UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Reply
 
Thread Tools
Old 29th Oct 2018, 5:28 am   #1
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Passive Cathode ray tube modulation

This is for any one interested in putting in a z axis connection on your old scope that doesn't have one very simple hack too.
So the procedure in the Trio scope schematic example is brake the connection for the cathode pin 3 and connect a spare mains transformers primary between that and pin 22 rest explained below ...Should work for any oscilloscope just an example here as i have this scope .
Pretty much a very simple idea i don't know if it has been used in the past by any one else , the idea was and is just using a small mains transformer the primary 240 volt side is used between the cathode and its negative 1500 volt supply ...so only 2 connections .
The secondary which i think would of been 12 volts side is used for the modulation input just connected right from the lap top to ground and signal output connection just another 2 connections to that secondary .
So small signal about a 1 volt or so at best can modulation your scope if you don't have a Z axis connection .
It does rely on how high the video level is but every thing i tried seemed to work some files better than others but had not used any thing apart from this passive induction amplifier .
A speaker audio transformer i think would also work more than likely better,i was surprised it still worked without the one transistor amplifier i also recalled i used back 2 decades ago when i first tried this .
So results below with a 32 line video upside down and reversed ...least its not inverted I will add a video amplifier transistor to this but not in the scope just to it ,but for using a little transformer and 4 connections a quick and dirty z axis modulation which i can't see why it can't be used on any old scope with out or a broken z axis connection as i had and just made it would with this idea .
Attached Thumbnails
Click image for larger version

Name:	IMG_0107.JPG
Views:	147
Size:	88.3 KB
ID:	171742   Click image for larger version

Name:	IMG_0039.JPG
Views:	150
Size:	73.2 KB
ID:	171743   Click image for larger version

Name:	IMG_0037.jpg
Views:	151
Size:	59.6 KB
ID:	171744   Click image for larger version

Name:	IMG_0112.jpg
Views:	162
Size:	45.6 KB
ID:	171745   Click image for larger version

Name:	IMG_0121.jpg
Views:	138
Size:	45.5 KB
ID:	171746  

dalekmoore2007 is offline   Reply With Quote
Old 29th Oct 2018, 1:35 pm   #2
dominicbeesley
Octode
 
dominicbeesley's Avatar
 
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,678
Default Re: Passive Cathode ray tube modulation

I've never tried a transformer in that position - it works well! I think your idea of trying an output transformer is good - a valve output transformer for a pentode would probably give a good step up!

D
dominicbeesley is offline   Reply With Quote
Old 29th Oct 2018, 3:15 pm   #3
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 5,128
Default Re: Passive Cathode ray tube modulation

Hi Gents, it should work well, but you may stress the transformer insulation with the voltages involved.
The larger transformers may also have significant inductance and limit the mod signal rise time.
A small HV pulse transformer may work better.

Ed
Ed_Dinning is offline   Reply With Quote
Old 29th Oct 2018, 4:03 pm   #4
dominicbeesley
Octode
 
dominicbeesley's Avatar
 
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,678
Default Re: Passive Cathode ray tube modulation

Good point Ed, that would need to be considered.

How well do pulse transformers do at low frequencies - for the NBTV signals that Harry's using they need, preferably, to go down to single digit Hz [frame rate is 12.5Hz, highest frequencies of interest in the 10's of kHz]. I've always just used a bigish HV cap to either grid or cathode...
dominicbeesley is offline   Reply With Quote
Old 29th Oct 2018, 10:35 pm   #5
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 5,128
Default Re: Passive Cathode ray tube modulation

Hi Dom, small transformers do not normally work well at low frequencies, big ones may have too much self capacitance.

It would be possible to "burst fire" a small transformer as was done to imitate DC gating on SCRs.

Ed
Ed_Dinning is offline   Reply With Quote
Old 30th Oct 2018, 6:03 am   #6
TonyDuell
Nonode
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 2,714
Default Re: Passive Cathode ray tube modulation

I've not tried it, but what about using an optoisolator?

