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VT100 CRT Alignment Guide

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Adjustment controls on the Ball Brothers monitor board (DEC, EK-VT100-TM-003, Figure 5-6)

Display alignment on the VT100 monitor covers the brightness range, yoke rotation, height, width, centring, linearity, vertical hold and focus. The procedures below follow section 5.3 of DEC's VT100 Series Video Terminal Technical Manual (EK-VT100-TM-003), which gives separate procedures for the two monitors fitted in production.[1]

Warning: DEC's procedure is carried out with the terminal running and the top cover off, close to the flyback transformer and the CRT anode lead at about 12 kV. Use the non-metallic alignment tool on the coils, read CRT Discharge Procedure before starting, and discharge the CRT before touching anything near the anode with the power off.[2][3]

Identify the monitor

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VT100 monitors
Monitor DEC part How to recognise it
Ball Brothers 30-14590 Flyback transformer on the horizontal brace above the CRT neck; small connector board on the yoke
Elston 30-16080 Flyback transformer on the side plate opposite the monitor board; no connector card on the yoke

[4][1]

Early VT100s have the Ball monitor and later ones the Elston.[4] DEC gives one procedure for the Elston and DEC-built monitors; DEC's own monitor board (5414131) uses the same control designators, R109, R334, R342 and R477.[5][6]

Before you start

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DEC's conditions for every adjustment:[1]

  • Normal video, white characters on a dark background (SET-UP B, switch 1-3 = 0).
  • 80 characters per line.
  • ANSI mode (SET-UP B, switch 2-3 = 1).
  • The CRT alignment template taped to the screen.
  • The screen filled with E's, after the brightness has been set.

To fill the screen with E's, put the terminal in ANSI mode and LOCAL, then type ESC # 8. The pattern is generated inside the terminal and is not sent to the host.[7]

Check each adjustment in the order given, because many of them interact. If a check is already correct, skip that adjustment and go to the next.[1]

Templates

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DEC's mylar templates are 94-03220-03 for the display outline and 94-03270-03 for character height and width. Without them, make a paper template from the dimensions in Figure 5-4 of the manual: mark the centre of a piece of stiff card, measure out to the top, bottom and sides, draw the inner box and an outer box the width of the red shaded band, and cut out the band, leaving a short support in the middle of each side.[8]

To fit the template, mark the four edges of the screen on the glass with a water-soluble felt-tip pen, using the bezel as a guide. Remove the top cover and tape the template to the screen with its four corners the same distance from the marks. The template stays on for every step except, with a paper template, brightness and linearity.[8] When finished, remove the template and wipe the marks off the screen.[5]

The width and linearity coils need a non-metallic monitor alignment tool, DEC part 29-23190-00.[9][10]

Ball Brothers monitor

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Yoke collar clamp screw and vertical centring rings on the Ball monitor (DEC, EK-VT100-TM-003, Figure 5-7)
  1. Brightness. Let the terminal warm up for at least five minutes. In SET-UP, press the up-arrow key until the brightness stops rising. Turn R117 until the raster appears, then turn it back until the raster just disappears. Return the brightness to a medium level with the down-arrow key.[11]
  2. Yoke rotation. Check that the four sides of the display are parallel to the red shaded area of the template. If not, loosen the yoke collar clamp screw and turn the yoke until they are. Push the yoke fully forward towards the face of the CRT, hold it there and tighten the clamp screw.[12]
  3. Vertical height. The top and bottom lines should fall inside the red shaded area. Adjust R110.[9]
  4. Horizontal width. The first and last columns should fall inside the red shaded area. Adjust coil L103 with the alignment tool.[9]
  5. Centring. Turn A103 fully anticlockwise, then centre the display in the red area by rotating the centring tabs on the yoke. A103 shifts the display a little horizontally; set wrongly, it can distort the left side of the screen.[9]
  6. Vertical linearity. Using the character height template, check that characters near the four corners and in the centre fall between its minimum and maximum marks. Adjust A102, then recheck the height and the vertical hold.[9]
  7. Horizontal linearity. Check character widths in the same places with the width template. Adjust coil L102 with the alignment tool, then recheck the width.[9]
  8. Vertical hold. Turn A101 fully anticlockwise. If the display rolls or tears, turn A101 clockwise until it is stable and note the position; keep turning until it becomes unstable again, then set A101 halfway between the two points. Recheck height and vertical linearity.[5]
  9. Focus. Raise the brightness to maximum in SET-UP, then press the down-arrow key eight times. Every dot in the vertical stroke of each E should be visible at the four corners and the centre. If not, adjust R122 for the best overall result.[5]

DEC prints the tool number in the horizontal linearity step as 29-231290-00; the width step, the Elston width step and the spares list give 29-23190-00.[9][10]

