Re: tek 7xx4 series brightness dimming after power on

Chuck Harris

There is a 5uf, wet tantalum capacitor, C171, in the z-axis output amplifier
that sometimes degrades to a condition between open and shorted. It has direct
control over the intensity.

Also everything down stream of that capacitor is the very high impedance DC
restorer circuitry. Any resistor 1M or higher is a reliability problem... as
is any capacitor that it services.

-Chuck Harris

Alberto, IZ2EWV wrote:

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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