This one :

https://uk.rs-online.com/web/p/optocouplers/1713892/

has an isolation voltage of 10kV (which should be enough) and is in a package that keeps the 2 ends well apart. And I would think it had enough bandwidth for this application. There are some higher voltage ones (up to 50kV) at RS too, but they are more expensive.
TonyDuell is offline   Reply With Quote
Old 30th Oct 2018, 6:34 am   #7
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

Thanks for the replies every one Dom yes it works very well when you think a mains transformer was made for 50hz or 60hz .
I knew it worked from memory 20 years ago one of those things had a broken z connection and needed one !
How good is it its junk every one has laying around and would work on any scope .
Steve Anderson mentioned on the NBTV forum mains transformers are flash tested to 2.5kv so here its working at half that .
From my tests it works great ,no electronics 4 connections to worry about .
It was mentioned to me also best use a newer transformer not some rusty one where its falling to bits ! I would be interested if any one else gives it a go ,i plan on trying the transformer passive modulation on my magnetic FSC colour monitor i am building ,as i have only ever tried it on a electostatic tube ...
So any one trying this with something else remember 1.5Kv or so volts negative can jump wires i only know it works on a mains transformer.
dalekmoore2007 is offline   Reply With Quote
Old 30th Oct 2018, 7:13 am   #8
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

Here's the one i am using /
Attached Thumbnails
Click image for larger version

Name:	IMG_0126.jpg
Views:	75
Size:	52.4 KB
ID:	171822   Click image for larger version

Name:	IMG_0127.jpg
Views:	53
Size:	46.8 KB
ID:	171823  
dalekmoore2007 is offline   Reply With Quote
Old 30th Oct 2018, 7:16 am   #9
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

Procedure i did was lower the scopes luminance so its trace line is just off being seen and i tapped a low voltage cross the secondary, i had a resistor wheel here too to see how low i could go and see an effect on the intensity...and does not need much at all a lap top you are lucky if it can light a led.
Pretty easy add on for any scope without a z axis connection ,
dalekmoore2007 is offline   Reply With Quote
Old 30th Oct 2018, 10:23 pm   #10
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 5,128
Default Re: Passive Cathode ray tube modulation

Hi Gents, yes, if you can get a split bobbin transformer it will have better isolation properties.

Optos should also work well, but they have pretty complex data sheets to decipher correctly.

Ed
Ed_Dinning is offline   Reply With Quote
Old 31st Oct 2018, 5:29 am   #11
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

There are of cause the optos but i was interested here in a simple transformer doing the job with no extra electronics just electrics of it ! data sheet not needed if you can work out a primary and a secondary that's all you need to know.
A split bobbin transformer is a good idea Ed.
dalekmoore2007 is offline   Reply With Quote
Old 3rd Nov 2018, 9:50 am   #12
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

I have been experimenting some more with the 240v main's transformer CRT cathode modulation .
Something i was not expecting was if i reverse the secondary connections to the input the video go's from normal to inverted that was a new one to me .
Also swapped to a 6 volt secondary transformer and the video modulation doubles.
Click image for larger version

Name:	IMG_0429.jpg
Views:	39
Size:	88.2 KB
ID:	172051

Click image for larger version

Name:	IMG_0465.JPG
Views:	45
Size:	97.8 KB
ID:	172052

Before i found this out about swapping the input transformer connections invert normal i made a circuit for the lower signal video files for displaying better .I now know ...you have other options ....as is just the passive way via the transformers secondary a 6 volt one will give you a fine enough display better than a 12 volt one , for a touch better the circuit shown in the pictures it will work but will also work just with a say 33 uf cap on pin 6 of the Tl071 other end of cap to on secondary connection other to ground ...same gos for the class A transistor amplifier secondary of transformer to its 1uf cap other to ground as is for the rest in the circuit ...so you can use it as it or split the circuit .
Attached Thumbnails
Click image for larger version

Name:	IMG_0262.jpg
Views:	44
Size:	100.5 KB
ID:	172053   Click image for larger version

Name:	IMG_0457.JPG
Views:	49
Size:	32.5 KB
ID:	172054   Click image for larger version

Name:	addtext_com_MDcxMDQ2NTA2MzI.jpg
Views:	31
Size:	59.1 KB
ID:	172055  
dalekmoore2007 is offline   Reply With Quote
Old 3rd Nov 2018, 6:31 pm   #13
merlinmaxwell
Dekatron
 
merlinmaxwell's Avatar
 
Join Date: Jul 2008
Location: Maidenhead, Berkshire, UK.
Posts: 7,809
Default Re: Passive Cathode ray tube modulation

