VT100 Troubleshooting Guide

Fault-finding on the VT100 starts with its built-in self-test, which reports a failure on the keyboard LEDs or as a character on the screen, and with DEC's symptom table in the VT100 Series Video Terminal Technical Manual (EK-VT100-TM-003). The same manual has a component-level section for the terminal controller, keyboard, power supply and monitor, and the VT100 print set (MP00633) has the H7831 power supply parts list.

DEC's VT100 interconnection diagram, with the power supply, controller, monitor and keyboard connector voltages (EK-VT100-J1-001, p. 7-3)

The VT100 has a switching mains power supply and a CRT with an anode at about 11.5 to 12 kV. Unplug it and read CRT Discharge Procedure before working inside the case. DEC's field procedure discharges the anode either with a meter lead clipped to chassis ground and touched to the anode, or with an insulated screwdriver slid under the anode cap while its shaft is held against the unpainted CRT frame.

Before you start

DEC's first checks, with the power switch off:

1. Check that the power cord is firmly connected at both ends and that the wall outlet works.
2. Check that the voltage selection switch is in the right position and that the fuse is good.
3. Check that the keyboard coiled cord is plugged into the keyboard connector at the back of the terminal.

Then switch on and watch the power-up self-test. Setting the voltage switch to 115 V and connecting the terminal to 230 V stresses the power supply; DEC lists it among the causes of failed switching transistors.

Power-up self-test

At power-on the VT100 tests the RAM on the terminal controller and on the advanced video option (AVO) if fitted, checks the nonvolatile RAM (NVR) and each ROM against stored checksums, then lights all the keyboard LEDs, sounds the bell for a quarter of a second and waits for the keyboard's end-of-scan character. To repeat the test, enter SET-UP and press 0, or in ANSI mode send ESC [2;1y for one pass or ESC [2;9y to run it until it fails.

A ROM or main RAM failure is fatal and is shown on keyboard LEDs L1 to L4:

Keyboard LED error codes (DEC Table 5-5)

Error	L1	L2	L3	L4	Replace
ROM 1 checksum	Off	Off	Off	On	Terminal controller
ROM 2 checksum	Off	Off	On	Off	Terminal controller
ROM 3 checksum	Off	Off	On	On	Terminal controller
ROM 4 checksum	Off	On	Off	Off	Terminal controller
Main data RAM	Off	On	Off	On	Terminal controller

Other failures are shown as a character in the top left corner of the screen under the cursor. When the test runs continuously and finds an error, the whole screen flashes between white and black about once a second.

Screen error characters for single faults (DEC Table 5-2)

Character	Fault detected	Replace
1	Advanced video option	Advanced video board
2	Nonvolatile RAM	Terminal controller
4	Keyboard	Keyboard
8	Data loopback	Terminal controller
@	EIA test	Terminal controller

Combinations of faults give other characters (3, 5, 6, 7, 9, ;, :, <, =, >, ?, and A to O); DEC's Table 5-2 lists each with the boards to replace. The data loopback and EIA tests need a loopback connector: 12-15336 for EIA and 70-15503-00 for 20 mA current loop. Send ESC [2;2y for one pass or ESC [2;10y to repeat.

DEC points out what the self-test does not check: writing to the NVR (tested only by saving), most of the video processor (use the SET-UP screen test), and the accuracy of the baud rates. If the bell sounds with all LEDs lit and then the screen fills with garbage or stays blank, suspect the video processor (garbage) or the monitor (blank).

Symptoms

Symptom, probable cause and action (from DEC Table 5-3)

Symptom	Probable cause	Action
No response at switch-on; CRT filament and LEDs off	No mains; main fuse; power supply; line cord; DC power harness	Check the outlet and fuse. A fuse that blows again points to a short (see the power supply section below)
No response; CRT filament lit	Power supply; terminal controller; DC harness	Measure the supply outputs at the controller (table below)
No tones at switch-on; LEDs lit	Controller, keyboard, keyboard cable or speaker	Check the keyboard cable first
No tones and no LEDs	Keyboard disconnected; keyboard cable; keyboard; connectors; controller	Reconnect, then substitute
No cursor; CRT filament lit	Brightness set too low; controller; monitor board; flyback transformer; DC harness; CRT and yoke	Raise the brightness in SET-UP and on the monitor board before anything else
No cursor; CRT filament not lit; keyboard works	Monitor fuse open; monitor board; DC harness; CRT and yoke	DEC replaces the monitor board when its fuse is open
Random characters on screen	Controller	Substitute the controller
Horizontal or vertical line on screen	Monitor connectors; monitor board; CRT	Reseat the monitor connectors. A horizontal line means the vertical oscillator has failed
Display distorted; characters narrow on one side	Monitor misadjusted; monitor board; flyback; CRT and yoke; controller	See VT100 CRT Alignment Guide
Screen rolling	Ball monitor vertical hold misadjusted; DC harness; controller; monitor	Adjust vertical hold A101 on the Ball board
Display jumpy	Interlace on; power (line frequency) setting wrong; controller; power supply; monitor board; flyback	Turn interlace off and set the power (line frequency) feature to match the mains before replacing parts
SET-UP settings lost; several bells at power-up or recall; error 2	Settings not saved; controller	Save the settings; if they are still lost, the fault is on the terminal controller, which carries the NVR
Wrong characters when on line	Speed, bits per character or parity set wrong	Match the host's settings
No response to escape sequences	ANSI/VT52 setting wrong	Set the correct mode in SET-UP B

Power supply

Output voltages

The H7831 has no voltage adjustments. Measure at the terminal controller connector J6 with the terminal powered:

H7831 outputs (DEC specifications and connector pinout)

Rail	Rated load	Total regulation	Ripple	Controller J6 pins	Power supply connector pins
+5 V	1.5 to 11.0 A	±5%	150 mV p-p	18–22	14–18
+12 V	1.0 to 2.75 A	±5%	240 mV p-p	7	2, 3
−12 V	0 to 0.5 A	±5%	240 mV p-p	9	5
−23 V	10 mA	±10%	500 mV p-p	10	6

Ground is on J6 pins 2, 8 and 13 to 17 and on power supply connector pins 1, 4 and 8 to 13. The keyboard takes +12 V on pin 3 of its connector and has its own 5 V regulator; if the keyboard has power, either the ON LINE or the LOCAL LED is lit. The +5 V output must carry at least its minimum load for the +12 V output to regulate, so DEC says not to run the supply without a load.

