video monitor with DG7-32

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

video monitor with DG7-32

Postby Didi » Sat Jun 22, 2013 1:47 am

Hi to all friends of the old TV technology,
since yesterday I'm a new member in this group. For 20 years I'm interesting in the old radios from the 50's and I'm restoring them too.For some months I'm interested in the old TV technology too.
Some weeks ago I started to build a very simple TV. My intension is to put a videosignal in it and
watch it on the screen of the DG7-32.
I attached a link of the circuit to the mail.

https://www.dropbox.com/s/i2721v54xykvdqf/1.jpg?m
https://www.dropbox.com/s/kfqw1a2fahovv0y/2.jpg?m

The deflection part is made with 2 EF80
An aditional syncronization part can be attached to the schematic too.
The 3rd part of the schematic is the CRT related circuit.
I fed a video signal ( with my digicam) into the grid of the video amp ( EL84) and was able to syncronize it with the x and y pots by hand.
But the pic I could see on the CRT was very very bad. Perhaps the CRT related part is made too simple to get a useful pic, I tried a lot of adjustments but I wasn't sucessful.
The x,y sawtooth signals are ok ( 64 us and 20 ms signal ) so I wonder where the problem could be.
Because some of you are building similar constructions I think someone could be able to give me some advises to get a better pic on the CRT.
It would be great if you're able to give me informations about your experiences with the circuits you use for this purpose or send me the schematic you've used for the TV.
Perhaps someone of you have made experiences with the same schematic too.
Thank you very much for all kind of advices.

kind regards

Didi
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Postby Steve Anderson » Sat Jun 22, 2013 12:42 pm

Hi Didi, and welcome to the forum.

I have also used the DG7-32 in quite a few TV and NBTV applications without any major problems.

When you say 'But the pic I could see on the CRT was very very bad...', what was wrong with it?

I have looked at the two drawings you linked to, both seem intrinsically correct, though awful drawings and no voltages or other useful information.

If you can take a couple of snapshots of what you are getting and post them here that would make things a lot easier to figure out.

Steve A.
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Postby DrZarkov » Sat Jun 22, 2013 4:58 pm

Hello Didi, this is your "Dutch TV with 5 valves", you've posted at the "Dampfradioforum" and at the "Wumpus-Gollum-Forum"? Nice to see you are making progress. :)
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video monitor with DG7-32

Postby Didi » Sat Jun 22, 2013 8:48 pm

Hi to all,
I'm very pleased that you're trying to help me cause at the moment I don't know how to find the mistake. It's very hard to find someone who's familar with making experiments with a CRT.
In the afternoon I'll send you some pics of the screen and the sawteeth of the X and Y deflection
to give you aditional informations.
In the most CRT schematics I saw that both x and y deflection plates are connected with the deflection circuit.
In my case only one x and one y deflection plate is connected with the deflection schematic.
I'm confused about this.
Another fact is that my circuit don't use a negative voltage fot the supply of the grid 1.

kind regards
Didi
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Postby Steve Anderson » Sat Jun 22, 2013 10:29 pm

Didi, you don't have to supply opposite polarity sweep waveforms, where the spot lands on the screen is a function of the voltage difference between the x (or Y) plates. However, in most CRTs this will 'work', but often (mostly) you'll end up with a raster that isn't square but trapezoidal in shape. Also now the average voltage of the deflection plates isn't constant you get de-focusing at the edges.

Put simply, single-ended drive to the plates of a CRT which was designed to have a differential drive (DG7-32) isn't recommended. It's for those that are building toys...

If you study the diagram you will see the grid is actually negative w.r.t the cathode, but it's such a bad drawing it isn't obvious.

Steve A.

I attach one tube deflection arrangement I used with a DG7-32, the bit's in red weren't 100% fixed at the time I did the drawing. The CRT grid/cathode sat at around 450V negative so there was about 650V in total across the tube. A DG7-32 run at under 600V or so isn't that good...don't believe the datasheets!
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32 Line 'Club Standard' 3.gif
32 Line 'Club Standard' 3.gif (12.05 KiB) Viewed 7869 times
ECC91 Deflection 1.jpg
ECC91 Deflection 1.jpg (289.44 KiB) Viewed 7868 times
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video monitor with DG7-32

Postby Didi » Sat Jun 22, 2013 11:16 pm

Hi Steve,
I see you know a lot of the CRT schematics.
After I read your answer I try to understand the deflection of the DG7-32:
Please correct me if I'm wrong:
The DG7-32 is made for a schematic that uses 2 X and 2 Y deflection wires.
Do you want to say it would be better to use another CRT and deflection schematic
to get a good pictue or is it possible to make some changes in my schematic to
get it to work better?
Thank you very much for your efforts I'm very glad that someone is able to give me
very useful informations.

kind regads
Didi
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Postby Steve Anderson » Sat Jun 22, 2013 11:29 pm

No, there's nothing wrong with the DG7-32 it's just those circuits you are using aren't very good...OK, they're crap.

Even CRTs designed for single-ended drive (usually in one axis only though) still exhibit de-focusing at the edges, e.g. 1CP1...this single-ended drive arrangement was usually only used on the smallest of tubes where it was hard to see the de-focusing...1" (2.5cm) tubes being the most common...but of course there's always exceptions...

Steve A.

I attach what can be achieved with a DG7-72 using only 72 lines...four separate pictures...not all at once!!

