Re: 7904 excessive shadow/flare

 

Hi Max,
The expansion mesh was an engineering compromise which had significant drawbacks but all the alternatives had worse drawbacks.
Tek scopes were always known for their razor sharp trace. When the 7000 series was being designed it became apparent that the high frequencies the scope had to attain were going to require a new breakthrough if the length of the CRTs were going to be reasonable. The result was the expansion mesh which, unfortunately traded the super sharp trace for higher bandwidth.

An alternative is available however. The 7104 does not use the expansion mesh. It uses a box lens so the trace is finer. It also has the Micro Channel Plate which improved the brightness of the trace so much it was now possible to see a single 300pSec pulse in normal room light at 200pSec/Div sweep speed.

You need to get yourself a 7104. There is so much more it can do than a 7904.

Dennis Tillman W7PF

-----Original Message-----
From: unclebanjoman, Sent: Friday, May 24, 2019 6:21 AM

O.K.! Thanks to everyone for the comprehensive answers, very instructive to me!

Undoubtedly the driving circuits of the 7904 and 7804 CRTs are completely different and this could partly explain the different behavior regarding the halo / flare effect.
Over time I will get used to the remarkable brilliance of my new 7904 (and his associated halo /flares) which, moreover, is really in excellent condition and after careful calibration it matches perfectly the manual's specifications.

My 7904 has serial number B269718; reading the manufacture date of a pair of TI SNxxxx ICs, it should be from 1979.

If another 7904 to be repaired happens to me, I will be curious to observe the halo / flare effect, whether it is identical or not. I will let you know if the case.

Thanks again,
Max




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Dennis Tillman W7PF
TekScopes Moderator

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