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CRT ANALYSIS, RESTORATION & REJUVENATION

All CRTs received undergo a series of tests before any restorative work is performed.  Once accurate information about the gun's condition is obtained, normal output of the CRT is then determined.  The on-site unit used at Ataricade provides for seven levels of cathode restoration if the CRT electron gun is found to have one or more problems.  This ensures that only enough restoration needed to do the job is performed rather than doing too much and damaging the CRT further, or needlessly shortening its life.

Preliminary CRT testing is comprised of the following:

  • G1 Short: The G1 Short test checks for shorts and leakage between the first grid (G1 a.k.a. the control grid) and the cathode, and between G1 and the second grid (G2 a.k.a. the screen grid)

  • H-K Short: The H-K Short test checks for leakage path exists between the filament or heather (H) and cathode (K)

  • Cutoff Test & Lo Tracking: Together, the Cutoff & Lo Tracking tests thoroughly test the dynamic range of the electron gun (read more below)

  • Emission & Hi Tracking: Together with the Cutoff and Lo Tracking tests, the Emission and Hi Tracking tests thoroughly check the dynamic operating condition of color and monochrome CRTs (read more below)

  • Emission Life:  The Emission Life test provides an indication of the cathode's approximate remaining useful emission (read more below)

 

RESTORE FUNCTIONS

 

G1 SHORT: If a G1 short is found to exist it is burned away by discharging a large capacitor through the short.  This method ensures that an otherwise good electron gun does not get damaged because the capacitive discharge is self-limiting; the CRT does not draw current once the short opens.  For added safety, the capacitor will not discharge until the cathode has had time to cool. 

GUN RESTORATION: Restore functions are performed on only one gun at a time.  The is important because all three guns of a color CRT don't always need restoration, or the same amount of restoration.  Restoration is commenced at the bottom and then goes up from there.  After a gun receives an application of restoration, the gun is then retested.  If the gun has not been adequately restored, then the same level of restoration is then applied (and subsequently tested) before stepping-up to the next level.  The levels are comprised of:

  • Re-Activate
  • Low
  • Normal
  • High
  • Extended

REJUV:  Rejuvenation is the most severe form of cathode restoration, and is only used if none of the other progressive restoration steps produce restore current.  Rejuvenation works by discharging an RC network between the cathode and G1 with normal filament voltage applied.  This produces momentary, high positive bias. 

Rejuvenation often successfully breaks the contamination layer on severely contaminated cathodes to allow normal restore currents to occur.  Rejuvenation is NEVER applied to a cathode that is able to produce restore current, as the sudden, high positive bias surge can easily strip emitting material from the warm cathode. 

 

RESTORATION TIPS

 

For those that attempt to restore their own CRTS please keep in mind:

  • RESTORATION IS A SUBTRACTIVE PROCESS:  It does not add "new" emitting material to the cathode.  Removing the contaminating layer exposes fresh emitting material on the surface, allowing normal emission.

  • ALWAYS TEST A CRT BEFORE RESTORING IT - NEVER RESTORE A CRT THAT TESTS GOOD:  A poor picture can be caused by a bad CRT or by a chassis defect.  Needlessly restoring a CRT wastes time and can shorten the CRT's life.

  • MORE IS NOT BETTER: A CRT will be damaged by too much restoration.  Only use enough restoration to bring the CRT back to an acceptable level of performance.

  • DO NOT RESTORE A BRAND NEW OR NEWLY RE-BUILT CRT:  If a new or rebuilt CRT tests poorly, let it operate for at least 1 hour with normal filament voltage applied, and then retest it.

 

THEORY

 

MORE ABOUT CUTOFF: The Cutoff test dynamically reproduces the point where the electron gun just comes out of cutoff and begins to conduct current (this is the normal "black" picture level).  The Cutoff test applies a negative bias (respect to the cathode) to the control grid (G1), while the amount of positive (G2) voltage is adjusted.  The minimum cutoff point is specified by the CRT's manufacturer to be within a certain range of bias and G2 voltage.

