Re: A possibly sick 7904A
Your description is pretty accurate.toggle quoted messageShow quoted text
The more modern scopes have compensation circuitry
that changes the focus voltage with the instantaneous
intensity setting.... and changes the instantaneous
intensity settings for the different beam uses...
So, the intensity is separately controllable for the A
horizontal plugin, the B horizontal plugin, the readout,
and in some cases, the "intensified" horizontal A vs B
settings... but FOCUS, and ASTIGMATISM, each have only
one user control.
Each of these settings result in a different beam current,
which require a different focus/astigmatism voltage.
For the 7904A, which was one of the earlier adopters of
this enhancement, they have a circuit called Auto Focus,
and in the Performance check and Adjustment procedure,
page 5-12 of my manual, they explain how it gets adjusted.
The do the main focus adjustment with the beam very dim,
and then they adjust the Auto Focus circuit with the beam
at full intensity control position.
Roger Evans via groups.io wrote:
I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.