The Tube in my 576 displayed a similar behavior. Though no read out on the
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576. It was confirmed by others here as a high hour / weak tube.
On Mon, Feb 3, 2020, 5:04 PM <firstname.lastname@example.org> wrote:
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
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.