A CRT gun can fail the Cutoff test two ways: 1) The gun cannot be brought up to the "Cutoff" area, or 2) The reading is uncontrollable, and quickly pegs full scale, or drifts drastically.  A gun that cannot be brought up for the "Cutoff" area has a worn or contaminated cathode.  The CRT will have poor contrast (too dark black & grays) if all three guns are weak, or a bad gray scale if just one or two guns are weak The second cutoff test failure (uncontrollable reading that pegs full scale or drifts drastically) is caused by either an open G1 grid, or by air that has leaked into the CRT.

(Weak cutoff can often be improved through restoration)

MORE ABOUT LO TRACKING: In addition to specifying that each gun reaches cutoff within a certain Bias and G2 voltage range, color CRT manufacturers specify the ratio of the highest to lowest G2 voltage needed to produce cutoff.  The Lo Tracking test compares the ratio of the G2 voltages to adjusted Cutoff levels.  If all three guns are within range and the G2 voltages are within 1:25:1.  Failure occurs if any of the guns are outside of the specified range or if the G2 voltage ratio is too great (i.e. weak guns require more G2 voltage to reach cutoff). 

A color CRT that fails the Cutoff test will also fail the Lo Tracking test.  Color CRTs that pass the Cutoff test but then fail the Low Tracking test may not be able to be adjust for a good gray scale, depending on the adjustment range of the chassis setup controls.

(Lo Tracking failure can often be improved through restoration)

MORE ABOUT EMISSION: The Emission test measures how much current the electron gun produced at zero bias.  During the Emission test the negative bias is removed from the control grid (G1).  This allows the G2 voltage that was set during the Cutoff test to pull electrons from the cathode and stimulate maximum video drive (white picture levels).

The emission test of the unit employed at Ataricade uses an exclusive "sliding Good/Bad" scale which is based on the manufacturer's specified operating bias.  The test compares the gun's actual output to the normal expected output.

(Low Emission can often be improved through restoration)

MORE ABOUT HI TRACKING: All three guns in a color CRT must be properly balanced to produce a proper gray scale.  Computer monitors and color television receives have setup adjustments to balance the three guns, but these adjustments have a limited ranger.  If the emission from one gun is too much higher or lower than the others, the chassis adjustments will not have enough range to properly balance the guns.  The result is a picture with wrong colors and colored grays and white.  This is called poor gray scale or color tracking.

CRT manufacturers have established a ratio of 1:55:1 as the greatest variance between the strongest and weakest guns.  This simply means that the strongest gun cannot produce more than 55% more current than the weakest gun when all three guns are set to the same cutoff point.  During the Hi Tracking test the tracking ratio is calculated and emission readings for all three guns are displayed.  Failure is reported when one or two guns have weak emission, or if the ratio of emission currents is greater than 1:55:1.  A color CRT that fails the Emission test will also fail the High Tracking test. 

The Hi Tracking test is very similar to the Lo Tracking test in that both compare the conduction ratio of all three guns and the ability of the CRT to produce good gray scale (color) tracking.  The Lo Tracking test however, checks the guns at the point where they just begin to conduct.  A CRT that fails the Lo Tracking test will have gray scale problems in the darker (gray) levels.  The Hi Tracking test on the other hand,  checks the guns at full output.  A CRT that fails the Hi Tracking test will have gray scale problems in the brighter (white) levels.

(Poor tracking problems can often be improved through restoration)

MORE ABOUT EMISSION LIFE: The Emission Life test cools the cathodes slightly by decreasing the filament voltage by 25%.  A cooler cathode produces less emission, but most good cathodes produce more current than is needed for full beam current (white picture level).  Decreased filament voltage produces little or no reduction in the emission of good cathodes.  Cathodes that have lost emitting material and contaminated cathodes have little or reduced filament voltage .  A small amount of restoration often helps cathodes that fail the Emission Life test.

(A small amount of restoration often helps cathodes that fail the Emission Life test)


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Copyright 2004-2010 Francis Mariani. All rights reserved.