Re: 2465B - Weak Readout Intensity


flanneltuba@...
 

Really both the Readout and the Display intensities are weak. With the Display, that is, the sweeping trace, it is quite readable in normal light but maxes out at merely "readable"; as for the Readout, at its brightest level it is almost unreadable in normal room light. The Readout intensity control behaves as follows: In the Off, or center position of the control, there is of course no readout shown at all. As the Readout intensity pot is advanced either CW or CCW, it begins to display the Readout information, and achieves its brightest point at roughly 50% of the control's travel. Turning the Readout intensity control any further from this point does not increase the intensity, rather causes the Readout characters to become increasingly unfocused to the point of unreadability at the full CW or CCW stops.

As for the Display intensity control, it behaves in much the same fashion as the Readout control, where it will achieve peak intensity at about 50% of the control travel, and begin to lose focus (though not as badly as the Readout) through the reaming 50% of the range. It never gets to that point where the travelling trace begins for form a brightness corona from being turned all the way up, as normally functioning scopes tend to do.

I have probed the signal transition through this adjustment range from every point in the circuit from the DAC signal and hold op amps, through to the Display Sequencer and then the Z-Axis Amplifier and HV circuit, and have observed that the levels at all points follow the full range of change in the front panel Readout intensity control, from Off through Full without any noticeable deviations through this range. Hence, the arrival at the conclusion that the CRT may be at fault.

I already have the original CRT removed in anticipation of the arrival of the replacement. When the replacement arrives, I'll pop it in and the verdict should be obvious. Either I have a weak CRT, or I have a whole lot more head scratching to do! I tool a number of pictures, which I intend on posting when I can free up some time...perhaps this weekend.

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