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'[EE]: CRT biasing (was [EE]: Fun with nested poten'
2005\10\21@190548 by Philip Pemberton

face picon face
 OK, I think I've just about nailed the biasing for my DG7-32 CRT, but the
Intensity control doesn't seem to have much of an effect on the brightness of
the trace.
 If I set it all the way to one end of its travel, I get an unfocussed
5mm-ish ragged elliptical spot on the screen that seems to fade (brightest at
the bottom left of the ellipse, darker towards the top right). At about
25%-100% position the brightness does vary but not by any perceptible amount.
There is no setting where the spot will extinguish completely.

Here's a schematic of the resistor chain as it is now:

            100k    120k        68k      220k    680k
-500V <<<---/\/\/---/\/\/---*---/\/\/----/\/\/---/\/\/----------------> 0V
             ^             |              ^
             |             |              |
             |             |              |
             |             |              |
         Vg1 (inten)   Vk (cathode)   Vg3 (focus)
      -522V to -465V     -396V        -366V to -276V

Anode (Vg2,g4) voltage: +243V

At one point I had a 47k pot between +240 and GND, with the wiper connected
to the CRT anode as a primitive "astigmatism" control. This has since been
removed because I thought it served no useful purpose. The anode is wired
straight to the +240 supply.

The voltage ranges were taken relative to 0V and were measured with the CRT
connected. They all appear to be within range of the figures in the CRT
datasheet...
The only thing I can think of is that it might be related to Vg3. The
datasheet says:
Limiting Values (design center values)
======================================
 Vg3: Max 200V (note 4)
 Note 4: For calculation of the grid 3 potentiometer a grid 3 current of
         min. -15uA and max. +10uA must be taken into account.
 Note 5: Negative grid No.1 voltage for visual extinction of the focused
         spot.

Max Circuit Values
==================
 RD: Max 5 M[ohms]
 Rg1: Max 0.5M[ohms]

Now based on my calculations, there should be 0.4mA available for that.
Here's what I did:
 G3 section: 68k + 220k + 680k = 968k
 Ig3 = 400V / 968000 Ohms = 0.413 mA

Can any CRT experts who might be lurking here please tell me what I'm doing
wrong?

Thanks.
--
Phil.                              | Acorn RiscPC600 SA220 64MB+6GB 100baseT
spam_OUTphilpemTakeThisOuTspamphilpem.me.uk              | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/          | Panasonic CF-25 Mk.2 Toughbook
... Today is cancelled due to lack of interest!

2005\10\21@201030 by Marcel Duchamp

picon face
What effect does adjusting the Vg3 voltage do?

Philip Pemberton wrote:
{Quote hidden}

2005\10\21@202024 by Philip Pemberton

face picon face
In message <.....4359840E.4080709KILLspamspam@spam@sbcglobal.net>
         Marcel Duchamp <marcel.duchampspamKILLspamsbcglobal.net> wrote:

> What effect does adjusting the Vg3 voltage do?

Standard focus/defocus. One way focusses the beam spot, the other way causes
the spot to get larger. When the Vg1 is in the "dead zone" (big spot), it's
impossible to focus the spot on the CRT.
The -500 and +240 remain stable all the time.

Later.
--
Phil.                              | Acorn RiscPC600 SA220 64MB+6GB 100baseT
.....philpemKILLspamspam.....philpem.me.uk              | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/          | Panasonic CF-25 Mk.2 Toughbook
... Where's my 640Meg SIMM? I want to run Excel!

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