Elston and DEC monitors

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Controls on the Elston monitor board (DEC, EK-VT100-TM-003, Figure 5-8)
  1. Brightness. Raise the brightness to maximum in SET-UP. Turn R109 until the raster appears, then back until it disappears. Return the brightness to normal with the down-arrow key.[5]
  2. Yoke rotation. As for the Ball monitor: loosen the yoke collar clamp screw, square the display to the template, push the yoke fully forward and tighten.[13]
  3. Vertical height. Adjust R334 until the top and bottom lines fall in the red shaded area.[13]
  4. Horizontal width. Adjust coil L403 with the alignment tool until the first and last columns fall in the red shaded area.[13]
  5. Centring. Rotate the frontmost centring rings on the CRT neck to centre the display in the red area.[14]
  6. Vertical linearity. Check character heights at the corners and centre with the height template and adjust R342, then recheck the height.[14]
  7. Focus. Raise the brightness to maximum, then press the down-arrow key eight times. Adjust R477 for the best overall characters, then adjust the focus rings furthest from the yoke for the best focus at all four corners, readjusting R477 if needed.[14]

The Elston procedure has no horizontal linearity or vertical hold step.[5][14]

DEC notes that some operators prefer the focus set slightly soft; if every dot is already visible, leave the control alone.[5]

Faults that adjustment will not cure

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Display faults needing parts
Symptom Cause Source
Display bows in or out at the edges Yoke pincushion misadjusted; DEC replaces the CRT and yoke assembly DEC Table 5-3[15]
Characters narrow on the left or right Monitor board, flyback transformer, CRT and yoke, or controller DEC Table 5-3[15]
Horizontal line only Vertical oscillator failed DEC section 5.6.4[16]
Blank white raster, ballooned display C439 and CR408 (Elston and DEC boards) Randy Hick, community[17]
Narrow screen L402 Randy Hick, community[17]
Very wide characters CR409 Randy Hick, community[17]
Overlap in the centre, narrow right side C437 Randy Hick, community[17]

See VT100 Troubleshooting Guide for the rest of DEC's symptom table.

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References

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  1. ↑ 1.0 1.1 1.2 1.3 DEC, EK-VT100-TM-003, pp. 5-24 to 5-25, sections 5.3 and 5.3.2. Adjustment conditions and order; Ball monitor identification.
  2. ↑ DEC, EK-VT100-TM-003, pp. 4-104 to 4-106, sections 4.8.1.10 and 4.8.2. Anode voltage about 12 kV.
  3. ↑ DEC, EK-VT100-TM-003, p. 5-63, section 5.4.25. CRT anode discharge.
  4. ↑ 4.0 4.1 DEC, EK-VT100-TM-003, p. 4-99, section 4.8. Monitors 30-14590 and 30-16080.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 DEC, EK-VT100-TM-003, p. 5-31, sections 5.3.2.8, 5.3.2.9, 5.3.3 and 5.3.3.1. Ball vertical hold and focus; Elston identification and brightness.
  6. ↑ Digital Equipment Corporation, VT100 Field Maintenance Print Set, MP00633, February 1982, VT100 monitor parts list 5414131-0-DBP, sheet A1 (PDF p. 55) (File:VT100 Field Maintenance Print Set (MP00633, Feb 1982).pdf). Potentiometers R109, R334, R342 and R477.
  7. ↑ DEC, EK-VT100-TM-003, pp. 5-20 to 5-21, section 5.2.7.7. Video adjust test.
  8. ↑ 8.0 8.1 DEC, EK-VT100-TM-003, pp. 5-25 to 5-27, sections 5.3.1.1 and 5.3.1.2, Figures 5-4 and 5-5. Template part numbers, paper template, attaching the template.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 DEC, EK-VT100-TM-003, p. 5-30, sections 5.3.2.3 to 5.3.2.7. Ball height, width, centring and linearity; alignment tool numbers as printed.
  10. ↑ 10.0 10.1 Digital Equipment Corporation, VT100 Mini Maintenance Hardware Guide, EK-VT100-J1-001, September 1978, p. 1-10, recommended spares list (File:VT100 Mini Maintenance Hardware Guide (EK-VT100-J1-001).pdf). Monitor alignment tool 29-23190-00.
  11. ↑ DEC, EK-VT100-TM-003, p. 5-28, section 5.3.2.1 and Figure 5-6. Ball brightness.
  12. ↑ DEC, EK-VT100-TM-003, p. 5-29, section 5.3.2.2 and Figure 5-7. Ball yoke rotation.
  13. ↑ 13.0 13.1 13.2 DEC, EK-VT100-TM-003, p. 5-32, sections 5.3.3.2 to 5.3.3.4 and Figure 5-8. Elston yoke, height and width.
  14. ↑ 14.0 14.1 14.2 14.3 DEC, EK-VT100-TM-003, pp. 5-33 to 5-34, sections 5.3.3.5 to 5.3.3.7. Elston centring, vertical linearity and focus.
  15. ↑ 15.0 15.1 Digital Equipment Corporation, VT100 Series Video Terminal Technical Manual, EK-VT100-TM-003, third edition, July 1982, pp. 5-5 to 5-6, Table 5-3 (File:VT100 Series Technical Manual (EK-VT100-TM-003).pdf). Display symptoms.
  16. ↑ DEC, EK-VT100-TM-003, p. 5-78, section 5.6.4. CRT monitor troubleshooting.
  17. ↑ 17.0 17.1 17.2 17.3 Randy Hick, "VT100 trouble analysis of video monitor board", text file hosted at bitsavers.org. Community repair notes.
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