Re: 7904 excessive shadow/flare


Hi Chuck and Max,
I have a more complete answer about the cause of the halo / flare after speaking to my friend. He designed the vertical deflection plates of the 7104.

A very important consideration with CRTs 30 or more years ago was their photographic writing rate since photographic film was the only way to capture fast transient events for further study. Photographic writing rate could be increased by raising the accelerating voltage of the CRT. Faster scopes, such as the 7904, required higher HV to transfer enough energy to each electron in the beam since the beam was sweeping across the CRT at faster sweep settings in the 7904.

Inside the CRT the electrons ae accelerated by the HV field. The first thing they strike is a very thin conductive aluminum layer. The aluminum is so thin most of the electrons pass through it and strike the phosphor where they excite the electrons in the phosphor atoms causing them to emit visible photons. The purpose of the aluminum layer is to draw off the charge which would otherwise accumulate on the phosphor layer and repel the beam of electrons.

Unfortunately, as the electrons in the beam are passing through the layer of aluminum there is a small chance some of them will encounter a nucleus. When that happens the electron is repelled (scattered at a random angle) with some force back out of the aluminum layer and into the vacuum of the CRT. It quickly begins to feel the pull of the HV field again and it starts accelerating towards the aluminum again, but it is no longer part of the original focused beam of electrons. It can be scattered fairly far from the original beam by the nucleus it hit. These back scattered electrons are now accelerated toward the aluminum some distance away from the main beam and in a completely unfocused form. This is what is causing the halo/flare you are seeing.

The halo / flare is the same shape as the main focused beam but it is fuzzier and removed from the main trace on the CRT.

Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of Chuck Harris
Sent: Wednesday, May 22, 2019 9:11 PM
Subject: Re: [TekScopes] 7904 excessive shadow/flare

The 7904, because of its very high anode voltage, and its mesh lens is one of the worst scopes in existence for reflections and flare inside of the CRT. But I like it anyway.

-Chuck Harris

unclebanjoman wrote:
No, the artifact is visible with any type of signal.
It also appears using the signal standardizer. I used the latter today to perform some calibrations on my 7904 and I was very irritated by this halo / flare effect.
With sweep rates of 0.1-0.5 sec and no signal applied is clearly visible the usual "globus" leading/lagging the spot.
Already with moderate brightness the artifact is visible.


Dennis Tillman W7PF
TekScopes Moderator

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