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VT320 Troubleshooting Guide

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The DC OK indicator inside the top vents and the four keyboard LEDs (DEC, EK-VT320-PS-001, Figure 1-1)

Fault-finding on the VT320 follows chapter 1 of DEC's VT320 Pocket Service Guide (EK-VT320-PS-001): a flowchart built around the power supply's DC OK indicator, a symptom table, the built-in self-tests and their error codes. DEC services the terminal at the level of three main units: the power supply assembly, the monitor and logic with arc protection assembly, and the CRT/bezel/yoke assembly.[1][2]

Warning: the CRT anode holds a high voltage even after switch-off. DEC's method is to clip the anode discharge tool (29-24717-00) to the grounding braid on the rear of the CRT and hold its probe under the rubber anode cap, against the connector, for at least three seconds. See CRT Discharge Procedure. DEC also asks for a static protection kit (29-26246-00) when handling internal parts.[3][4]

Normal power-up

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At switch-on the keyboard LEDs flash on and off, the keyboard sounds a bell, all the LEDs go out, and "VT320 OK" appears on the screen. The message clears when a character other than XON, XOFF or NULL arrives, when a key is pressed, or after 30 minutes.[1][5] The green DC OK LED on the power supply can be seen through the ventilation slots on top of the terminal.[1]

Nothing on the screen

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DEC's flowchart:[6]

  1. Check the installation and run the power-up self-test. If it passes, run the loopback tests and use the symptom table below.
  2. If it fails, remove the access cover, check the internal connections against the diagram in Appendix C, and power up.
  3. If DC OK is on and an error code appears, go to the error codes below. If DC OK is off, or no error code appears, continue.
  4. Switch off and unplug. Disconnect the power supply harness from the power supply.
  5. Plug in and switch on with the harness disconnected.
  6. If DC OK still does not light, replace the power supply assembly.
  7. If DC OK lights, the supply is working without its load: replace the monitor and logic with arc protection assembly, and if that does not cure it, the CRT/bezel/yoke assembly.

Symptom table

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Troubleshooting the VT320 (from DEC Table 1-1)
Symptom Probable cause Action
Nothing on the screen Power supply; CRT/bezel/yoke Follow the flowchart above
VT320 OK shown, DC OK on, bell sounds, but no communication Loose cable; port circuits; wrong baud rate Check the cable; run the loopback tests; check the speeds in Communications Set-Up
Horizontal or vertical line Monitor and logic assembly; CRT yoke harness connector Reseat the yoke connector, then replace the monitor and logic assembly
Display distorted Monitor out of alignment; monitor and logic assembly; CRT and yoke Align the monitor (VT320 CRT Service Guide), then replace in that order
Display jittery Monitor and logic assembly Replace it
No bell, but VT320 OK appears Keyboard cable; keyboard transducer Check the cable at the right side of the terminal; replace the keyboard
Wrong characters in local mode Alternate character set; wrong keyboard language (international); keyboard; monitor and logic assembly Reset in the Set-Up Directory; check the language setting
Wrong characters on line, correct in local Speed, bits, parity or stop bits wrong Match the host in Communications Set-Up
Terminal appears locked – Select Clear Comm, then Reset, in the Set-Up Directory
Screen blank except a blinking block in the lower right corner CRT saver after half an hour without activity Press any key
Messages garbled XON/XOFF not enabled; monitor and logic assembly Enable XON/XOFF in Communications Set-Up
No response to escape sequences Wrong mode Select VT100 mode in General Set-Up
One or more keyboard LEDs stay on after power-up, or the LEDs do not run their power-up sequence Keyboard; monitor and logic assembly Replace in that order
Set-up cursor does not appear Monitor and logic assembly Replace it

[7]

Self-tests

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Before any test, enter set-up and set the terminal to Local and VT100 mode; restore the user's settings afterwards. Running the power-up test resets the set-up features to their saved values.[8][5]

