A 5" Multi-Purpose Monitor

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

Postby Steve Anderson » Thu Apr 03, 2014 12:17 am

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

Yep, all silvery/black - if there be such a colour, not a hint of white anywhere.

I'll know in short order when the corrected metalwork arrives.

In the meantime the video opto-coupler appears OK. The slowest part is the photo-diode, otherwise it's basically flat to 10MHz...that's currently the highest sine-wave I can produce here. The photo-diode gives up at around 6MHz at -3db-ish.

Harry, I need to find a source of orange cloured perspex for when I use the P7 variants. It'll bring back memories of when I worked at Decca Radar in the 70's...no frame stores then! Except that the CRTs used didn't have the blue flash associated with a P7 tube...no idea what phosphor they used...it was still orange though.

The attached photo shows an example that is due for a CRT change - note the burn in the centre - it takes a few years to get to this stage...

Steve A.
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Postby AncientBrit » Thu Apr 03, 2014 5:45 pm

Steve,

If you can't get coloured perspex you could always make a 'sandwich' of clear perspex and some coloured lighting 'gel' used in stage/tv lighting.

Cheers,

Graham
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Postby Steve Anderson » Thu Apr 03, 2014 6:18 pm

AncientBrit wrote:If you can't get coloured perspex you could always make a 'sandwich' of clear perspex and some coloured lighting 'gel' used in stage/tv lighting.

Yep, an idea, I'll ask management if she can get an off-cut from the lighting department - she works as a group studio manager for one of the local TV chains here.. I only need something around 15cm (6") square.

Though I do wonder about internal reflections - a case of suck it and see...

Steve A.
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Postby AncientBrit » Thu Apr 03, 2014 6:29 pm

Steve,

Maybe also use a circular polarising filter (as opposed to a straight polariser)

I've used them in the past, they work well.
The reflection will be an opposite handed rotation and therefore get blocked.

Cheers,

Graham
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Postby Harry Dalek » Thu Apr 03, 2014 9:50 pm

That one must have a fair few valves in that beast Steve.
Where did that come from and having a P7 CRT must be some sort of radar.
Bet the wife wondered why you wanted that one any case i see the beauty init :wink:
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Postby Steve Anderson » Fri Apr 04, 2014 4:40 pm

Graham, I will certainly ask the question r.e. circular polarization but I suspect I'll be greeted with blank stares...but you never know. I know Lee Filters in the UK, but whether the locals use that sort of quality supplier is unknown.

Harry, a radar display unit it is, but not the ones I worked on, I suspect that it was a precursor to the versions I came into contact with. I wasn't able to find any data regarding the later ones from the 70s. I would guess that unit is from the late 60s, perhaps early 70s.

When I joined Decca the tubes (apart from the CRT) where virtually gone. A marine radar system usually has three major components, the display/control unit (as per above), the transceiver and the rotating antenna array. In the smaller (3-6kW units) the transceiver was often part of the antenna assembly - that made servicing a real headache as you had to climb the mast to fix anything - usually just replacing the mixer diodes - usually zapped by another vessel berthing near yours.

The units I worked on used the original 7400 series logic chips which guzzled power - but nothing compared to the magnetron used to deliver the RF pulse. Very closely related to the one in your microwave.

In 1975 all there was left as far as devices with cathodes were concerned was the CRT in the display unit, the 10mW Klystron used as a local oscillator for the receiver, the Magnetron transmitter and a wave-guide T/R cell that prevented the Magnetron from burning out the aforementioned mixer diodes...supposedly.

These were replaced eventually by Gunn-diode oscillators and more robust mixer diodes.

In the long-range C-band versions, it's not easy keeping 250kW of peak power from zapping mixer diodes. The average power was much less of course but enough to fry a sea-gull at a few metres.

Steve A.

There is a story r.e. the ASMI (Aircraft Surface Movement Indicator) equipment at Heathrow (also by Decca)...but that's even more OT than this has been so far...
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Postby AncientBrit » Fri Apr 04, 2014 6:15 pm

Hi Steve,

re the circular polarising filter I gather they are made as a screw in filter for Digital SLR cameras so you might investigate that source.

Not sure whether the diameter will suit your tube though.

Apparently a linear polariser causes problems with the auto focus.

Regards,

Graham
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Postby Steve Anderson » Fri Apr 04, 2014 8:31 pm

Auto-focus....don't even mention it! The Olympus camera I bought some years ago cannot. It's useless. 1000 quid of garbage. Impulse buy - error - the Canon I had stolen only had 60% of the pixels but the resolution. colour rendition and sheer quality was/is far superior.

It's a pile of shite...

Sorry guys, but how can someone make and honestly sell such a pile of crap?

My Samsung G5 phone takes better pictures....most of the recent ones I have posted here are taken with the phone...

Steve A.
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Postby AncientBrit » Fri Apr 04, 2014 9:29 pm

Steve,

Thinking about the size of your CRT I don't think a standard camera filter will cover the area.

I'll research and see if they still do it in sheet form,

Graham

try
http://www.polarizingsheet.com/circularpolarizingsheet.php
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Postby Steve Anderson » Fri Apr 04, 2014 10:11 pm

Graham. don't spend time on this, the P7 thing is a whim, I'll more than likely use a P1 with a frame store. I'll certainly do a P7 version, but it's just a tube-change - if nothing else to show how it was done many years ago..

Steve A.

