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DECmate III Troubleshooting Guide

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DECmate III system unit (Centre for Computing History)

Fault-finding on the DECmate III works module by module: the H7845 power supply, the 54-16489 system board, the RX50 drive, and the VR201 monitor and keyboard.[1] DEC's DECmate III Owner's Manual (EK-DM3BU-OM) and DECmate III Pocket Service Guide (EK-DM3BU-PS-002) are catalogued on Manx, but no scan of either is online.[2] The PDP-8 FAQ describes the DECmate III as using the same technology as the DECmate II, whose self-test codes and checks are on DECmate II Troubleshooting Guide. No DEC document online confirms that the DECmate III reports the same codes.[3]

The VR201 monitor contains a CRT and the system unit has a mains switching supply. Read CRT Discharge Procedure before opening the monitor.

Before you start

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  1. Check which supply is fitted. DEC built the DECmate III with an H7845-AA for 120 V or an H7845-AB for 240 V, so a machine moved between countries may have the wrong one.[4]
  2. Check the monitor and keyboard cables. On the DECmate II the keyboard plugs into the VR201 monitor, not the system unit.[5]
  3. Listen for the fan and the RX50 at power-on.

Power checks

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The system board takes power through J2. Measure with the board connected and the supply running.[6]

DECmate III system board power connector J2
Pin Supply
8 +12 V
7 +5 V
5 −12 V
1, 2, 3, 4 Ground

[6]

The DECmate III print set gives no tolerances for these rails. C31 and C50, 10 µF 20 V tantalums, sit beside J2.[6][7]

The H7845 is rated 78 W (85 W for the -BA and -BB alternatives).[4] Its schematic is not in the print set. For the general approach to a dead DEC switching supply of this period, including testing with a series lamp and a dummy load, see the repair record on DECmate II Power Supply Service, which covers the H7842 used in the DECmate II.

Symptom, probable cause and action
Symptom Probable cause Action
No fan, no drive activity No mains, wrong supply for the mains voltage, or a dead H7845 Check the cord and outlet. Confirm the H7845 variant. Measure J2 on the system board.[4][6]
Fan runs, no picture Monitor, monitor cable, or a system board fault Check the VR201 on another DECmate II, DECmate III or Rainbow if available. Check J2 voltages.
Picture shimmers or the background is too bright Ageing VR201 electrolytics See DECmate III Capacitor Replacement Guide.[8]
Cloudy patches round the screen edge Deteriorated glue between the VR201's safety glass and tube ("cataracts") The glass can be removed with gentle heat and refitted on spacers. Discharge the CRT first.[8]
Drive will not read any diskette Spindle not turning Check the spindle turns freely by hand. On one RX50 a misaligned cone lift cover bound the spindle.[8]

RX50 drive

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The RX50 slides out of the front after the spring tab on the base chassis is pressed.[9] Notes from a Rainbow restoration apply to the same drive: lost door pins let a door come adrift, and the drive cable can be crushed against a corner of the power supply when parts are refitted, giving an intermittent fault.[8] See Floppy Drive Repair.

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References

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  1. ↑ Digital Equipment Corporation, PC238 DECmate III print set, MP-01968-01, unit assembly wiring interconnection (PDF p. 7) (File:DECmate III PC238 Print Set (MP01968-01).pdf).
  2. ↑ Manx catalogue of online computer manuals, search for "DECmate", manx-docs.org. Lists EK-DM3BU-OM and EK-DM3BU-PS-002 with no online copy.
  3. ↑ Douglas W. Jones, "PDP-8 Summary of Models and Options", alt.sys.pdp8 FAQ, posted 8 June 2001, section "What is a DECmate III?", faqs.org.
  4. ↑ 4.0 4.1 4.2 DEC, print set MP-01968-01, BA25-H small system box parts list (PDF p. 39).
  5. ↑ Digital Equipment Corporation, DECmate II Specification, revision 4, 18 January 1983, section 1, bitsavers.org. The keyboard connects to the monitor.
  6. ↑ 6.0 6.1 6.2 6.3 DEC, print set MP-01968-01, system board schematic, power connector J2 (PDF p. 23).
  7. ↑ DEC, print set MP-01968-01, system board parts lists 5416489-1-DBP and -2-DBP (PDF pp. 45 and 55).
  8. ↑ 8.0 8.1 8.2 8.3 Rob Jarratt, "Rainbow 100A Restoration", Rob's Old Computers, 29 July 2025, robs-old-computers.com. VR201 and RX50 repairs on a Rainbow 100A.
  9. ↑ DEC, print set MP-01968-01, BA25-H small system box drawing, note 2 (PDF p. 37).
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