Re: 7904 - Very Dim Trace

Ed Breya
 

There are typically two ways to get DC-coupled Z-axis control on scope CRTs. One is to generate a high voltage, similar to that used for the cathode supply, and stack it onto the ground-referenced Z-axis signal. This is what's applied to the control grid. The other way is to use a DC-restorer circuit, which is basically an AC-coupled charge pump, with the AC swing proportional to the ground-referenced Z-axis signal. It's coupled capacitively and rectified, and stacked onto the cathode supply, then applied to the control grid.

Either way, the control grid is indeed usually negative with respect to the cathode, by maybe up to 100V or so, and it controls the beam current. There are pluses and minuses to each approach.

Ed

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