A great example of giving it a go and it works, I bet many theorists would say it wouldn't.
__________________
Cats have staff, it's dogs that have owners.
merlinmaxwell is offline   Reply With Quote
Old 3rd Nov 2018, 11:36 pm   #14
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

Thanks i remembered i tried it 20 years ago sort of !, one of those things i had a need and tried it again to find out my memory was correct, now knowing more what i am doing it seemed logical.
Comes in useful testing circuits as my hobby is NBTV.
dalekmoore2007 is offline   Reply With Quote
Old 4th Nov 2018, 2:03 am   #15
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 6,869
Default Re: Passive Cathode ray tube modulation

Quote:
Originally Posted by merlinmaxwell View Post
A great example of giving it a go and it works, I bet many theorists would say it wouldn't.
Well, going right back to first principles, the more negative a grid is with respect to the cathode, the fewer electrons get through it. So by superimposing a voltage between the grid and cathode of the tube in the oscilloscope, you can vary the strength of the electron beam and therefore the brightness of the line on the screen. That's just secondary-school physics.

And there's nothing on that diagram that looks as though it might be damaged by wiring a transforner there. The stand-off voltages involved are more than the transformer is likely to see in normal operation, but still within the ratings to which it should have been tested.

So bearing in mind both the above points, it's at least worth a punt. In the absolute worst case, you can treat yourself to a brand spanking new digital 'scope that will allow you to change not just the brightness of the trace, but the colour as well .....
__________________
Julie {formerly known AJS_Derby}
julie_m is offline   Reply With Quote
Old 4th Nov 2018, 6:22 am   #16
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

Yes the mains transformers are tested way above their operating voltage just about twice whats going into it now .
Bandwidth is way above what i was expecting testing so far up to 64 line pictures below and i had one 128 line vampira still to test so gos up to that line rate .
i was thinking of using a valve audio transformer but i think the mains transformer is safer and seems to work well as is ,not sure the audio transformer are tested to the degree of the mains version.

Yes a digital scope sounds good , i do like crt's and analog scopes as i can understand them what the hell is happening in the digital gear some times is beyond me ...here i am using my very first little scope from 1980 wonder if the digital scopes now will still be working in around the late 2050's
Attached Thumbnails
Click image for larger version

Name:	IMG_0243.jpg
Views:	24
Size:	32.0 KB
ID:	172111   Click image for larger version

Name:	IMG_0510.JPG
Views:	25
Size:	106.4 KB
ID:	172112   Click image for larger version

Name:	IMG_0529.JPG
Views:	27
Size:	114.3 KB
ID:	172113   Click image for larger version

Name:	IMG_0502.JPG
Views:	25
Size:	106.4 KB
ID:	172114   Click image for larger version

Name:	IMG_0526.JPG
Views:	24
Size:	120.8 KB
ID:	172115  

dalekmoore2007 is offline   Reply With Quote
Old 4th Nov 2018, 9:37 am   #17
dalekmoore2007
Pentode
 
Join Date: Aug 2008
Location: Melbourne, Australia
Posts: 101
Default Re: Passive Cathode ray tube modulation

Below i had 2 other 128 line videos i made a while back i had to display , so what i had handy here and what photos came out .
i used here just the original idea just the lap top to the mains transformer secondary winding's to modulate the cathode ...remember depending on which of the windings are on earth or output audio of the lap top the video will be either inverted or normal with no electronics doing this only magnetism varying current direction in the secondary coil effecting the primary a varying electromotive force i suppose in a positive or negative direction effecting the voltage to the cathode /
Attached Thumbnails
Click image for larger version

Name:	IMG_0565.jpg
Views:	22
Size:	47.6 KB
ID:	172120   Click image for larger version

Name:	IMG_0612.JPG
Views:	20
Size:	142.3 KB
ID:	172121  
dalekmoore2007 is offline   Reply With Quote
Old 4th Nov 2018, 10:18 am   #18
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 5,128
Default Re: Passive Cathode ray tube modulation

Hi Dalek, audio transformers may be better due to lower winding capacitance; they are probably tested to 2KV as are mains transformers.

I'll be out in Melbourne next year, so could probably bring some bits out. Drop me a PM

Ed
Ed_Dinning is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 5:32 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.