Component-level faults

DEC's notes on the H7831, with part types from the print set:

• The switching transistor Q7 (2N6547) and its companion diode D19 (800 V, 1 A) fail most often, because they carry the most stress. When they go, check D41, R22 (a 1 Ω fusible resistor), the input rectifiers and the two thermistors R1 and R50, which heavy current can crack.
• If the fuse blows and Q7 is good, check the reservoir capacitors C9 and C14 (590 µF 200 V), the input rectifiers (the two connected to the thermistors are used on 115 V), and the bleeder resistors R19 and R20.
• If Q7 fails repeatedly, the control circuit may be driving it wrongly. Lift one end of R22 to disable the power stage and examine the control circuit; the start-up regulator Z2 runs hot in this state but is safe. Look for a strong pulse on the collector of Q2, a ramp on Q5's collector, a square wave on Q1's collector, a negative-going pulse on pin 2 of E3 and a 12 to 13 µs square wave on pin 3.
• The −12 V output is current-limited by its three-terminal regulator and the −23 V output by a series resistor (R28, 300 Ω 10 W). An overload on +5 V or +12 V shuts down the whole supply. Check for shorts by measuring the voltage across the current-sense resistors R37, R44 and R45 (0.01 Ω).
• If there is no short and the supply will not start although the start-up voltage (10.8 V at the banded end of D25) is present, lift D10, which carries SHUTDOWN L from the current limiters to pin 9 of the 3524, to isolate the comparators E1. For the +12 V comparator, pin 5 should be positive with respect to pin 6; for the +5 V comparator, pin 3 should be positive with respect to pin 2.
• The slow-start circuit can also hold the supply off. Lift D39 to disconnect it and bring the supply up gradually on an autotransformer.
• Oscillation in the +5 V loop: check C31 (3800 µF 6.3 V), L2, C17, R31 and R30. Oscillation in the +12 V loop: check C26 (560 µF 20 V), L1, R4, C1 and R5. Over-sensitive current limiting can also look like output instability.

Electrolytic capacitor values for the whole board are on VT100 General Maintenance.

Monitor

DEC says most monitor faults are blown horizontal output transistors and diodes. A working monitor makes only a slight 15 kHz whistle; if that sound from the flyback is normal, the high voltage is probably present.

• Dark screen with normal sounds: turn up the brightness control on the monitor board, enter SET-UP and raise the intensity to maximum with the up-arrow key. If the display is faint or absent, check the horizontal drive, the CRT filament and voltages (control, focus and screen grid) and the video signal from the terminal controller.
• Horizontal line: the vertical oscillator has failed.
• Rolling or tearing: check that the horizontal and vertical drive signals leave the controller and reach the monitor board. On a Ball monitor, adjust the vertical hold.

The drive signals arrive on monitor connector P101: pin 6 horizontal drive, pin 7 +12 V, pin 8 video, pin 9 vertical drive, pins 5 and 10 ground.

Randy Hick's notes on the VT100 video monitor board link these symptoms to parts. They are community repair experience. Hick's designators (C439, CR408, Q414, CR409, CR102) are the ones DEC uses for the Elston monitor in its circuit description; the Ball board numbers its parts differently.

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, and fits a 1N4004 for CR408 and 75 µF 16 V for C439
White lines on the left side; 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

In DEC's description of the Elston circuit, CR408 and C439 work with a winding on the flyback primary to boost the horizontal output stage supply, and C438 and C441 carry the yoke current; CR409 rectifies the screen and focus supply of about 370 V and CR102 the 38 V cathode driver supply.

Terminal controller

DEC's component-level notes for the controller board:

• Check the supply voltages and ripple first, check that the voltage switch is not on 230 V on a 115 V supply, and check that every board edge contact is clean and seated.
• No valid LED code: reseat the ROMs and check their chip selects. Check the 8080 clocks from the 8224 (E32 pins 6, 10 and 11), READY and RESET high (E32 pins 4 and 1) and SYNC (E32 pin 5) every 3 to 5 clock periods. HOLD REQUEST (E35 pin 13) should go high for about 60 µs every 635 µs; if it stays high, the video processor is faulty.
• RAM error on the LEDs: there is no simple way to tell which of the six RAM chips has failed. Check the memory decoders first, then replace the chips one at a time.
• The NVR (E24) is the only device on the −23 V supply, at E24 pin 2, and needs the LBA 7 clock with a 12 V swing at E24 pin 6.

Keyboard

If the keyboard fails the self-test, the terminal becomes receive-only and shows 4 under the cursor. DEC says the fault is more likely in the cable, the connectors or the keyboard than in the terminal. The terminal sends command bytes every 1.28 ms; check for the clock and data on the keyboard interface, then the keyboard UART and scan counter signals described in the manual. A failure of the LEDs or bell during the self-test does not fail the keyboard, but the self-test is the time to check them.

Related pages

• VT100
• VT100 General Maintenance
• VT100 CRT Alignment Guide
• VT100 Documentation
• CRT Discharge Procedure
• Capacitor Failure Symptoms