Shorts Removal; Cathode Restoring - B+K precision 467 Instruction Manual

SHORTS REMOVAL
Picture tube guns are constructed with extremely small spacing between the
cathode rfnd the G1 (control) electrode. This close spacing, together with the
possibility of the cathode coating material becoming loose and bridging the gap
between the cathode and the control grid often results in a fairly low resistance
short between these elements. The presence of such a short will be immediately
detected in going through the normal test sequence with the Model 467, because
the leakage indicator lamps will glow brightly showing which cathode is shorted to
Gl. Upon moving the function switch to the RESTORE position another neon
lamp (Gl-K Short) will glow brightly indicating that a low resistance short does
exist (Fig. 5).
Such a short can usually be eliminated by means of a high energy discharge from
a capacitor. Pressing the REMOVE SHORTS button of the Model 467 places a
capacitor (previously charged to a high voltage) across the short (Fig. 6). When the
short has been successfully burned out, the Gl-K SHORT indicating lamp will not
glow.
51K
Fig. 5.
Gl-K short indicator lamp (simplified).
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Fig.
6. Gl-K short removal circuit (simplified).
CATHODE RESTORING
The most effective technique for cathode rejuvenation involves elevating the
heater voltage and simultaneously applying a high positive voltage between G1 and
cathode, thus causing a much greater than normal cathode current to flow.
Unfortunately, unless this process is very carefully controlled, the cathode can
easily be damaged and instead of improving performance, the emission capability of
the cathode is lowered or destroyed.
This problem is avoided in the Model 467 by pre-heating the cathode, and then
initiating the rejuvenation cycle by means of a pushbutton which applies the high
rejuvenation voltage to the control electrode, and simultaneously removes the
heater voltage (Fig. 7). This permits a high rejuvenation current to begin flowing
immediately as the rejuvenation button is pressed, but permits the current to flow
only as long as the cathode retains sufficient heat to maintain the temperature
required for emission. The rejuvenation cycle is therefore automatically terminated
when the cathode cools below the point at which emission can be sustained. This
time interval will depend upon both the condition of the cathode emitting surface,
and the thermal characteristic of the cathode structure. The larger, more rugged
cathodes will result in a longer rejuvenation cycle and, of course, they are the ones
that need and can withstand the longer time interval with less danger of being
damaged. Therefore, the cathode structure itself serves as the automatic timing
element that adjusts the timing interval according to its own needs. This technique,
which prevents simultaneous application of heater and rejuvenate voltage is an
exclusive (Patent Applied For) feature of the Model 467's cathode restoring system.
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