DECmate II Troubleshooting Guide
Fault-finding on the DECmate II follows chapter 7 of DEC's DECmate II Owner's Manual (EK-DECM2-OM-001): the power-up self-test, the routine checks DEC asks for before any repair, and the System Test diskette.[1] The system unit has four replaceable modules (system board, power supply, RX50 drive, and the power switch and fan assembly), so most faults come down to finding the module.[2] Power supply repair is covered on DECmate II Power Supply Service.
The VR201 monitor contains a CRT. Read CRT Discharge Procedure before opening it.
Stop at once
[edit | edit source]DEC tells owners to unplug the system immediately if they notice a burning smell, smoke, no sound of the fan or diskette drive, a screeching noise, or liquid spilled through the vents.[3] A burning smell with the circuit breaker tripped is how one H7842 supply failure first showed itself; the restorer found a destroyed switching transistor after powering it again.[4]
Routine checks
[edit | edit source]DEC's list of checks to make before anything else:[5]
- Test the wall outlet with a lamp that is known to work.
- Turn the power switch off, wait about 4 seconds, and turn it on again. This reboots the system and repeats the self-test.
- Check that all cables and the power cord are firmly plugged in at both ends.
- Check the circuit breaker on the back of the system unit. If it has tripped, push it in and switch off and on again. If it trips a second time, unplug the system and check the voltage selection switch.
- Run the System Test diskette.
The circuit breaker trips if the voltage switch is set to 115 V and the unit is connected to 230 V.[6]
Power-up self-test
[edit | edit source]The self-test checks the system board every time the DECmate II is switched on and ends with one of three messages: no problem found, wrong diskette in drive 0, or an error code.[7] The Compose LED on the keyboard glows when the self-test finds a non-fatal error.[8]
An error code is a sum of the values below, so 12 means errors 04 and 08 together. Only 32 is fatal. The message stays on screen until a key is pressed.[9]
| Code | Name | What DEC says it means |
|---|---|---|
| 01 | Printer port failure | The CPU gets no response from the printer circuitry. The machine works but will not print. |
| 02 | Real time clock failure | The clock circuit has a problem. The machine works but cannot log the time. |
| 04 | Control interrupt failure | The circuitry that handles incoming messages has a problem. Some components or modules may not respond. |
| 08 | Keyboard interface failure | The CPU cannot communicate with the keyboard. The machine may still work as a read-only terminal. |
| 16 | Actual keyboard failure | The keyboard's own self-test found a problem, and all the keyboard LEDs glow steadily. |
| 32 | Diskette drive interface failure | The CPU cannot communicate with the diskette drive system. Fatal: the DECmate II cannot be used. |
For any code, DEC's procedure is to record the number, remove the diskettes, switch off, and then use the DECmate II Repair Guide or the help line.[11]
System Test diskette
[edit | edit source]The System Test diskette (BL-T345A-MA in the owner's manual) is for faults the power-up test does not report. It tests the system board, the RX50 drive and its interface, the printer and printer interface, and the CP/M board, RX01/RX02 interface and second RX50 if fitted. Each item ends as OK, THIS ITEM NOT FOUND IN SYSTEM (check cables and that external devices are switched on), or PROBLEM HERE.[12] An image of revision BL-T345B-MA is on archive.org.[13]
The CP/M board has its own APU test LED on the connector panel. It flickers during the board's self-test and glows steadily if the board has a problem.[14]
Power faults
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| System goes off suddenly | Mains interrupted | Check the cord and whether other equipment lost power. Switch off and wait for steady power before switching on.[15] |
| Screen blanks for a moment, fan and drive keep running | Momentary power interruption | Restart the system and carry on.[15] |
| System stops responding to typing during a brownout | Low mains voltage | Wait for full power before continuing.[15] |
| Circuit breaker pops at once | Voltage switch at 115 V on a 230 V supply, or a short in the supply or a load | Check the switch. Then disconnect the supply from the system unit and test it on its own. See DECmate II Power Supply Service.[6][4] |
| Pop from the supply when the mains cable is plugged in, before switching on | Shorted mains filter in the power entry module | On an H7842-A in a Rainbow, the Corcom F2987A power entry module measured short across its input after such a pop.[16] |
| No outputs, fan and drives silent | Failed switching transistor, mains bridge rectifier, start-up regulator or PWM controller | See the repair record on DECmate II Power Supply Service.[4][17] |
Monitor faults
[edit | edit source]The VR201 monitor is also used with the Rainbow 100, and repair notes from Rainbow owners apply to it.[16] DEC's VR201 print set is hosted here.[18]
- A shimmer in the picture, or a background that grows too bright before the text is bright enough, points to the monitor board electrolytics. One restorer found bulging or high-ESR capacitors and replaced C24, C30, C41, C303, C404 and C39.[16] DEC's parts list gives C24, C30, C41 and C303 as 1000 µF 16 V, and C39 and C404 as 100 µF 25 V.[18] The full list is on DECmate III Capacitor Replacement Guide.
