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VT52 General Maintenance

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VT52 intensity control and fuse locations (EK-VT52-MM-002 Figure 5-1)
Rear of the VT52: intensity control (left) and fuse (right) (DEC, EK-VT52-MM-002, Figure 5-1)

The VT52 has few parts that wear: DEC says the keyboard and the key-click mechanism are its only moving parts and need no preventive maintenance.[1] Maintenance is cleaning, keeping the vents clear, and safe access to the Monitor/Power Supply module, which carries both the CRT drive and the linear power supply.

Warning: the CRT anode runs at 11,000 V, and the power supply has large filter capacitors. Unplug the terminal and discharge the CRT and the capacitors before handling the module; see CRT Discharge Procedure.[2][3]

Cleaning

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DEC's user maintenance instructions:[1]

  • Clean the shell with soap and water or any mild detergent. Do not use cleaners containing solvents.
  • Do not use excessive water. The case is not weatherproof, and liquids, coins or paper clips falling through its openings can upset the electronics.
  • The keyboard electronics sit close to the outside of the case. Rub the keys with a dry or barely damp cloth.
  • Do not remove the keycaps to clean them; the switch contacts can be damaged if they are refitted wrongly.

Ventilation

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The VT52 has no fan. Its two logic boards in the base and the monitor/power supply board at the rear are cooled by convection, and documents stacked on the flat top could block the vents and cause overheating.[4] DEC's later terminals have sloped tops to discourage this.[5] Keep the top clear.

Mains and fuse

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The US model runs on 100–126 V at 60 Hz. The European model is switch-selectable for 220 or 240 V nominal (191–238 V or 209–260 V) at 50 or 60 Hz. The voltage and power requirements are printed on the ON/OFF switch plate on the right-hand side.[6][7] The fuse is on the rear panel (Figure 5-1).[8]

DEC traces a flickering display, with the cursor present, to jumper W7 on the RUT module: remove W7 for 60 Hz mains and fit it for 50 Hz.[8]

Opening the terminal

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  1. Unplug the terminal and set the ON/OFF switch to OFF.
  2. Place the unit upside down on a bench.
  3. Remove the sliding cover over the line cord opening (two screws) and the base front screen (three screws).
  4. Remove the screws holding the rear louver plate (up to ten) and the ground leads.
  5. Slide the interface cable and its grommet out of the slot in the rear screen and undo the ground screw at the rear of the base screen.
  6. Turn the five quarter-turn fasteners 90° anticlockwise with a ¼ inch nut driver and lift the base off. The fasteners stay in place.

[9] Terminals up to Westfield serial number 3657 and Phoenix serial number 5400 have screws in place of the quarter-turn fasteners.[9]

With the base off, the RUT and DP modules are visible; the keyboard plugs into RUT connector J5, the monitor signals into DP J8 and the power supply into DP J9. The RUT and DP modules come out together and are separated on the bench; keep the small plastic clip that holds their corners.[9]

Monitor/Power Supply module

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To remove the module, take out the base and the RUT/DP pair, remove the louvered heat sink plate and the ground leads, then disconnect the 9-pin Mate-N-Lok connector, the CRT neck socket, the ground strap, the 6-pin yoke connector and the four wires to the two large capacitors.[3]

  • Discharge the filter capacitors first, with a screwdriver or clip lead to the chassis, connecting the ground end before touching the capacitor.[3]
  • The red wires go to the positive terminals of the two large capacitors; reconnect them the same way.[3]
  • Keep the small plastic spacer between the board and the shell and refit it with the board.[3]

The module's outputs are +15 V, −12 V, −5 V and +5 V. DEC's voltage table and the component tests for each rail are on VT52 Troubleshooting Guide; the display adjustments are on VT52 CRT Alignment Guide.[10]

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References

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  1. ↑ 1.0 1.1 DEC, EK-VT52-MM-002, section 3.3, pp. 3-9 to 3-10, "User Maintenance".
  2. ↑ DEC, EK-VT52-MM-002, section 4.3.3.2, p. 4-24. Flyback transformer, 11,000 V anode.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 DEC, EK-VT52-MM-002, section 5.3.3, pp. 5-15 to 5-18. Monitor/Power Supply module removal, capacitor discharge and wiring.
  4. ↑ "VT52", Wikipedia, en.wikipedia.org/wiki/VT52. Convection cooling and blocked vents.
  5. ↑ Paul Flo Williams, "25 Years of DEC Video Terminals", VT100.net, vt100.net/dec/vt_history. Sloped tops on later terminals.
  6. ↑ Digital Equipment Corporation, VT52 DECscope Maintenance Manual, EK-VT52-MM-002, July 1978, section 1.3, pp. 1-3 to 1-4 (File:VT52 DECscope Maintenance Manual (EK-VT52-MM-002).pdf). Line voltage and frequency.
  7. ↑ DEC, EK-VT52-MM-002, section 2.2. Voltage and power requirements on the ON/OFF switch plate.
  8. ↑ 8.0 8.1 DEC, EK-VT52-MM-002, pp. 5-3 to 5-7, Table 5-1 and Figure 5-1. Fuse location; W7 jumper.
  9. ↑ 9.0 9.1 9.2 DEC, EK-VT52-MM-002, sections 5.3.1 and 5.3.2, pp. 5-12 to 5-14. Base and RUT/DP removal, older serial numbers.
  10. ↑ DEC, EK-VT52-MM-002, section 4.3.3.3, p. 4-25, Table 4-6. Supply outputs.
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