Re: 475 Grid Bias problem

Jeff Davis

To recap:

The beam finder doesn't work, although all other functions of the scope do. When pressed, the beam finder blanks the display. After determining that the correct voltage comes out of the Z-axis amplifier when the beam finder is pressed, we proceeded down the path of looking into whether the DC restorer was getting the right voltage to the grid. It appears that it is.

In order to get a visible trace, the intensity setting has to be much higher than normal - the exact value depending on the value of the grid bias. At this point, I cannot set the grid bias per the calibration procedure because at the recommended intensity setting (+15V), no setting of the grid bias brings the dot into view.

I've confirmed the cathode, grid, anode, vertical shield, vertical deflection, horizontal deflection, and mesh voltages.
I haven't checked focus, astig, or geometry voltages. However, all of those functions appear to be OK on the trace.
From: <> on behalf of David [TekScopes] <>
Sent: Sunday, August 20, 2017 5:29 PM
Subject: Re: [TekScopes] Re: 475 Grid Bias problem

The voltage which matters is the difference between the control grid
and cathode voltage which comes out to 79 volts in both cases.

Note that you can measure this voltage without a high voltage probe
using a handheld voltmeter connected to the grid and cathode while
being isolated. Its 10 megohm input resistance will shunt 22 megohm
resistor R1378 however which may disturb the measurement.

Are the voltages at the other CRT elements correct?

At this point, I am confused about what the original problem was.

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