VT220 Troubleshooting Guide
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Fault-finding on the VT220 uses the terminal's own self-tests, its screen and keyboard-LED error codes, and the troubleshooting chart in DEC's VT220 Pocket Service Guide (EK-VT220-PS-003). DEC services the terminal at the level of field-replaceable units: the terminal controller board, the power supply/monitor board, the flyback transformer (models A to C) or mains transformer (models D to F), the CRT/bezel/yoke assembly and the LK201 keyboard.[1][2][3]
Warning: with the access cover off, the CRT anode is exposed and may hold a stored charge; the flyback produces 12,500 V for it. DEC's discharge method is to clip the anode discharge tool (part 29-24717-00) to the metal chassis, push the probe under the soft anode cap until it touches the connector, and hold it there for at least three seconds, taking care not to scratch the CRT. See CRT Discharge Procedure.[4][5][6]
Power-up self-test
[edit | edit source]A good power-up ends with the keyboard LEDs off, a bell tone, and the message VT220 OK in a box on the screen. The message clears when a character other than XON arrives, when Set-Up is pressed, or after 30 minutes.[1] If the test finds a fault, look at the screen first; if the screen cannot be used, read the keyboard LEDs.[7]
| Message | DEC's corrective action |
|---|---|
| VT220 NVR Error - 1 | Enter set-up, check the default values, power down and up; if the error stays, replace the terminal controller |
| VT220 EIA Port Data Error - 2 | Replace the terminal controller |
| VT220 EIA Port Controls Error - 3 | Replace the terminal controller |
| VT220 Keyboard Error - 4 | Check the keyboard is plugged in; then replace the keyboard, the terminal controller and the power supply/monitor board in turn |
| VT220 20 mA Port Error - 5 | Replace the terminal controller, then the power supply/monitor board |
| VT220 Printer Port Error - 6 | Replace the terminal controller, then the power supply/monitor board |
When the screen is unusable, the four keyboard LEDs (Hold Screen, Lock, Compose, Wait) flash an error pattern: on for one second, off for a third of a second. Every fixed pattern in DEC's Table 1-3 points to the terminal controller; a varying display that repeats one of the patterns points to the keyboard.[8][9]
Self-tests
[edit | edit source]Put the terminal in Local and VT100 mode in set-up, then type the sequence. Do not type the spaces shown in DEC's table.[10]
| Test | Once | Continuously | Needs |
|---|---|---|---|
| Power-up | ESC [4;1y | ESC [4;1;9y | – |
| EIA port data lines loopback | ESC [4;2y | ESC [4;2;9y | EIA loopback connector 12-15336-00 |
| Printer port loopback | ESC [4;3y | ESC [4;3;9y | Printer port loopback connector 29-24794-00 |
| EIA port control lines loopback | ESC [4;6y | ESC [4;6;9y | EIA loopback connector |
| 20 mA port loopback | ESC [4;7y | ESC [4;7;9y | 20 mA loopback connector 70-15503-00 |
| Screen alignment (screen of E's) | ESC # 8 | – | – |
| All except 20 mA and screen alignment | ESC [4;0y | – | Loopback connectors fitted |
After replacing or adjusting a unit, run all the tests again.[1]
Troubleshooting chart
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| No VT220 OK, power OK light off, no bell | No mains; power fuse; mains cord; power supply/monitor board | Try another outlet; replace the 2 A fuse; check the cord; then the board |
| No VT220 OK, power OK light on, no bell | Terminal controller | Replace the controller board |
| No VT220 OK, power OK light on, bell sounds | Brightness too low; terminal controller; monitor board; flyback transformer; CRT and yoke | Turn up contrast and brightness first; DEC replaces the whole terminal for a CRT and yoke fault |
| VT220 OK shown but no communication | Loose host cable; host port circuits; controller to power supply/monitor board connection | Check the cable; run the EIA or 20 mA loopback test; check the board-to-board connector |
| Random characters | Terminal controller | Replace the controller |
| Horizontal or vertical line | Monitor connectors; monitor circuit; CRT and yoke | Reseat the connectors, then replace the power supply/monitor board |
| Display distorted | Monitor out of alignment; monitor circuit | See VT220 CRT Adjustment Guide, then replace the board |
| Display jittery | Terminal controller; CRT and yoke; flyback transformer | Replace in that order |
| No bell | Keyboard speaker | Replace the keyboard |
| Wrong characters in local mode | Alternate character set selected; keyboard; controller | Clear with Reset in set-up |
| Wrong characters on line, correct in local | Speed, bits per character, parity or stop bits wrong | Match the host |
| Screen goes blank after half an hour, power OK light on | CRT saver | Press any key |
| No response to escape sequences | Wrong mode selected | Select VT100 mode in set-up |
Power supply
[edit | edit source]The power supply sits on the power supply/monitor board. On models A to C (board 5415651) it is a switching supply giving +5 V, +12 V and −12 V. Mains enters through an EMI filter, the on/off switch S1, the 115/230 V selector S2 and a 2 A 250 V fuse F1. A +12 V start-up circuit powers the pulse-width modulator until the +12 V output is running, and an overcurrent circuit watches the +5 V and +12 V outputs and +5 V for overvoltage.[12] DEC specifies a peak inrush of 25 A with the thermistors cold and an input of 60 W at most.[13][14]
A 26-pin flat cable joins the terminal controller to the power supply/monitor board, and a 2-wire cable runs to the power OK indicator on the front bezel. When the power OK light is off with mains present, check the 2 A fuse and the board. DEC's US and European fuse part numbers are 90-07215-00 and 12-19285-03.[15][2] Capacitor values for the board are on VT220 General Maintenance.