- Cloudy patches round the screen edge ("cataracts") come from the glue between the front safety glass and the tube deteriorating. The same restorer removed the glass with gentle heat, about 65–70 °C from a hair dryer, peeled off the old glue and refitted the glass on thin rubber spacers. Discharge the CRT first.[16]
Diskette faults
[edit | edit source]- A drive that will not read any diskette may not be turning the spindle. On one RX50 the spindle only twitched and was stiff by hand; a misaligned cone lift cover was binding it, and reassembling the drive cured it.[16]
- An RX50 door can come adrift when its mounting pins are lost.[16]
- The drive cable runs close to a sharp corner of the power supply and can be crushed when the drive or supply is refitted, giving an intermittent fault.[16]
See Floppy Drive Repair for head cleaning and alignment.
Related pages
[edit | edit source]- DECmate II
- DECmate II General Maintenance
- DECmate II Power Supply Service
- DECmate II Documentation
- CRT Discharge Procedure
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ Digital Equipment Corporation, DECmate II Owner's Manual, EK-DECM2-OM-001, December 1982, p. 73 (File:DECmate II Owner's Manual (EK-DECM2-OM-001).pdf). Chapter 7, "What to Do If Your DECmate II Doesn't Work".
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 8. System unit modules.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 74. When to call the help line at once.
- ↑ 4.0 4.1 4.2 Rob Jarratt, "DEC Rainbow H7842 Power Supply Failure", Rob's Old Computers, 20 November 2022, robs-old-computers.com. Repair of an H7842 from a Rainbow.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 79. Routine measures to take first.
- ↑ 6.0 6.1 DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 4. Circuit breaker and voltage selection switch.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, pp. 75 to 76. Self-test messages.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 54. Keyboard indicators.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 77. Combined codes; fatal and non-fatal errors.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 78, Table 7-1, "Self-Test Error Codes".
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 76. Procedure when an error code is shown.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, pp. 82 to 83. System test diskette results.
- ↑ "DECmate II System Test Diskette", BL-T345B-MA, disk image and label photograph, archive.org.
- ↑ DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 5. Connector panel and APU test LED.
- ↑ 15.0 15.1 15.2 DEC, DECmate II Owner's Manual, EK-DECM2-OM-001, p. 80. Problems related to power.
- ↑ 16.0 16.1 16.2 16.3 16.4 16.5 16.6 Rob Jarratt, "Rainbow 100A Restoration", Rob's Old Computers, 29 July 2025, robs-old-computers.com. H7842-A power entry module, RX50 spindle and door, VR201 cataracts and capacitors.
- ↑ Rob Jarratt, "Rainbow H7842 Finally Fixed", Rob's Old Computers, 29 June 2023, robs-old-computers.com. Shorted mains bridge rectifier.
- ↑ 18.0 18.1 Digital Equipment Corporation, VR201 monitor print set, MP-01410, May 1982, parts list 5415500, sheet A1 (PDF p. 6) (File:VR201 Monitor Print Set (MP01410).pdf).