HP9836C colour alignment (grey scale tracking)

[Tony Duell]

> 
> Tony Duell wrote:
> 
> > I am wondering about measuring the average beam current when displaying 
> > the level 15 raster of each primary colour and making sure it's 
> > reasonable. Beam current is near enough the final anode current, so 
> > measuring that would do. But that would mean having a microammeter 
> > floating at 25kV wrt earth. What fun...
> 
> I haven't seen the circuit, but I'm guessing you'll find some handy 
> resistor around the cathode to sample the voltage across.  Obviously 

There is a resisotor in series with each cathode, but it's hard to get 
to, and I suspect connecting a meter across it would have an effect on 
the (20MHz+) signals that exist around that area. 

Measuring the final anode current owuld be easier in that respect since 
the anode is part of th esmoothing capacitor anyway, so there are no 
really high frequency signals around. On the other hand the charging 
currenct for said capacitor comes from a flyback-like transoformer (not 
strictly a flyback transformer, there are separate output transsitors for 
the deflection yoke and the transformer, the latter is only driven if 
there are currents in both sets of deflection coils, thus preventing a 
line being burnt on the screen if something fails). And it's going to be 
a fairly 'peaky' waveform.. Maybe an extra smoothing capacitor on the 
'input' side of the meter -- the transofrmer side -- would be a good 
idea. It only has to be 100pF or so, but of course rated at 25kV or more.

> this will be somewhat high due to the grid currents, but my gut feeling 
> is that they won't be significant compared to the anode current.

I think the first and second anode currents are very small compared to 
the final anode current. Any electrons collected on the first/second 
anode are effectively wasted (they don't contribute to the light output 
of the CRT), so it would be a good idea to keep them to a minimum ;-)

-tony