tek 7xx4 series brightness dimming after power on

Alberto, IZ2EWV

Hello dear friends,
I own three beautiful oscilloscopes from the series 7000: a 7904A, a 7104 and - the most recent arrival - a 7854.
Some of them had common issues, after powering on (failure at the RC snubber and the AC line filter - which I replaced with new components and everything come back to normality.
I ask you about a behavior that is common to the 7904A and the 7854: at power on, the traces and the readouts are bright, but after some minutes they dim: to restore the brightness it’s a matter of little correction on the related controls.
By the way, I ask you if it is a normal situation (e.g. “normal” at the time that the equipment was younger, by memory of somebody of yours) or if it is a sign of aging of some component in the main power supply or in the HV power supply. In this latter option, I ask you advice to let me get closer to the problem (I have the service manuals).

Tnx es 73 de Alberto, IZ2EWV

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