too bad when the datasheet for the LED gives a rise-time of 20ns and 100ns for the photo-diode - however, that's into a resistive load.

Without finding a faster photo-diode I doubt it's worth spending any more time on this, the gains would be so marginal.

Results below...

The next hard part is boosting this signal to some 50V p-p to drive the CRT grid whilst still preserving the bandwidth...

Steve A.
Attachments
Square Wave 2.gif
Square Wave 2.gif (6.39 KiB) Viewed 19523 times
PAL Video  2.gif
PAL Video 2.gif (7.33 KiB) Viewed 19523 times
HB Opto 4.jpg
HB Opto 4.jpg (53.06 KiB) Viewed 19523 times
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Postby AncientBrit » Mon Mar 24, 2014 6:39 pm

Very neat Steve.

Any chance of a circuit or is it still WIP?

Regards,

Graham
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Postby Steve Anderson » Mon Mar 24, 2014 8:05 pm

Graham, it is still WIP and I like to get the whole device completed before I publish. Although this is but a small snippet who knows what may happen when I glue on the output stage. Instability or other such horrors? I will publish, but I prefer to ensure that it's reproducible by others first...not that anyone will probably want to copy this!

I hope that the sub-circuits or chunks of it are at least are useful to others, even if just a source of ideas.

Steve A.
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Postby AncientBrit » Mon Mar 24, 2014 11:44 pm

Understood Steve.

I'm particularly interested in the Z mod coupling to the CRT.
I await further progress (no pressure there!).

Kind regards,

Graham

ps If WW still existed that's the type of cct. that would have appeared in their Circuit snippets.
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Postby Panrock » Wed Mar 26, 2014 6:03 am

I too am watching this with great interest.

Steve Anderson wrote:The next hard part is boosting this signal to some 50V p-p to drive the CRT grid whilst still preserving the bandwidth...

Hard? Well I can't think of anyone better up to the task than you! :lol:

Steve O

PS. One of these days (or years!) I would like to try doing something similar - in a more limited way - with my Argus.
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Postby AncientBrit » Wed Mar 26, 2014 8:00 pm

Steve,

I came across this opto device, HCPL-4562, 9MHz.
Might be of interest although you've 'rolled your own' now..
There's access to the detector base which is helpful if using feedback.

Cheers,

Graham
Attachments
HCPL-4562 9Mhz opto data sheet.pdf
HCPL-4562 9MHz data sheet
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Postby Steve Anderson » Thu Mar 27, 2014 12:27 am

Thanks for the vote of confidence Steve. It is as expected proving tricky. If i were content with a bandwidth of 2MHz I would already be done. But I do want that 5MHz+ for use in the future and to save myself the same sort of exercise again.

Graham, I was aware of the HCPL4562 and have that very same datasheet., it's the same old problem, getting hold of them here. Farnell have them but their shipping charges are obscene. Three quid (approx.) per device - 15 quid for shipping two items that together weigh less than a gram. Forget it!

Thanks all...

Steve A.
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Postby Steve Anderson » Tue Apr 01, 2014 7:03 pm

For the first time in probably 60 years the heater of this first 5ADP1 was energized. I need to try the other one as well as the two 5ADP7s I have...Harry - SSTV stuff in the offing...

Things are generally moving on but I do keep getting interrupted by 'real work'.

Steve A.

I'll try and take another photo once the sun has gone down...
Attachments
It Glows 1.jpg
It Glows 1.jpg (59.54 KiB) Viewed 19465 times
5ADP1 1.jpg
5ADP1 1.jpg (25.13 KiB) Viewed 19465 times
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Postby Steve Anderson » Tue Apr 01, 2014 8:34 pm

All four of these 5ADPx's I have I bought unopened from the factory. The last of which I haven't opened is attached. If anyone can shed some light on a date that would be useful info to know....or any other data too..

Steve A.
Attachments
Unopened 1.jpg
Unopened 1.jpg (56.63 KiB) Viewed 19460 times
Unopened 2.jpg
Unopened 2.jpg (52.15 KiB) Viewed 19460 times
Unopened 3.jpg
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Postby Steve Anderson » Tue Apr 01, 2014 9:35 pm

As much as I like semi-conductors, there is nothing that can compare with this...

Steve A.
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Glow Babt Glow 1.jpg
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Beam Structure 1.jpg
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Postby Steve Anderson » Tue Apr 01, 2014 10:30 pm

OK, one last one...

Steve A.
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Postby Harry Dalek » Wed Apr 02, 2014 10:14 pm

Steve Anderson wrote:For the first time in probably 60 years the heater of this first 5ADP1 was energized. I need to try the other one as well as the two 5ADP7s I have...Harry - SSTV stuff in the offing...

Things are generally moving on but I do keep getting interrupted by 'real work'.

Steve A.

I'll try and take another photo once the sun has gone down...


Yes thats a fine looking crt but a p7s after glow brings happiness to my heart :D
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Postby AncientBrit » Wed Apr 02, 2014 10:15 pm

Steve,

How's the tube getter, still a good vacuum?

Graham
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