CRT General Discussion.

[Panrock]

In some other experiments I've done with neons, using them as the timebase in an NBTV experiment before I'd found this forum:

Odd how fascinating this is - but it it is, isn't it?

Reminds me of my own very first ultra-crude televisor which I made at school in 1966. I used a whole load of tiny 90v neons in series to get sufficient deflection for my 3-inch VCR139A tube and the EHT was only 400 volts.

Steve O

[dominicbeesley]

Reminds me of my own very first ultra-crude televisor which I made at school in 1966. I used a whole load of tiny 90v neons in series to get sufficient deflection for my 3-inch VCR139A tube and the EHT was only 400 volts.

Steve O

Now hows that for a coincidence guess what I've been doing all day today - winding a mini-lopt to fire up a VCR139A! BTW: How small a spot size can I expect on one of these - best focus I can get is about 2mm with an EHT of 600V....Just finished winding anothe LOPT with 3x more turns....wait for it fizz bang swear!

And yes it really is fascinating and fun!

Cheers

Dom

[Steve Anderson]

- best focus I can get is about 2mm with an EHT of 600V....Just finished winding anothe LOPT with 3x more turns....wait for it fizz bang swear! Cheers. Dom

Even with these older tubes you should be able to get a spot size of less than 1mm, probably around 0.6mm. But it does require the correct voltages, 600V does seem a little low, 800V is probably nearer the mark. I've been doing some 'googling' for this tube and come up with virtually zero including the equivalents, CV1588, E4205 and the ACR10.

The other thing to check is that the average voltage of each pair of deflection plates is the same as the final anode, there is no PDA in this tube.

Not wishing bad luck on you, the focus problem could be the tube going slightly 'soft', but I would try the HT first. Also I suggest you check for the amount of ripple on the HT, this can affect focus too.

Steve A.

I did find a small amount of data here:-

http://www.r-type.org/static/webairf.htm

Also here:-

http://www.r-type.org/exhib/aaa0981.htm

Another equivalent is the ECR30, pdf attached...

[dominicbeesley]

Thanks for all that Steve,

After posting last night I wound a new LOPT (well EHT tranny) the tripler now outputs between 100 and 1500V variable and still I can't get the spot size down, even with all four deflection plates strapped to the EHT. The one thing I've not experimented with yet is the a3 voltage....

Possibly the tube is a bit soft, when it was first switch on the defocused spot was actually a ring that gradually "burnt in", its hard to describe but it looked a bit like when film gets stuck in a projector and burns through. Though the two getter spots like nice and healthy.

I tried all sorts with the neons - including a limiting resistor to reduce the current when they strike and smaller caps (can't go too small or hand capacitance and hum play havoc) still similar results. Even running them at pure DC through a high resistance caused quick deterioration. The neons I've used are tiny new ones from RS (the only ones I could find). I bet some bigger old ones as used in mains sockets or old cooker indicators would stand up to more abuse!

Anyway I'm going to try to throw together a telly using all this stuff to try out different spot wobble patterns (to go with the aperture experiments) if I get this working but for it to work well I need to get the spot size down a bit.

Cheers

Dom

[Steve Anderson]

...The one thing I've not experimented with yet is the a3 voltage.... Cheers. Dom

I used to have a VCR138 (not 139) tube decades ago and as I recall I'm sure I was able to get the line width/spot size down to less than 1mm as I used it as part of a flying snot scanner for SSTV.

It is a different tube, a slightly larger screen and much longer but comes from the same period and uses very similar voltages.

Now I can't find any data for the VCR139 specifically, but referring to the equivalent ECR30 datasheet a1 and a3 are tied together and would be at ground or slightly positive, say +150V which would also be the average potential of the deflection plates. The cathode would sit at -650V giving a total of 800V 'across' the tube. The grid (brightness) would be up to 20V more negative still.

a2 is the focus electrode which should sit some 100 to 170V positive wrt the cathode (-550V to -480V wrt ground). All of the above I'm sure you're aware of, but put it in for completeness.

Most of these oscillographic tubes were used in this manner. I have seen somewhere that 'odd things' can happen when these older tubes are used with the cathode at or near ground and the anodes and deflection plates operated at a high positive voltage. One I have seen myself is that when touching the screen the trace moves and distorts. Who knows, if you are using this tube in this manner it might explain the poor focus?

Later tubes like the DG-xxx series had an aluminiumized coating applied to the rear of the phosphor which eliminated this and increased the brightness. Don't forget these older tubes were nowhere near as bright as more modern ones. Even the DG7-32 display elsewhere on this forum had to be viewed in subdued light, even with its aluminiumized coating.

