DECmate I Troubleshooting Guide

VT278 rear panel, showing the POWER O.K. and CPU O.K. indicators, main fuse and selector switches (DEC, DECmate System Architecture Manual, Figure 3-2)

Fault-finding on the DECmate I (VT278) starts with its power-up self-test, which DEC documents in the DECmate System Architecture Manual (EK-VT278-HR-001): two LEDs on the rear panel, seven LEDs on the keyboard and the SET-UP message on the screen together identify the failing test. The DECmate Owner's Guide adds a problem-solving chart for installation faults, and the VT278 print set (MP-00900) has the module parts lists and the CPU schematics.

The VT278 contains a CRT and a mains switching power supply. Read CRT Discharge Procedure before working inside the case.

Before you start

1. Check the voltage selector and line frequency selector switches beside the power cord connector. DEC's owner's guide requires 120 V/60 Hz in North America and 220–240 V/50 Hz in Europe.
2. Check the main fuse on the rear panel. DEC's chart tells the user to look for a burnt fuse or a broken filament when the system does not start or make any sound.
3. Check that the keyboard cable is plugged into the rear of the terminal. DEC lists a missing keyboard cable as a cause of the self-test not running.
4. If the RX02 drive unit has its own power switch, check that it is on.

Power-up self-test

DEC divides the self-test into four stages.

1. Power test. The power supply checks that its outputs are within tolerance. If they are, PWR OK on the rear panel lights. If PWR OK stays dark, no further tests run.
2. CPU OK tests. T1 checks the control panel ROM checksum, T2 the control panel RAM, and T3 the keyboard's response to a scan command. If any fails, CPU OK lights continuously and testing stops.
3. General tests T4 to T13, reported on the keyboard LEDs (table below).
4. A pass is shown by CPU OK blinking once a second, the word SET-UP blinking at the bottom of the screen, and only ON LINE lit on the keyboard.

VT278 general self-test summary (DEC Table 3-1)

Test	Description	CPU OK	Keyboard LEDs	Screen
Fatal errors
T4	Display buffer memory	On	O X X X X X X	None
T5	CRT controller	On	O X X X X X O	None
T6	Main memory read/write	On	O X X X X O X	None
T7	Second main memory test	On	O X X X X O O	None
T8	HLT instruction	On	O X X X O X X	None
T9	Panel request failure	On	O X X X O X O	None
Non-fatal errors
T10	Printer port loopback	Blinks	O X X X O O X	SET-UP
T11	Real-time clock	Blinks	O X X X O O O	SET-UP
T12	Printer and real-time clock	Blinks	O X X O X X X	SET-UP
T13	Undefined interrupt	Blinks	O X X O X X O	SET-UP

X is a lit LED and O an unlit one, read left to right across the seven keyboard LEDs (ON LINE, LOCAL, the data-set-ready LED and L1 to L4). A fatal error stops the machine; a non-fatal error leaves it usable, but a program that uses the failed printer port, clock or interrupt may misbehave.

To run the self-test again, enter SET-UP mode and press the 0 key. The VT278 returns to SET-UP when the tests finish.

Reading the result

Symptom, probable cause and action

Symptom	Probable cause	Action
PWR OK dark, no fan, no sound	No mains, wrong selector setting, blown main fuse, or a dead H7836 supply	Check the outlet, both selector switches and the fuse. Then measure the supply outputs (+5 V, +12 V, −12 V)
PWR OK lit, CPU OK on continuously, no keyboard pattern	T1, T2 or T3 failed: control panel ROM, control panel RAM, or no response from the keyboard	Check the keyboard and its cable first, since T3 needs the keyboard to answer. Control panel memory is on the M8436 CPU module
CPU OK on continuously with a keyboard pattern	Fatal error T4 to T9	Look the pattern up in the table. The display buffer, main memory and CRT controller tested by T4 to T7 are all on the CPU board
CPU OK blinking, SET-UP shown, LEDs other than ON LINE lit	Non-fatal error T10 to T13	Printer port, real-time clock or interrupt fault. The machine still works for programs that do not use the failed function
Fans run and ON LINE lights, but no cursor or SET-UP	Self-test passed but the display did not come up	Switch off and on and wait about 3 seconds. Then hold CTRL and press SET-UP

Video monitor board

The VT278 uses the VT100 monitor board (drawing 5414131), so faults catalogued for the VT100 display apply. Randy Hick's VT100 monitor board notes, hosted on bitsavers, link these symptoms to parts. They are community repair experience, not DEC data.

VT100 monitor board symptoms (Randy Hick)

Symptom	Part named
Blank white raster, ballooned display	C439 and CR408. Hick calls this a very common failure, with C439 blown and CR408 burnt or blown. He replaces CR408 with a 1N4004 and C439 with 75 µF 16 V
White lines on the left side	C103
Misshapen characters	C103
Small characters on the top row	CR304
Skinny letters on the right side	Q414 and C441
Jumpy screen, alternating fuzzy and clear	C486 and C487
Very light, almost blank screen	CR102
Very wide characters	CR409
Narrow screen	L402
Screen overlap in the centre, narrow right side	C437

DEC's parts list fits C439 as 75 µF 6 V. Hick's replacement is a higher-voltage part of the same value. The full capacitor list is on DECmate I Capacitor Replacement Guide.

Diskette faults

The RX02 drives connect through a single 37-pin connector at the rear of the VT278. DEC's owner's guide lists these checks for the diskette system test:

• The System Test diskette must be in drive 0 with the drive 0 door closed.
• With the diskette in place, hold CTRL and press SET-UP, then press 2 to bootstrap from drive 0.
• For a rapid clicking sound after pressing 2, DEC's remedy is to check that the System Test diskette is in drive 0, inserted the right way, with the door closed, and to try again.

For the drives themselves see Floppy Drive Repair.

Related pages

• DECmate I
• DECmate I General Maintenance
• DECmate I Capacitor Replacement Guide
• DECmate I Documentation
• CRT Discharge Procedure
• Capacitor Failure Symptoms