Re: A possibly sick 7904A

Dave Daniel

AFAIK, the 7904A does *not* use and MCP CRT. The 7104 and 2467 do use MCP CRTs.


On Aug 19, 2020, at 15:24, Eric <ericsp@...> wrote:

Looks like my frame needs a bit of a tune up then. Ill be working on this once I get the current plugin done. I have shied away from the MCP CRT as read some where that those tubes were all on borrowed time as even at very low levels on intensity there was some slight phosphor damage. Not sure if this turned out to be the case.


On 8/19/2020 8:48 AM, Chuck Harris wrote:
Your description is pretty accurate.

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.

-Chuck Harris

Roger Evans via 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.

I tried putting a fast rise square wave into my 7904 at 1kHz and qualitatively I see what you see. I have to turn up the brightness a lot to see the trace at 1ns/div and it does defocus compared with the readout. I can improve the focus slightly and that worsens the focus of the readout significantly.

Maybe if we get some more comments we can decide who is unrealistic and who is easily satisfied. I have no idea what the factory specifications were.



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