Jump to content

VT100 General Maintenance

From RetroTechCollection
DEC VT100 interior with CRT and anode cap
A VT100 with the top cover removed. The anode cap on the CRT carries about 12 kV (Amber, CC BY-SA 4.0)

The VT100 needs little routine care: DEC says the keyboard keys are its only moving parts and need no preventive maintenance.[1] Work inside the case is mostly about the two assemblies that age, the H7831 switching power supply and the CRT monitor, and about checking them safely.

Warning: the CRT anode is charged to about 11.5 to 12 kV, and the power supply's reservoir capacitors C9 and C14 hold about 300 V DC. Unplug the terminal and read CRT Discharge Procedure before opening it.[2][3]

Cleaning

[edit | edit source]

DEC's user maintenance instructions:[1]

  • Clean the case with soap and water or a mild detergent. Do not use cleaners containing solvents.
  • Use as little water as possible; the case is not sealed and liquid or metal objects falling through the vents can upset the electronics.
  • Clean the keys with a dry or barely damp cloth. DEC advises against removing keycaps for cleaning, since the switch contacts can be damaged if they are refitted wrongly.
  • Keep the ventilation slots clear. Objects on top of or under the terminal can make it overheat.

Inside the terminal, check that every board edge contact is clean and firmly seated; DEC lists this among the first checks when the controller misbehaves.[4]

Opening the case

[edit | edit source]
  1. Disconnect the mains plug.
  2. Release the two front pop fasteners under the front lip with a small flat-blade screwdriver, then the two rear pop fasteners on the lower rear edge of the bottom cover.
  3. Lift the top cover straight up.

[5] To remove the power supply, take off the top and bottom covers and pull the plungers of the three pop fasteners on the side of the chassis, then draw the supply out by its end plate. On refitting, the grounding tab at the back of the chassis must enter the slot in the end of the supply's switch plate.[3]

VT100 power supply capacitor discharge points (EK-VT100-TM-003 Figure 5-25)
Shorting points for C9 and C14 on the removed power supply (DEC, EK-VT100-TM-003, Figure 5-25)

DEC's instruction for the reservoir capacitors is to leave the power supply plugged into the terminal for at least four minutes after removing the mains cord, so that the circuit discharges them, and to short them with an insulated wire once the supply is out.[3] Bleeder resistors R19 and R20 (100 kΩ) are fitted across them.[6][7]

Power supply

[edit | edit source]

Mains setting and fuse

[edit | edit source]

The 120/230 V switch on the power supply selects a voltage doubler (120 V) or a bridge rectifier (230 V). DEC lists connecting a supply set for 115 V to 230 V mains among the stresses that make the switching transistor fail.[6][8] The mains fuse is 3 A, DEC part 90-07217; DEC's documents disagree on its type, the technical manual saying normal blow and the field interconnection diagram fast blow.[9][2][10]

H7831 capacitors

[edit | edit source]

The H7831 (module 54-12996) carries these electrolytic and tantalum capacitors. Values are from the February 1982 print set; earlier boards may differ.[7]

H7831 electrolytic and tantalum capacitors (DEC parts list 5412996-0-DBP rev. P)
Reference Value Rating Type DEC part
C9, C14 590 µF 200 V Aluminium electrolytic 1014861-00
C18 20 µF 200 V Aluminium electrolytic 1012607-05
C20 270 µF 40 V Aluminium electrolytic 1012607-04
C22 1 µF 100 V Aluminium electrolytic 1005507-00
C26 560 µF 20 V Aluminium electrolytic 1012607-00
C31 3800 µF 6.3 V Aluminium electrolytic 1014277-00
C33 470 µF 50 V Aluminium electrolytic 1014883-00
C17 1 µF 35 V Solid tantalum 1001776-00
C34 15 µF 20 V Solid tantalum 1004812-00

[7]

C9 and C14 are the reservoir capacitors on the rectified mains. C31 smooths the +5 V output and C26 is in the +12 V control loop; DEC names both when the outputs oscillate. C33 filters the start-up supply that feeds the +12 V regulator Z2.[6][8] See Capacitor Failure Symptoms for what aged parts look like, and check the board you have against the parts list before ordering.

Checking the outputs

[edit | edit source]

The supply has no voltage adjustments. Measure the rails at the controller connector J6 and compare them with DEC's limits: +5 V ±5% on pins 18 to 22, +12 V ±5% on pin 7, −12 V ±5% on pin 9 and −23 V ±10% on pin 10.[11][2] Do not run the supply on the bench without a load on +5 V, because the +12 V output cannot regulate without it.[8] Fault-finding is on VT100 Troubleshooting Guide.

