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Can't get a small CRT display to focus

Galgso

Member
I have a 2AP1 CRT (datasheet attached) that I wish to use to make an oscilloscope eventually. The CRT was bought off Ebay and may potentially be faulty although the seller claimed that the CRT was fully functional. The main issue I am having is that I cannot get the beam to focus at all. To make sense of the following information, please note that I have the grid connected to 0v and that the cathode is positive with respect to the grid. I did this to avoid needing a split supply.

Here are the DC voltages across the various electrodes, all referenced to the grid:
Cathode: 0v (the screen is completely invisible at 20v)
Anode 1: 200v (varying this from 80v to 540v seems to change the focus somewhat but there is nothing vaguely resembling a dot. See image 2)
Anode 2: 1kV
D1: 40v (Adjusting this seems to help make the shape on the screen more focused to some extent)
D2: 0v
D3: 20v (at 80v the beam is completely prevented from reaching the screen and the screen becomes dark)
D4: 0v
Heater (separate supply, not referenced to grid): roughly 6.3v DC

The issue is that I cannot get anything vaguely resembling a dot. I don't know if my adjustments are correct because I just see a giant blob on the screen and all I can tell when I twist the knobs on my supplies is that something is changing.

An additional issue is that the screen is pretty dim. I initially had A2 at around 500v and thought that was the reason it was dim so I brought in my other supply which goes up to a higher voltage and set it to 1kV which did help but not the extent I thought it would. It might look bright in the pictures attached but that is in near complete darkness; in a well-lit room I can hardly tell that the display is on.
 

Attachments

  • 2ap1.pdf
    92.6 KB · Views: 59
  • Image2.jpg
    Image2.jpg
    1 MB · Views: 66
  • Image1.jpg
    Image1.jpg
    1 MB · Views: 71
  • crt.jpg
    crt.jpg
    2.1 MB · Views: 85
Solution
I thought you meant higher than the first anode which is at 260v?

No, each anode and the plates need to be (considerably) higher then the previous one in order to accelerate the electron beam.

I've currently got D2 at 520v and D1 at around 20v. The whole screen is glowing faintly (looks very similar to "Image1.jpg" in the original post with the 2AP1). The glow is unfortunately too dim to capture with my phone's camera.

What voltages are on A1 and A2?, you mentioned 900V on A2 in the first post, if so 520V is FAR too low for the deflection plates, they want to be 1000V or so. Also, having a 500V difference between D1 and D2 is going to deflect the beam off the screen, even if it is working. You want D1, D2, D3 and D4...
I finally got it to work. I connected A2 to 500v and connected all the deflection plates to 700v and I got a nice dot after after adjusting A1's voltage. Thank you Nigel.
 

Attachments

  • Dot.jpg
    Dot.jpg
    1.8 MB · Views: 22

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