DG7-32 CRT questions

Hi, I'm trying to wire up a DG7-32 CRT. I've managed to get a visible spot, which is more-or-less focussable down to a 0.5mm dot. Maybe not as good as my oscilloscope, but pretty good for a 1950s CRT.

The tube itself is a Tungsram of unknown age, and was bought new-old-stock. I'm using the Philips DG7-32 datasheet because I can't find a copy of the Tungsram datasheet anywhere. Given that it's a standard tube, I doubt the specs vary that much between manufacturers.

Now, the problem is, I can't get the spot to extinguish completely. Here's the circuit I'm using to derive the G1, cathode and G3 voltages:

======

100k 120k 68k 220k 680k

-500V

You need to have a DC path from the deflection plates to anode, otherwise you get stray charge building up on the plates, causing all sorts of unexpected effects. _Never_ leave deflection plates floating.

According to the data I have, beam cutoff should occur with g1 between 50 and 100V negative wrt cathode, with 500 volts on the anode. g1 voltage

*does* affect focus. Google for "Wehnelt cylinder" for an explanation of why. According to what you say, you can get good focus, so that would appear to rule out an O/C g1 electrode.

ISTR these tubes were always soggy on g3 voltage. As long as the spot focuses, don't worry too much.

That's how you implement an astigmatism control with this type of CRT. Setting the anode voltage equal to the mean voltage on the plates. You'll need this when you drive the plates from amplifiers, and they're somewhere between 0V and B+ voltage.

Get those plates connected to anode via 100K resistors for a start, and see if some of your problems go away.

Any particular reason for this? I'd try to get hold of a B+ mains transformer and voltage double the output of that.

Do you have a DC path from heater to cathode? Don't rely on cathode insulation (only rated at 125V), and remember that your heater transformer insulation needs to withstand cathode voltage. Heater windings going leaky to frame used to be quite common in the days when 'scopes used mains transformer fed heaters.

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Well, that explains that. I've redesigned the biasing circuitry and it now works fine. The focussed spot is less than 0.5mm in size.

I've built up some deflection amps based on Sascha Ittner's scope clock design. Problem is, the damn thing doesn't seem to have enough of a span in the "X pos" and "Y pos" controls to get the spot moved to the correct position on the screen :-/ The same basic circuit is used in the "OZ2CPU" CRT clock, though with different component values.

The big problem now is getting the deflection amplifiers going. I'm having problems where the Y deflection is fine, but the X deflection won't go to the edge of the screen. What I want to do is tweak the gain of the amplifier so that it'll run off 5V and the HT, and take an input of either 0-5V or 0-2.5V, where 0V=full left,

5V (or 2.5V) = full right, 0.5V = spot centered.

I wanted to run it off a 12V supply.

Yes. One end of the heater is wired directly to the cathode.

Let's see.. The transistor consists of: Plastic bobbin - good to a few kV Primary windings A layer of Kapton tape Two layers of Kapton-coated tinfoil, grounded A layer of Kapton tape Secondary winding

I'm not sure what its isolation voltage is, but it should be good enough for a CRT.

Later.

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