27 Sep
2008
27 Sep
'08
1:58 p.m.
Tony Duell wrote:
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.
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 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