Another point is focus current (a2), in these older tubes (pre-1945) the focus current was comparatively high. With the development of the Einzel focus system it dropped to well under 50uA (of either polarity!) It might be worth checking the divider chain for the focus anode that it has sufficient standing current.

Putting a small capacitor from a2 to the cathode would eliminate any hum/ripple on it, say 10uF/250V.

Magnetic fields....make sure there are no (50Hz) transformers, motors etc. nearby, also the LOPT might be the culprit. I might face this problem with 'The Brute'.

Otherwise at this point I'm out of ideas.

Steve A.

[AncientBrit]

Dominic,

The conventional way to configure tubes is with deflection plates near ground, say around 100v, with the cathode up in the air at around -1000v and one side of heater strapped to cathode.

You can drive with cathode at deck, plates at +1000v and scans ac coupled via a caps.
But as Steve says electrostatic problems occur when a hand approaches the tube face.

I agree with Steve, 1mm or less is normal for spot size. Is your EHT impedance low, ie when beam current is drawn to produce the spot does the voltage dip?

The halo sounds like secondary emission.
I'd give the tube a burn-in for a few hours. Add a bit of 50Hz to the plates to save spot burn.
The getter may get to work to clean up things.

(Dominic, off topic, sorry. Did you see EPE magazine's offer for Microchip's PICkit2. £9.99. Programmer, development board with 16F887, ICE debug and software? I know you said in a recent post you were returning to PICs. Offer for UK only, closes at end of month. You have to quote a coupon number to get the discount. I've bought one. I'm impressed)

Regards,

Graham

[Steve Anderson]

Dominic, The conventional way to configure tubes is with deflection plates near ground, say around 100v, with the cathode up in the air at around -1000v and one side of heater strapped to cathode. Regards, Graham

The attached diagram illustrates this point where the a1 and a3 are at ground potential. This would have been for an AC coupled 'scope whereas having the anodes and deflection plates at a hundred or so volts allows DC-coupled amplifiers to be used.

From the National Valve web-site. Shame the component values are omitted.

Another point, this diagram shows single-ended drive to the plates which can lead to distortion of the waveform/raster and de-focusing at the edges. Always better to drive a tube like this with a symmetrical waveform, either a tube or semiconductor differential stage.

There are tubes designed for asymmetric drive on one set of plates, the small 1" DH3-91 (no focus anode too) and the 3" DG7-6 (in various phosphors). Many of the datasheets for European tubes do recommend that a3 is grounded, or at least nearby.

Steve A.

[Viewmaster]

8

...The one thing I've not experimented with yet is the a3 voltage.... Cheers. Dom

I used to have a VCR138 (not 139) tube decades ago and as I recall I'm sure I was able to get the line width/spot size down to less than 1mm as I used it as part of a flying snot scanner for SSTV.

It is a different tube, a slightly larger screen and much longer but comes from the same period and uses very similar voltages.

Now I can't find any data for the VCR139 specifically,
Steve A.

Steve,
In Babani's International valve manual the VCR139A=CV1588=
Cossor 23D
The details for this Cossor CRT are as follows....

Vf = 4
If = 1.1
Va1 = 800
Va2 = 120
Va3 = 800
-Vg1 = 12
Deflection sensitivity (mm/V) on the X plates is .217
Same on the Y plates.

I don't know the difference, if any, between the VCR139 and the VCR139A

If you need the base pin connections for the VCR139A just shout.
Albert.

[Steve Anderson]

Steve, In Babani's International valve manual the VCR139A=CV1588=Cossor 23D. The details for this Cossor CRT are as follows....

I don't know the difference, if any, between the VCR139 and the VCR139A. Albert.

Thanks for that Albert, it confirms that it's the same as the Mullard ECR03 at least electrically.

I don't know if this applies to European tubes but in the US the difference between a (for example) 5CP1 and a 5CP1-A was the improved focus arrangement which used a lot less current on the focus anode, otherwise there was no difference.

I have a 3FP7 that positively guzzles current on the focus anode, been looking around for a 3FP7-A for years, but to be honest don't really need it now.

Steve A.

[AncientBrit]

Dominic,

Just a wild thought but do the deflection plates have dc return to deck, ie direct drive from a valve/transistor or a bleed resistor?

If they were floating dc wise they would eventually drift reducing the overall CRT potential.

Graham

[dominicbeesley]

Wow thanks for all the help lads,

Firstly I've tried isolating the tube physically from all the sources of mains and other hum but no difference.

