Re: 7904 excessive shadow/flare

Chuck Harris

The dome mesh is an anode, though it is called a "Slot Lens", on
the schematic. It is at pin 4 on the CRT.

I don't know if "Slot Lens" is the more formal name, or if perhaps
it isn't a dome?

The traditional dome mesh lens, as I have seen them is a super fine
stainless steel window screen with a pronounced bow in the screen
direction. Maybe the 7904 is using a special lens that only magnifies
the Y direction, and not the X direction, using slots instead of holes,
like the mesh lens?

I haven't broken a 7904 CRT as yet, so I haven't seen the lens.

They use two quad pole lenses to focus the beam before and after
the vertical plates.

I looked at my 7904 and 7854 more closely, and the 7904 is adjusted
to have a much hotter beam, just about always, than the 7854. The
way I can tell, is the beam spot, although P31 phosphor, is always
a lot bluer than the 7854, which is simply a boring green spot on a
green trace.

The 7854 is also 100MHz lower bandwidth than the screaming 500MHz
7904. It is different, even though it uses the same CRT.

And, (there is always an and with me...) the 7854 is 10 years newer
than the 7904. Surely there were some CRT improvements in that time?

-Chuck Harris

unclebanjoman wrote:

On Thu, May 23, 2019 at 02:42 PM, Chuck Harris wrote:

I know that, but my 7904's beat my 7854 in the flare department
by easily double. They also have much brighter maximum intensity
than does the 7854.

I haven't explored the differences, but the 7904's all have a worse
time than *any* other scope I have ever seen.
O.K. indeed the artifact is two times more pronounced with my 7904 with respect to my 7854 (grossly speaking).
The question that arise is: why?
Both the two models share the same tube type.
The post acceleration voltage is the same (21 kV).
Catode voltage is the same.(I presume).

If this is due to the dome mesh, where is it located on the diagram?
is there a regulation that affects it?
On the 7904 scheme I also see an adjustment for the preset shield volt (I adjusted as per manual): what's the use?


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