Monitor

[edit | edit source]

VT100s have one of two monitors. The early Ball Brothers monitor (30-14590) has its flyback transformer on the brace above the CRT neck and a small connector board on the yoke; the later Elston monitor (30-16080) has the flyback on the side plate opposite the monitor board and no yoke connector card. DEC's own VT100 monitor board, drawing 5414131, uses the Elston adjustment designators.[12][13][14]

The 5414131 board carries these electrolytics and tantalums:

VT100 monitor board 5414131 electrolytic and tantalum capacitors
Reference Value Rating Type DEC part
C102, C103 22 µF 50 V Aluminium electrolytic 1016555-00
C106, C434, C440 470 µF 16 V Aluminium electrolytic 1016557-00
C312, C437 4.7 µF 25 V Aluminium electrolytic 1016553-00
C321 1000 µF 16 V Aluminium electrolytic 1016548-00
C439 75 µF 6 V Aluminium electrolytic 1016554-00
C441 10 µF 25 V Aluminium electrolytic 1016558-00
C315 1 µF 35 V Solid tantalum 1001776-00
C318, C319 4.7 µF 35 V Solid tantalum 1016496-00

[14]

The board's own fuse, F401, is a 2.5 A 125 V sub-miniature part, and the horizontal output transistor Q414 is a BU407D.[14] C439 is the part most often named in community repair notes: Randy Hick describes a blown C439 with a burnt CR408 as a very common cause of a blank white raster and a ballooned display, and fits a 75 µF 16 V part.[15] The same board is used in the DECmate I; see DECmate I Capacitor Replacement Guide, which lists it from the DECmate print set.

Adjustment of height, width, linearity, centring and focus is on VT100 CRT Alignment Guide.

Nonvolatile memory

[edit | edit source]

SET-UP features and the answerback message are kept in an ER1400 electrically alterable memory (E24). DEC rates its data retention at 10 years or more if it is written fewer than 1000 times, and at 1 year or more for 1000 to 10,000 writes. It needs 35 V to write and erase, and it is the only part of the terminal that uses the −23 V supply.[16][17]

Keyboard

[edit | edit source]

The keyboard is powered from +12 V through its coiled cable (pin 3 of the keyboard connector) and has its own 5 V regulator; if it has power, the ON LINE or LOCAL LED is lit. DEC says keyboard faults are more often in the cable, the connectors or the keyboard itself than in the terminal, because the keyboard is more exposed.[2][17]

[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 Digital Equipment Corporation, VT100 Series Video Terminal Technical Manual, EK-VT100-TM-003, third edition, July 1982, p. 3-3, "User Maintenance" (File:VT100 Series Technical Manual (EK-VT100-TM-003).pdf).
  2. ↑ 2.0 2.1 2.2 2.3 Digital Equipment Corporation, VT100 Mini Maintenance Hardware Guide, EK-VT100-J1-001, September 1978, p. 7-3 (File:VT100 Mini Maintenance Hardware Guide (EK-VT100-J1-001).pdf). Fuse, connector voltages, CRT anode 11.5 kV.
  3. ↑ 3.0 3.1 3.2 DEC, EK-VT100-TM-003, pp. 5-56 to 5-57, section 5.4.20 and Figure 5-25. Power supply removal and capacitor discharge warning.
  4. ↑ DEC, EK-VT100-TM-003, p. 5-73, section 5.6.1. Board edge contacts and supply checks.
  5. ↑ DEC, EK-VT100-TM-003, p. 5-51, section 5.4.14. Top cover removal.
  6. ↑ 6.0 6.1 6.2 DEC, EK-VT100-TM-003, pp. 4-107 to 4-111, section 4.9. H7831 input circuit, bleeders, start-up supply, outputs.
  7. ↑ 7.0 7.1 7.2 Digital Equipment Corporation, VT100 Field Maintenance Print Set, MP00633, February 1982, H7831 parts list 5412996-0-DBP revision P, sheets A1 to A3 (PDF pp. 23, 24 and 26) (File:VT100 Field Maintenance Print Set (MP00633, Feb 1982).pdf).
  8. ↑ 8.0 8.1 8.2 DEC, EK-VT100-TM-003, p. 5-78, section 5.6.3. Power supply stress, output loop components, minimum load.
  9. ↑ DEC, EK-VT100-TM-003, p. 4-112, section 4.9.5.1. Input specification and fuse.
  10. ↑ Digital Equipment Corporation, VT100 Illustrated Parts Breakdown, EK-VT100-IP-004, December 1980, figure 3 (File:VT100 Illustrated Parts Breakdown (EK-VT100-IP-004).pdf). Fuse 3 A 250 V, 90-07217-00.
  11. ↑ DEC, EK-VT100-TM-003, pp. 4-113 to 4-114, section 4.9.5. Output specifications.
  12. ↑ DEC, EK-VT100-TM-003, p. 4-99, section 4.8. Monitors 30-14590 and 30-16080.
  13. ↑ DEC, EK-VT100-TM-003, pp. 5-25 and 5-31, sections 5.3.2 and 5.3.3. Identifying the Ball and Elston monitors.
  14. ↑ 14.0 14.1 14.2 DEC, print set MP00633, VT100 monitor parts list 5414131-0-DBP revision S, sheets A1 and A2 (PDF pp. 55 and 56).
  15. ↑ Randy Hick, "VT100 trouble analysis of video monitor board", text file hosted at bitsavers.org. Community repair notes.
  16. ↑ DEC, EK-VT100-TM-003, pp. 4-44 to 4-46, section 4.5. ER1400 data retention and support circuits.
  17. ↑ 17.0 17.1 DEC, EK-VT100-TM-003, p. 5-77, sections 5.6.1.4 and 5.6.2. NVR on −23 V; keyboard power and faults.
Retrieved from "https://retrotechwiki.com/index.php?title=VT100_General_Maintenance&oldid=24723"