Hi Aaron
You've confirmed what I suspected, that the focus voltage is moving
enough to unfocus the image as the beam current is varying. I will take
some measurements of beam current, focus current, focus and cathode
voltages and try to feed the focus from a more stable voltage, perhaps
using a mosfet in the control rather than the simple potential divider.
The aim is automatic brightness control, probably not using the LDR in
the grid circuit directly but using another DAC (or PWM) to control grid
voltage and hence brightness. The LDR would be read by the AVR and
acted on. Brightness control by the AVR is something I'm working
towards so I can try to draw any object, no matter how big, only once
rather than drawing larger objectes many times to achieve equal
brightness with small objects.
I have the final anode voltage set at the midpoint voltage on the
deflection amplifiers. So a centred undeflected spot has
D1=D1'=D2=D2'=A3 voltage. I can trim the A3 voltage for best
astigmatism which I have found is slightly off the equal voltage rule, I
guess because one set of deflection plates are further down the tube as
you say. But I have not tried varying the voltages on the deflection
plates so D1=D1' != D2=D2' for a centred spot. The deflection
amplifiers are identical and fully interchangable at the moment.
Of course, I've not yet progressed to CRTs with a PDA.
Another change I'm planning to try is the way the octant information is
sent to the blanking circuits. At the moment I hae an 8 bit bus
straight from one AVR into a second that carries out the blanking
decisions. This is just like yours and Davids done in discrete logic
ICs. I'm going to use the SPI bus to serially send the data to the
second AVR. There are a couple of advantages, it frees up a lot of pins
on the main AVR that I can use for other things, but also the data sent
is then not limited to 8 bits. It become easy to, for example send 16
blanking bits and so a circle can be divided up into 16ths rather than
8ths. It does look like there is time to do this easily in the object
drawing cycle. The second AVR become a SPI slave and any data can be
sent to it.
My test trafo has been shipped from the USA and I'm hoping it'll be with
me before xmas so I can build and test the new PSU. Working towards
Scope Clock 2 Version 2 with ideas building for Scope Clock 3 (as
above). I also want to build and case a clock for the house so I have
something to show.
Sent one set of PCBs to the states, another set are promised. A guy on
Malta has asked me for a set, you can see is collection of CRTs and
radio equipment here
http://www.qrz.com/db/9H1GT
But I'm waiting for the new PSU before supplying my home made PCBs. All
good fun.
Cheers Grahame
On 12/12/2012 10:32, Oscilloclock wrote:
Wow, wow, wow!!
Grahame, I think if you are varying the grid current well within the design
range and you see focus vary significantly enough to notice, you have a PSU
problem.
Either you aren't supplying the right voltage to the anode, the deflection
plates are not at the right potential (in many tubes, X and Y plates should be
at slightly different potentials w.r.t. cathode due to their placement along
the neck of the tube), or your PSU is not regulating well (not able to supply
enough current).
I would measure all the voltages while you vary the grid bias and see if
anything changes significantly.
(Of course, even professional oscilloscopes and X-Y monitors exhibit this focus
change to some degree, at extreme intensities.)
(My <a href="http://oscilloclock.com/protoype ">Prototype</a> has this problem
in a big way, due to the super poor job I did winding my own transformer!
Aaron
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