I've also tried running with the a3 etc at ground and the cathodes at -800 still no joy (and a good hefty belt - it meant having my two sitting at -800 only a matter of time before I forgot and touched one!)

Whent he spot is moving about the definition was a little better but not much, anyway I went looking for some bits and found by accident an ACR10 CRT which I'd always shied away from as it sounds like a baby's rattle when you move it, anyway I checked and its got similar specs to VCR139A, same pin-out and looks identical looking in through the neck. Plugged that in and get a much better picture so must conclude that the other tube is a bit tired! It requires a lot more negative grid volts to dim it too and more drive to get any picture.

When the set is running I'll want to have a conventional telly rather than scope arrangement so I can couple directly to the cathode or grid to feed in video - I'll have to check the current to screen though I've already decoupled that with a small cap as there was already one on the focus pot I'm using. I don't think it draws a lot of current though but that migh affect whether I use grid or cathode feed for the video.

If having the a3 at high potential causes problems I may try putting aquadag on the neck and grounding it?!?

As to the DH3-91 only one plate is asymmetrical and it still give a bit of distortion (see links below copied from split thread). Driving the other plate asymettrically gives pretty rubbish results. In previous attemtps I used an extra valve to make a phase splitter this time I might try and make a transformer instead...

I've checked and the EHT doesn't dip but the first tube did wobble about if the grid was modulated by a signal, the second is steady as a rock - I reckon the first tube really must have had it!

Anyway thanks Graham, Steve and Albert for all the information....no doubt I'll be back soon! This is all just practice for trying to get a colour tube to work - again with little or no data, or clue - now that will be fun!

Another tube I've got which I could try is the DG7-3, trouble is I can only find data for the DG7-5 and am not sure how different they are?

Cheers

Dom


http://www.vintage-radio.net/forum/showthread.php?t=31105
http://www.vintage-radio.net/forum/showthread.php?t=24304

[Steve Anderson]

OK, it seems that the VCR139 is past its 'sell by' date if the ACR10 is that much better. Partly the clue is in the grid voltage....moving on....

DG7-3, no worries Bruce, see attached, and a line width of 0.7mm is mentioned! There is a layout error in the persistence data, the DG7-3 has a standard 'scope-type green medium persistence phosphor.

Voltages are slightly different to the VCR139, heater is 6.3V not 4V but others in the same sort of range. It's also a 'faster' tube with less capacitance, but of no issue in NBTV, but will help with 405/525/625 standards.

It's worth downloading the pdf at the end as it has current curves within it.

Steve A.

[dominicbeesley]

Wow quick service! Thanks for that steve - a 6.3V heater is a bonus too - one less bench supply needed to power the thing!

I've etched the EHT generator onto a PCB and drilled it so it should be easier to test things out - at the moment its all held together with connector blocks and lots (about 30) of croc-clip leads....very easy to pick up the wrong one when fiddling!

Tomorrow I'll try subbing these tubes into the circuit I made up for the DH3-91s in the NBTV link and see what the results are like. Then I'll have a look at making a spot wobble circuit, any ideas how to do this I want a spot that wobbles over two lines but with a profile that falls off linearly from the middle.

I've tried a simple sine wave but of course that gives edges brighter than the middle - All I can think of (that is easy to implement) is a sinewave modulated by a ramp, I'll try this with solid state devices first but in the final (if the experiment works/proves worthwhile) I'd like something that used a single valve or could somehow be derived from the EHT circuits. (I'll probably only enable it on NBTV repro though not 625/405 as that would lead to patterns unless I made it very high frequency).

Cheers

Dom

Dom

[Steve Anderson]

I've tried a simple sine wave but of course that gives edges brighter than the middle -Cheers. Dom.

For NBTV applications I would suggest an op-amp in a relaxation oscillator arrangement. Buy setting the 'trip levels' low at say +/-1V on a +/- 12V supply would ensure a fairly good approximation of a triangle wave to be mixed in with the frame deflection.

At 405/525/625 frequencies this is not so easy. Research is needed into how they did it in 'the old days'. Steve O might have some input here, although his 405-line stuff is usually electromagnetically deflected, but it might be a source of food for thought.

Steve A.

[dominicbeesley]

I've just tried a quick simulation in software and unfortunately a simple

y = ramp(t) * sin(N*t)

Doesn't give a nice linear brightness fall off at the edges it is more of a curved spike. Simple sine waves are more like a sort of "bath-tub" profile of brightness

[I worked this out by sampling how long the spot would spend at each point - I'm guessing this is right]

Looks like more thought needed, I've tried a few different ideas and none give the right kind of curve!

Dom