DECmate III Troubleshooting Guide
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]
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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
edit- 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]
- Check the monitor and keyboard cables. On the DECmate II the keyboard plugs into the VR201 monitor, not the system unit.[5]
- Listen for the fan and the RX50 at power-on.
Power checks
editThe system board takes power through J2. Measure with the board connected and the supply running.[6]
| Pin | Supply |
|---|---|
| 8 | +12 V |
| 7 | +5 V |
| 5 | −12 V |
| 1, 2, 3, 4 | Ground |
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 | 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
editThe 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.
Related pages
editReferences
edit- ↑ 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).
- ↑ 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.
- ↑ 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.
- ↑ 4.0 4.1 4.2 DEC, print set MP-01968-01, BA25-H small system box parts list (PDF p. 39).
- ↑ Digital Equipment Corporation, DECmate II Specification, revision 4, 18 January 1983, section 1, bitsavers.org. The keyboard connects to the monitor.
- ↑ 6.0 6.1 6.2 6.3 DEC, print set MP-01968-01, system board schematic, power connector J2 (PDF p. 23).
- ↑ DEC, print set MP-01968-01, system board parts lists 5416489-1-DBP and -2-DBP (PDF pp. 45 and 55).
- ↑ 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.
- ↑ DEC, print set MP-01968-01, BA25-H small system box drawing, note 2 (PDF p. 37).