VT220 CRT Adjustment Guide
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The VT220 monitor is aligned with the procedure in chapter 4 of DEC's VT220 Pocket Service Guide (EK-VT220-PS-003). The procedure, the control designators and the target dimensions are the same for every VT220 model.[1][2]
Warning: the adjustments are made with the access cover off and the terminal running, close to the flyback transformer and the CRT anode lead at 12,500 V. Stand the terminal on a non-conductive surface as DEC requires, use the non-metallic tuning wand on the width coil, and read CRT Discharge Procedure before starting.[3][4]
Controls
[edit | edit source]| Label | Control | Designator |
|---|---|---|
| VL | Vertical linearity | R307 |
| VH | Vertical height | R303 |
| FO | Focus | R221 |
| HH | Horizontal hold | R203 |
| PH | Horizontal centring (phase) | R211 |
| HW | Horizontal width coil | L201 |
| – | Brightness and contrast | User controls with the tilt adjustment (Figure 4-2) |
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DEC's technical manual describes R203 as the adjustable control of the horizontal hold network, R211 as the horizontal centring (phase) control, L201 as the adjustable coil that sets the horizontal trace width, and R307 as the vertical linearity control.[7][8][9] There is no vertical hold adjustment.[10]
Tools in DEC's list: a video alignment tool (29-23190-00), a tuning wand (29-23189-00) for the width coil, a flexible metric ruler (29-25342-00) and an alignment template (29-24371-00).[11]
Preparation
[edit | edit source]- Remove the access cover.
- Place the terminal on a non-conductive surface, as shown in DEC's Figure 4-1.
- Reconnect the keyboard cable and the power cord and switch on.
- In set-up select reverse video (black characters on a white background), 80 columns, VT100 mode and Local.
- When VT220 OK appears, type ESC # 8. The screen fills with capital E's.
[2][3][5] Check every adjustment in order, because many of them affect each other; skip any that is already right.[2]
Character quality
[edit | edit source]- Brightness. Let the terminal warm up for at least five minutes. Turn the brightness and contrast controls to maximum, then reduce the brightness until the white diagonal raster lines just disappear.[12]
- Contrast. Set the contrast for the screen intensity the user wants.[12]
- Vertical linearity. Check the character height at the four corners and the centre; all should be 3.7 ± 0.4 mm. Adjust VL (R307) until they match.[10]
- Focus. Check that the individual dots in the vertical strokes of each E are visible at the corners and centre, with the screen clean, since dirt can look like poor focus. Adjust FO (R221) for the best overall result. DEC allows the focus to be set to the user's preference.[10]
Horizontal hold
[edit | edit source]Look for tearing or other horizontal instability, and adjust HH (R203) until the display is stable.[10]
Centring
[edit | edit source]- Horizontal. With the flexible ruler, measure from the middle of the left edge of the pattern to the bezel, then the same on the right. If the two differ by more than 6 mm, adjust PH (R211) and measure again.[13]
- Vertical. Measure from the centre character of the top row to the bezel, then from the bottom row. If either distance is less than 3 mm, centre the display by turning the centring rings on the CRT neck with their tabs, then repeat the horizontal centring to bring PH back into range.[13]
Size
[edit | edit source]- Width. Measure one row of the E pattern. If it is not 203.2 mm, turn the width coil L201 with the tuning wand (29-23189-00) in the hexagonal hole of the coil.[6]
- Height. Measure one column of the pattern. If it is not 127 mm, adjust VH (R303), then recheck the vertical linearity.[14]
The 203.2 × 127 mm targets are the active display area in DEC's specification (8 × 5 in).[15]
When adjustment is not enough
[edit | edit source]DEC's troubleshooting chart sends a display that stays distorted after alignment to the power supply/monitor board, and a jittery display to the terminal controller, the CRT and yoke, or (models A to C) the flyback transformer.[16] See VT220 Troubleshooting Guide.
Related pages
[edit | edit source]- VT220
- VT220 General Maintenance
- VT220 Troubleshooting Guide
- VT220 Documentation
- CRT Discharge Procedure
References
[edit | edit source]- ↑ DEC, EK-VT220-PS-003, p. ix. Alignment procedures are the same for all models.
- ↑ 2.0 2.1 2.2 DEC, EK-VT220-PS-003, p. 57, section 4.1. Order of adjustments and set-up conditions.
- ↑ 3.0 3.1 3.2 DEC, EK-VT220-PS-003, p. 58, Figure 4-1a and section 4.2.1. Control locations, models A to C; initial preparations.
- ↑ DEC, EK-VT220-TM-001, p. 8-11, section 8.2.3. Anode 12,500 V.
- ↑ 5.0 5.1 DEC, EK-VT220-PS-003, p. 59, Figure 4-1b. Control locations, models D to F; displaying the pattern.
- ↑ 6.0 6.1 DEC, EK-VT220-PS-003, p. 63, section 4.2.5.1 and Figure 4-4. Horizontal width.
- ↑ Digital Equipment Corporation, VT220 Technical Manual, EK-VT220-TM-001, November 1984, p. 8-4, section 8.2.2.1 (File:VT220 Technical Manual (EK-VT220-TM-001).pdf). Horizontal hold and centring networks.
- ↑ DEC, EK-VT220-TM-001, p. 8-8, section 8.2.2.3. L201 and L202 in the horizontal output circuit.
- ↑ DEC, EK-VT220-TM-001, p. 8-14, section 8.2.4. Vertical linearity control R307.
- ↑ 10.0 10.1 10.2 10.3 DEC, EK-VT220-PS-003, p. 61, sections 4.2.2.3, 4.2.2.4 and 4.2.3. Vertical linearity, focus, horizontal hold.
- ↑ Digital Equipment Corporation, VT220 Pocket Service Guide, EK-VT220-PS-003, third edition, August 1986, p. x, "Tools Required" (File:VT220 Pocket Service Guide (EK-VT220-PS-003).pdf).
- ↑ 12.0 12.1 DEC, EK-VT220-PS-003, p. 60, sections 4.2.2.1 and 4.2.2.2 and Figure 4-2. Brightness and contrast.
- ↑ 13.0 13.1 DEC, EK-VT220-PS-003, pp. 62 to 63, section 4.2.4 and Figure 4-3. Centring.
- ↑ DEC, EK-VT220-PS-003, p. 64, section 4.2.5.2. Vertical height.
- ↑ DEC, EK-VT220-TM-001, p. A-2, Appendix A. Active display size.
- ↑ DEC, EK-VT220-PS-003, pp. 17 to 18, Table 1-4.