I should apologize to the others as I have posted this picture at least once before...
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video monitor with DG7-32

Postby Didi » Sun Jun 23, 2013 11:26 pm

Hi Steve,
the pics you've posted look great, I'm amazed about the quality that can be achieved.
I'll try to use the deflection you posted

(http://www.taswegian.com/NBTV/forum/fil ... _3_473.gif

for my TV project.
Can I use the following schematic for the power supply of the DG7-32 ?

http://www.taswegian.com/NBTV/forum/fil ... 3b_201.gif

I'm a little confused because of the several DG7-32 PSU schematics that are published on this
site.


kind regards
Didi
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Postby Steve Anderson » Mon Jun 24, 2013 12:20 am

There are many ways that a DG7-32 (and others) can be operated, they are quite flexible, as long as there's 600-800V across the tube (cathode-anode & deflection plates) all will be well. The grid of course needs to be somewhat negative to the cathode

So you could have this...

Final anode and deflection plate average voltage +700V
Cathode at +100V,
Grid between +100V and 0v

or...

Final anode and deflection plate average voltage +200V
Cathode at -400V,
Grid between -400V and -500V

..or any combination you might choose.

It's often dictated by the transformers and other components users have available. This also is dictated by the required display. With NBTV with its low-frequency requirements, DC-coupling is to be preferred. Whereas standard (625/525-line TV) AC coupling should (just) be OK.

Steve A.

Of course the green hue of the DG7-32 was removed by converting the previous images to grey-scale.
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video monitor with DG7-32

Postby Didi » Mon Jun 24, 2013 2:29 am

Hi Steve,
thank you very much for your big efforts to help me. Now I think I'm ready to start a 2nd try
to build a useful monitor with the DG7-32.
Can I use the EL84 from my crap schematic :-)) to feed the videosignal with R11(10 K pot ) in the EL84???
The X and Y output( in the schematic they are called K and H ) would be connected to the vert. amps and horiz. amps of your deflection unit.

You're able to find the my video amp at the link below:

https://www.dropbox.com/s/6ysoao5sgu1gc5d/3.jpg

kind regards

Didi


video monitor with DG7-32
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Postby Panrock » Mon Jun 24, 2013 6:59 am

I'm inspired by your 72-line pictures too Steve A !

For me the task is much simpler - I already have a fully built-up electrostatic "Argus". So I now know what my next project must be: to make it into a multi-standard NBTV monitor!

All your fault! :lol:

Steve O
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Postby Steve Anderson » Mon Jun 24, 2013 4:54 pm

Didi, before going any further are you going to use this display for NBTV or standard (625/525) television? It does make quite a difference in both the video and deflection circuits.

Steve, (Steve A grinning here)...a multi-standard NBTV display? Hmm. I started out on one some time ago, it's got as far as the circuit design stage but nothing yet has physically been built. Should you need any help...

I hope (unlike myself) you'll keep the 'Argus' intact rather than plunder it for its parts and start out afresh. I know any sort of white-ish electrostatic deflection CRT is very hard to come by, I've been trying to get a hold of one for many years. A 7GP4, all electrostatic, would be perfect. Ho hum.

Steve A.
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Postby Panrock » Mon Jun 24, 2013 6:53 pm

Steve Anderson wrote:Steve, (Steve A grinning here)...a multi-standard NBTV display? Hmm. I started out on one some time ago, it's got as far as the circuit design stage but nothing yet has physically been built. Should you need any help...
As ever, very very kind. :) Many thanks, but read on.....

Steve Anderson wrote:I hope (unlike myself) you'll keep the 'Argus' intact rather than plunder it for its parts and start out afresh.

No need to worry about that. 'Hopefully' all I'll need to do is switch in and out a few caps, (adding a wafer switch) and I'm home and dry. A nice easy project after the mirror screw. Famous Last Words? :oops:

As for the provision of signals, of course I've now got Darryl's tasty World Converter...

Steve Anderson wrote:I know any sort of white-ish electrostatic deflection CRT is very hard to come by, I've been trying to get a hold of one for many years. A 7GP4, all electrostatic, would be perfect. Ho hum.

Before the current VCR517, I used a violet-behind green CV2810 which (sort of) did this.

Anyway, no chance of starting my next project until September. I'll get off the line now and start a new thread as and when.

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video monitor with DG7-32

Postby Didi » Tue Jun 25, 2013 2:32 am

Hi to all, hi Steve,
I want to use the TV circuit to feed a videosignal from a DVBT receiver into it.
Perhaps I use it for NBTV but at the moment I don't know much about the technical properties about the NBTV standard.
At the moment my intension is to use it for DVBT.
I think it's possible to use the following PSU schematic for the CRT:

http://www.taswegian.com/NBTV/forum/fil ... 3b_201.gif

You wrote that the deflection schematic have to be changed for the usage with a TV signal.
Cause of the TV norm, I'm living in Germany.
Can you give me informations what should be changed at the deflection circuit?

kind regards

Didi
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Postby Steve Anderson » Tue Jun 25, 2013 12:56 pm

Yes, Didi, you can use that PSU for what you wish, in fact you could use it for almost anything. For slow modes like SSTV or NBTV you'd need to get around those coupling caps to the deflection plates, but for standard-rate TV it should be OK.

Note: There are no 'position' controls so if you wish to have those you'll have to use some other arrangement. This was drawn up for one of our members here and he didn't need/want those controls.

If you tell me what transformer you intend to use (its output winding voltage(s)) I'll draw a circuit to suit. Once that's done we can move 'backwards' into thr deflection amplifiers. It would be useful to know what tubes/valves you have...assuming you wish to use them rather than transistors.

Steve A.
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