Monitor
[edit | edit source]Monitor faults in DEC's chart trace to the power supply/monitor board, the flyback transformer (models A to C), or the CRT and yoke, which DEC did not repair in the field.[11] Before replacing parts for a dim, distorted or unstable display, run the alignment in VT220 CRT Adjustment Guide; DEC notes that the alignment pattern may not be distinct at first, and that the alignment steps themselves correct this.[16]
The flyback transformer on models A to C is held by two nuts and connects to the board by a 7-wire connector. When refitting it, replace the tie wrap that holds the anode lead to the CRT.[5]
Keyboard
[edit | edit source]The LK201 runs its own self-test in under 70 ms at power-up: all four LEDs light, blink once, and go off if the test passes. LEDs that stay lit mean the keyboard has failed its test.[17] The keyboard takes +12 V from the terminal through the coiled cable.[17] For "VT220 Keyboard Error - 4", check the cable first, then substitute the keyboard.[8]
Related pages
[edit | edit source]- VT220
- VT220 General Maintenance
- VT220 CRT Adjustment Guide
- VT220 Documentation
- CRT Discharge Procedure
References
[edit | edit source]- ↑ 1.0 1.1 1.2 Digital Equipment Corporation, VT220 Pocket Service Guide, EK-VT220-PS-003, third edition, August 1986, p. 1, sections 1.2 and 1.3 (File:VT220 Pocket Service Guide (EK-VT220-PS-003).pdf). Self-tests and a successful power-up.
- ↑ 2.0 2.1 2.2 DEC, EK-VT220-PS-003, p. 43, Table 2-1, recommended spares, models A, B and C.
- ↑ DEC, EK-VT220-PS-003, p. 56, Table 3-1, recommended spares, models D, E and F.
- ↑ DEC, EK-VT220-PS-003, p. 24, section 2.2. Access cover warning.
- ↑ 5.0 5.1 DEC, EK-VT220-PS-003, pp. 30 to 33, section 2.5. Anode discharge and flyback transformer removal.
- ↑ DEC, EK-VT220-TM-001, p. 8-11, section 8.2.3. Anode 12,500 V.
- ↑ DEC, EK-VT220-PS-003, p. 3. Reading the screen and LED errors.
- ↑ 8.0 8.1 8.2 DEC, EK-VT220-PS-003, p. 15, section 1.11 and Table 1-2.
- ↑ DEC, EK-VT220-PS-003, p. 16, Table 1-3, "VT220 LED Error Codes".
- ↑ 10.0 10.1 DEC, EK-VT220-PS-003, p. 2, Table 1-1, "Self-Test Chart".
- ↑ 11.0 11.1 DEC, EK-VT220-PS-003, pp. 17 to 19, Table 1-4, "VT220 Troubleshooting Chart".
- ↑ DEC, EK-VT220-TM-001, pp. 9-1 to 9-4, sections 9.1 to 9.2.2. AC input components and conversion circuits.
- ↑ DEC, EK-VT220-TM-001, pp. 9-5 to 9-6, Table 9-1, "Input Voltage Specifications".
- ↑ DEC, EK-VT220-TM-001, p. A-2, Appendix A. Electrical specifications.
- ↑ DEC, EK-VT220-PS-003, pp. 26 to 29, section 2.4. Power supply/monitor board connections.
- ↑ DEC, EK-VT220-PS-003, p. 59, section 4.2.1. Alignment pattern note.
- ↑ 17.0 17.1 Digital Equipment Corporation, VT220 Technical Manual, EK-VT220-TM-001, November 1984, pp. 7-2 and 7-6 (File:VT220 Technical Manual (EK-VT220-TM-001).pdf). LK201 power and self-test.