The grid modulator looks more like the insides of a chip now, not a cap in sight, just a hell of a load of semis..13 and counting...chuck in a few diodes too...
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Postby Klaas Robers » Mon Apr 07, 2014 6:36 am

Steve, please continue posting on the driver of the CRT. I am making something comparable, with a 5BP1 that I have laying around for tens of years. I now first should make the -1750V supply, then I can fire the tube up to see the orientation of the deflection plates.

I made already balanced drivers for the deflection, with each a pair of EF80s. It is a simple long tailed pair, running on the 350V of an outboard general purpose tube power supply that I built some 40 years ago.

For the CRT filament I will be using a modern potted PCB transformer of 9V sec. with a series resistor in the primary. I hope that this will reduce the stray field of the transformer, so I can keep it on board.

I make this to show youngsters the principles behind TV. The tube can also display the wave form of the video signal and it demonstrates more or less the function and working of a CRT.

Next to that I will demonstrate the Nipkow disc as well of course. If I tell them that what they see is never seen by 99.99% of the population they think themselves much more special. And that is important.

There is a famous Dutch prize: "The silver Nipkow Disc", issued every year for the best Dutch TV program or series. Every body talks about it and nobody knows what it is or ever has seen it. And now these youngsters can say: "Oh yes, I know, I have seen it working and I understand how it functions....." And they have been told that no of all adults can repeat that.
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Postby Steve Anderson » Mon Apr 07, 2014 2:22 pm

Klaas, what I started out with is shown below, this was simply to check the bandwidth of the LED-Photo-Diode arrangement. In my case I was looking for more than 5MHz. The DC/LF part is easy so it was of no concern in this test, hence the input is AC-coupled. The scope traces above were obtained using this arrangement.

As noted above if a 5" (13cm) CRT is to be used and with their spot-size being around 0.03" (0.8mm) it's unlikely to be able to resolve any more than 150-200 pixels across its entire width. Even less on a 4:3 raster. I'm assuming you'll be using standard 625 video. This approximates to 2MHz of bandwidth required.

As noted by Graham above there are opto-couplers specifically designed for video signals, the HCPL-4562 for example - it's just I can't get them here. Even something like a 6N36 could well do the job when used in a similar manner for this bandwidth.

The circuit below had a gain of about 1.5 but this varies widely depending on the optical arrangement, one or two LEDs, distance between them and the photo-diode.. This of course needs to be followed by a suitable amplifier to present anything from 10-70V p-p of video to the CRT grid - this has been where I have been spending most of my time on this.

Starting with a simple grouded-base output stage with a collector load of 4k7 and a CRT/socket/strays/collector capacitance combined at an estimated 15pF yields a -3db point of approximately 2.3MHz - OK, just good enough. You could put a peaking inductor in series with the 4k7 which will help a bit. For much lower bandwidths (NBTV) this is of no consequence.

The contrast (gain) and bias-point (black level/brilliance) controls are on the low-voltage side of the coupler hence the requirement for DC-coupling in addition to good LF response (for NBTV and SSTV).

The transistors in the circuit are Fairchild devices, depending on the application any device found in TV IFs should do the trick.

The version I'm working on is way over-the-top even for 625 signals, but I will publish it in due course when I'm happy with it. As usual time has been short.

Steve A.
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Postby Klaas Robers » Mon Apr 07, 2014 5:43 pm

Dear Steve,

thanks for the total circuit diagram. This is a good beginning. So I need a 12 volts supply voltage at the -1750 volts level, that can be done starting from the filament voltage. And then the amplifier for the grid..... It needs also a supply voltage. I will see. I want to have the total chain DC coupled.

My plan is to make a short endless video clip in EPROM on 128 x 128 pixels, the definition of SSTV but displayable in switchable frame speeds until 50 Hz progressive scan. So a bandwidth of 1 MHz is enough.

May be I will make a circuit diagram of the deflection amplifier with 2 x EF80 for you and post it here.
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Postby AncientBrit » Mon Apr 07, 2014 5:51 pm

Morning gentlemen,

Klaas,

An alternative to a mains powered transformer for the CRT filament might be a small switched mode DC power supply.

These are available 'off the shelf' for a selection of dc outputs and a series resistor could be used to trim to the CRT heater used.

This should reduce stray magnetic fields but I'm not sure if the insulation on the secondary winding would be able to withstand a potential of 1kV, edit 1.75kV

Steve, thanks for the 'Work in Progress' opto circuit.

Kind regards,

Graham
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Postby Steve Anderson » Mon Apr 07, 2014 9:27 pm

As ever, CRTs often need a multitude of supplies. Not that much one can do to get around it.

A 1MHz bandwidth should be quite easy to obtain, simply avoid common-emitter stages unless they directly drive the emitter of a grounded-base stage. Don't let anything saturate or get cut-off. Though this should be less of a problem at 1MHz.

A note of caution r.e. heater droppers. If I'm thinking of doing this I always measure the heater current - especially for hard to get CRTs and even standard tubes.

The little 1" CRT (a 1CP1 variant) I used a few years back actually drew 300mA at 6.3V whereas every datashett said 600mA. If I had gone ahead and put in the dropper values for 600mA and applied 12V - goodbye CRT. Why I checked the current I do not know, but I'm glad I did.

Steve A.

Yes, I have checked the heater current for all of the 5ADP1s and 5ADP7s...it's as per datasheet.
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