VT320 self-tests (DEC Table 1-2)
Test Once Continuously Needs
Power-up ESC [4;1y ESC [4;1;9y –
RS-232 port data lines (international) ESC [4;2y ESC [4;2;9y RS-232 loopback 12-15336-00
RS-232 port control lines (international) ESC [4;6y ESC [4;6;9y RS-232 loopback 12-15336-00
Printer port ESC [4;3y ESC [4;3;9y DEC-423 loopback 12-25083-01
DEC-423 port ESC [4;7y ESC [4;7;9y DEC-423 loopback 12-25083-01
Screen alignment pattern ESC # 8 – –
All except screen alignment ESC [4;0y ESC [4;0;9y All loopback connectors fitted

[9]

A continuously running test stops only on an error or at switch-off, and the keyboard gives no bell during it.[5] A passing loopback test ends with a bell, the LEDs off and "VT320 OK".[10]

Error codes

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VT320 display error codes (DEC Table 1-3)
Message DEC's solution
VT320 NVR Error - 1 In the Set-Up Directory select Default, then Save; leave set-up; switch off and on. If the error continues, replace the monitor and logic assembly
VT320 RS232 Port Data Error - 2 Replace the monitor and logic assembly
VT320 RS232 Port Controls Error - 3 Replace the monitor and logic assembly
VT320 Keyboard Error - 4 Check the keyboard is plugged in; replace the keyboard; then the monitor and logic assembly
VT320 DEC-423 Port Error - 5 Replace the monitor and logic assembly
VT320 Printer Port Error - 6 Replace the monitor and logic assembly

[11]

If the screen itself is faulty, look at the keyboard LEDs instead. DEC replaces the monitor and logic assembly if one or more LEDs stay on at the end of a test, if the LEDs flash the same pattern repeatedly, or if they never light.[12]

Printer problems

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Run the power-up test and then the printer port loopback test; if both pass, the terminal is working. Then test the printer, check that its speed, data bits and parity match the Printer Set-Up screen and that "Printer Ready" shows on the status line, check the cable, and finally print the ESC # 8 test pattern with the Print Screen key.[13]

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References

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  1. ↑ 1.0 1.1 1.2 Digital Equipment Corporation, VT320 Pocket Service Guide, EK-VT320-PS-001, first edition, July 1987, pp. 1 to 2, section 1.2 and Figure 1-1 (File:VT320 Pocket Service Guide (EK-VT320-PS-001).pdf).
  2. ↑ DEC, EK-VT320-PS-001, p. 48, recommended spares list.
  3. ↑ DEC, EK-VT320-PS-001, p. 19, section 2.1. Static protection kit.
  4. ↑ DEC, EK-VT320-PS-001, p. 25, section 2.4. Anode discharge.
  5. ↑ 5.0 5.1 5.2 DEC, EK-VT320-PS-001, pp. 8 to 9, section 1.3.2. Power-up self-test.
  6. ↑ DEC, EK-VT320-PS-001, p. 3, Figure 1-2, "VT320 Troubleshooting Flowchart".
  7. ↑ DEC, EK-VT320-PS-001, pp. 4 to 6, Table 1-1, "Troubleshooting the VT320".
  8. ↑ DEC, EK-VT320-PS-001, p. 7, section 1.3.1.
  9. ↑ DEC, EK-VT320-PS-001, p. 10, Table 1-2, "VT320 Self-Test Chart".
  10. ↑ DEC, EK-VT320-PS-001, pp. 11 to 15, sections 1.3.3 to 1.3.6. Loopback tests.
  11. ↑ DEC, EK-VT320-PS-001, p. 18, Table 1-3, "VT320 Display Error Codes".
  12. ↑ DEC, EK-VT320-PS-001, p. 17, section 1.5. LED indications.
  13. ↑ DEC, EK-VT320-PS-001, pp. 16 to 17, section 1.4. Printer problems.
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