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VT320 CRT Service Guide

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VT320 monitor controls and the CRT maker jumpers (DEC, EK-VT320-PS-001, Figure 3-1)

CRT service on the VT320 means discharging the anode, replacing the CRT/bezel/yoke assembly, matching the monitor board to the CRT maker, and aligning the display. DEC describes all four in chapters 2 and 3 of its VT320 Pocket Service Guide (EK-VT320-PS-001).[1][2]

Warning: DEC's alignment procedures are done with the access cover off and the terminal running, and expose the technician to high voltage at the CRT anode. Read CRT Discharge Procedure before starting.[3][4]

Discharging the anode

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DEC's method, with the terminal switched off and unplugged:[1]

  1. Attach the alligator clip of the anode discharge tool (29-24717-00) to the grounding braid on the rear of the CRT.
  2. Slide the tool under the rubber anode cap until it touches the anode connector, and hold it there for at least three seconds.
  3. Remove the tool.
Anode discharge on the VT320 (DEC, EK-VT320-PS-001, p. 25)

To release the anode connector, push it in the direction of one arrow and then the opposite way, as drawn in DEC's figure, to free the clips.[5]

Replacing the CRT/bezel/yoke assembly

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The CRT, bezel and yoke are one replaceable unit: 70-24240-01 paper white, 70-24240-02 green, 70-24240-03 amber.[6]

  1. Remove the access cover: place the terminal face down on clean paper, loosen the captive screws at the base of the bezel, and pull the cover away from the base.
  2. Switch off, unplug and discharge the anode as above.
  3. Unclip the anode connector from the CRT.
  4. Disconnect the two ground connectors from the power supply shield.
  5. Remove the arc protection board from the neck of the CRT.
  6. Unplug the yoke wiring harness from the monitor and logic assembly.
  7. Release the connector tabs at the base and lift the base away from the CRT/bezel/yoke assembly.

[7][1][8] Reverse the steps to fit the new assembly. CRTs were made by Philips or Clinton, and the jumper on the monitor and logic board must match the label on top of the tube: J7 (PHIL) for Philips, J8 (CLIN) for Clinton.[9][10]

Handling CRTs

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DEC's rules for anyone handling a CRT:[11]

  • Wear safety goggles and gloves (DEC parts 29-16141 and 29-16146).
  • Never lift a CRT by its neck; use both hands on the sides near the face, and keep it away from your body.
  • Do not let the neck strike anything, and do not rest the tube on its neck.
  • Keep tools such as screwdrivers and soldering irons off the glass.
  • Store CRTs in a closed container or mounted in the equipment.

DEC's procedure for scrapping a tube, crushing the evacuation point inside its sealed carton, was for DEC personnel at DEC facilities only and not for customers.[11][12]

Alignment

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Yoke clamp screw and centring rings (DEC, EK-VT320-PS-001, Figure 3-2)

DEC asks for every adjustment to be checked in the order below, since many interact; skip any that is already correct.[2]

Preparation

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  1. Remove the access cover, reconnect the keyboard and power, and switch on. Wait for "VT320 OK".
  2. In set-up, select Local in the Set-Up Directory and VT100 mode in General Set-Up.
  3. For the width, height and centring steps, also select "Dark Text, Light Screen" and "No Status Display" in Display Set-Up.
  4. Leave set-up and type ESC # 8. The screen fills with capital E's.

