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DECmate III General Maintenance

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Layout of the DECmate III system unit from DEC's wiring drawing: power supply, RX50 drive and system board

Routine care and module handling for the DECmate III, from DEC's PC238 print set (MP-01968-01) and VR201 print set (MP-01410).[1][2] No DECmate III owner's manual is online, so the cleaning advice below is DEC's for the DECmate II, which uses the same VR201 monitor.[3]

The VR201 monitor contains a CRT. Read CRT Discharge Procedure before opening it.

Power supply variants

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DEC fitted one of two supplies according to the mains voltage: the H7845-AA for 120 V or the H7845-AB for 240 V, both 78 W. The parts list also allows the 85 W H7845-BA (120 V) and H7845-BB (240 V).[4] Read the label on the supply before connecting a machine of unknown history.

Inside the system unit

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DECmate III modules
Module DEC part
System board 54-16489-01
Diskette drive RX50-AA
Power supply H7845-AA or -AB

[5]

The BA25-H drawing notes give the handling details:[6]

  • To remove the RX50, press the spring tab on the base chassis and slide the drive forward.
  • The front bezel fits to the top cover, and the rear bezel to the base chassis.
  • DEC torques all screws to 4–5 in-lb (about 0.45–0.56 N·m).

Cleaning

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DEC's cleaning instructions for the DECmate II apply to the same monitor and keyboard:[3]

  • Switch off before cleaning any surface.
  • Clean the case with a little soap or mild detergent on a damp cloth. Never use a spray or solvent, and keep liquid out of the vents.
  • Clean the screen with a cloth moistened with alcohol or water.
  • Leave the keycaps in place; removing them can damage the switch contacts.
  • Keep the vents clear. Dust clogs the air passages and reduces the fan's cooling.[7]

Capacitors

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The system board's bulk capacitors are three solid tantalums (C31, C50, C36). The aluminium electrolytics are in the VR201 monitor, which has 21, and in the H7845 supply. The full list is on DECmate III Capacitor Replacement Guide.[8][2]

Monitor

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The VR201 came in three phosphor colours: VR201-A white (P4), VR201-B green (P31) and VR201-C amber (P134).[9] Clouding round the edge of the screen comes from the glue between the safety glass and the tube breaking down. One restorer removed the glass with a hair dryer, warming it to about 65–70 °C, peeled off the old glue and refitted the glass on thin rubber spacers.[10]

Routine checks

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  1. Confirm the supply variant matches the mains.
  2. Inspect the VR201 board for bulging or leaking electrolytics.
  3. Check that the RX50 spindle turns freely by hand with the drive out.[10]
  4. Power up and check +5 V, +12 V and −12 V at J2 on the system board (pins 7, 8 and 5; pins 1 to 4 ground).[11]
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References

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  1. ↑ Digital Equipment Corporation, PC238 DECmate III print set, MP-01968-01 (File:DECmate III PC238 Print Set (MP01968-01).pdf).
  2. ↑ 2.0 2.1 Digital Equipment Corporation, VR201 monitor print set, MP-01410, May 1982, parts list 5415500 (PDF pp. 6 to 9) (File:VR201 Monitor Print Set (MP01410).pdf).
  3. ↑ 3.0 3.1 DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, pp. 29 to 30. Caring for the DECmate II.
  4. ↑ DEC, print set MP-01968-01, BA25-H small system box parts list (PDF p. 39).
  5. ↑ DEC, print set MP-01968-01, unit assembly wiring interconnection (PDF p. 7).
  6. ↑ DEC, print set MP-01968-01, BA25-H small system box assembly drawing, notes 1 to 4 (PDF p. 37).
  7. ↑ Digital Equipment Corporation, DECmate II Owner's Manual, EK-DECM2-OM-001, December 1982, pp. 27 to 29 (File:DECmate II Owner's Manual (EK-DECM2-OM-001).pdf). Air quality.
  8. ↑ DEC, print set MP-01968-01, system board parts lists (PDF pp. 45 and 55).
  9. ↑ DEC, VR201 print set MP-01410, monitor assembly variation legend (PDF p. 2).
  10. ↑ 10.0 10.1 Rob Jarratt, "Rainbow 100A Restoration", Rob's Old Computers, 29 July 2025, robs-old-computers.com.
  11. ↑ DEC, print set MP-01968-01, system board schematic, power connector J2 (PDF p. 23).
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