[3][10]

VT320 monitor controls (DEC Figure 3-1)
Control Designator
Screen (G2) R225
Sub-contrast R316
Width coil L201
Vertical size R105
Vertical linearity R111
Focus R218
Brightness, contrast User controls on the right side

[2]

Steps

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  1. Screen control. Check the CRT maker jumper (J7 or J8). Set the brightness control to maximum and the contrast to minimum, then adjust G2 (R225) until the white background raster almost disappears but is still visible. Set the brightness back to the user's preference.[10]
  2. Sub-contrast. Set the contrast to maximum and turn the brightness down until the raster just disappears. Adjust SUB CONTRAST (R316) until the characters are almost blurred but still in focus, then set the contrast to the user's preference.[10][4]
  3. Rotation. Loosen the yoke clamp screw on the CRT neck and turn the yoke gently until the display is square and level. Take up the slack in the clamp before tightening, and do not overtighten it, or the CRT neck may crack.[4][13]
  4. Horizontal width. Measure the reverse-video pattern of E's with a metric tape; it should be about 225 mm wide. Adjust the WIDTH coil (L201) with the tuning wand.[13]
  5. Vertical height. The pattern should be about 160 mm high. Adjust V SIZE (R105), then recheck the vertical linearity.[13]
  6. Vertical linearity. The E's on the top and bottom rows should look the same height. Adjust V LIN (R111).[13][14]
  7. Centring. Turn the centring rings on the CRT neck until the reverse-video pattern sits centrally in the bezel.[14]
  8. Focus. With a clean screen, check that the dots in the vertical strokes of the E's can be seen at the four corners and the centre. Adjust FOCUS (R218).[14]

DEC's tool list gives the tuning wand as 29-23189-00 and the video alignment tool as 29-26128-00; the width step names the tuning wand but prints 29-26128-00.[15][13]

When alignment does not cure it

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DEC's troubleshooting table sends a display that stays distorted after alignment to the monitor and logic assembly and then the CRT and yoke; a horizontal or vertical line to the yoke harness connector and the monitor and logic assembly; and a jittery display to the monitor and logic assembly. See VT320 Troubleshooting Guide.[16]

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References

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  1. ↑ 1.0 1.1 1.2 DEC, EK-VT320-PS-001, p. 25, section 2.4. CRT/bezel/yoke removal and anode discharge.
  2. ↑ 2.0 2.1 2.2 DEC, EK-VT320-PS-001, p. 35, sections 3.1 and 3.2 and Figure 3-1.
  3. ↑ 3.0 3.1 DEC, EK-VT320-PS-001, p. 36, sections 3.2.1 and 3.2.2. Warning and preparation.
  4. ↑ 4.0 4.1 4.2 DEC, EK-VT320-PS-001, p. 38, sections 3.2.4 and 3.2.5 and Figure 3-2.
  5. ↑ DEC, EK-VT320-PS-001, p. 26, section 2.4. Anode connector removal.
  6. ↑ DEC, EK-VT320-PS-001, p. 48, recommended spares list.
  7. ↑ DEC, EK-VT320-PS-001, pp. 19 to 21, section 2.2. Access cover.
  8. ↑ DEC, EK-VT320-PS-001, pp. 27 to 28, section 2.4. Arc protection board, yoke harness, base.
  9. ↑ DEC, EK-VT320-PS-001, p. 33, section 2.5. CRT maker jumpers J7 and J8.
  10. ↑ 10.0 10.1 10.2 10.3 DEC, EK-VT320-PS-001, p. 37, sections 3.2.2 to 3.2.4.
  11. ↑ 11.0 11.1 DEC, EK-VT320-PS-001, pp. 28 to 29, section 2.4.1. CRT handling guidelines.
  12. ↑ DEC, EK-VT320-PS-001, pp. 30 to 31, section 2.4.1. CRT disposal at a Digital facility.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 DEC, EK-VT320-PS-001, p. 39, sections 3.2.5 to 3.2.8.
  14. ↑ 14.0 14.1 14.2 DEC, EK-VT320-PS-001, p. 40, sections 3.2.8 to 3.2.10.
  15. ↑ Digital Equipment Corporation, VT320 Pocket Service Guide, EK-VT320-PS-001, first edition, July 1987, p. vi, "Tools Required" (File:VT320 Pocket Service Guide (EK-VT320-PS-001).pdf).
  16. ↑ DEC, EK-VT320-PS-001, pp. 4 to 5, Table 1-1.
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