Editing VT100 Troubleshooting Guide

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[[File:VT100 interconnection diagram (EK-VT100-J1-001).png|thumb|right|360px|DEC's VT100 interconnection diagram, with the power supply, controller, monitor and keyboard connector voltages (EK-VT100-J1-001, p. 7-3)]]
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.<ref name="tm51" /><ref name="tm573" /><ref name="mp633" />

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.<ref name="mm73" /><ref name="tm563" />

== Before you start ==
DEC's first checks, with the power switch off:<ref name="tm52" />
# Check that the power cord is firmly connected at both ends and that the wall outlet works.
# Check that the voltage selection switch is in the right position and that the fuse is good.
# 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.<ref name="tm578" />

== 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.<ref name="tm513" /> 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.<ref name="tm513" />

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

{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+ '''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
|}
<ref name="tm51" /><ref name="tm513" />

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.<ref name="tm513" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''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
|}
<ref name="tm51" />

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.<ref name="tm51" /> 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.<ref name="tm513" />

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).<ref name="tm575" />

== Symptoms ==
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+ '''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
|}
<ref name="tm51" /><ref name="tm578" />

== Power supply ==
=== Output voltages ===
The H7831 has no voltage adjustments.<ref name="tm4113" /> Measure at the terminal controller connector J6 with the terminal powered:

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''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
|}
<ref name="tm4113" /><ref name="mm73" />

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.<ref name="mm73" /><ref name="tm577" /> 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.<ref name="tm578" />

=== Component-level faults ===
DEC's notes on the H7831, with part types from the print set:<ref name="tm577" /><ref name="tm578" /><ref name="mp633" />
* 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.<ref name="tm578" />
* 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.
<ref name="tm578" />

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.<ref name="mm73" />

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.<ref name="hick" /><ref name="tm4103" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+ '''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
|}
<ref name="hick" />

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.<ref name="tm4103" />

== Terminal controller ==
DEC's component-level notes for the controller board:<ref name="tm573" />
* 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.<ref name="tm577" />

== 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.<ref name="tm577" /> 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.<ref name="tm577" />

== Related pages ==
* [[VT100]]
* [[VT100 General Maintenance]]
* [[VT100 CRT Alignment Guide]]
* [[VT100 Documentation]]
* [[CRT Discharge Procedure]]
* [[Capacitor Failure Symptoms]]

== References ==
<references>
<ref name="tm51">Digital Equipment Corporation, ''VT100 Series Video Terminal Technical Manual'', EK-VT100-TM-003, third edition, July 1982, pp. 5-1 to 5-8, Tables 5-1 to 5-3 ([[:File:VT100 Series Technical Manual (EK-VT100-TM-003).pdf]]). LED and display error codes with the unit to replace; basic VT100 troubleshooting procedure.</ref>
<ref name="tm52">DEC, EK-VT100-TM-003, p. 5-3, section 5.2.2. Preliminary checks.</ref>
<ref name="tm513">DEC, EK-VT100-TM-003, pp. 5-13 to 5-16, sections 5.2.7.1 to 5.2.7.3, Tables 5-5 and 5-6. Self-test contents, error indications, starting the tests, loopback connectors.</ref>
<ref name="tm563">DEC, EK-VT100-TM-003, p. 5-63, section 5.4.25. CRT discharge and anode cap.</ref>
<ref name="tm573">DEC, EK-VT100-TM-003, pp. 5-73 to 5-74, sections 5.6.1 and 5.6.1.1. Terminal controller and microprocessor troubleshooting.</ref>
<ref name="tm575">DEC, EK-VT100-TM-003, p. 5-75, section 5.6.1.1.4. Faults the self-test does not detect.</ref>
<ref name="tm577">DEC, EK-VT100-TM-003, p. 5-77, sections 5.6.1.4 to 5.6.3. NVR, keyboard and power supply troubleshooting.</ref>
<ref name="tm578">DEC, EK-VT100-TM-003, p. 5-78, sections 5.6.3 and 5.6.4. Power supply troubleshooting continued; CRT monitor troubleshooting.</ref>
<ref name="tm4103">DEC, EK-VT100-TM-003, pp. 4-103 to 4-104, sections 4.8.1.9 and 4.8.1.10. Elston horizontal output circuit, high voltage and focus.</ref>
<ref name="tm4113">DEC, EK-VT100-TM-003, pp. 4-113 to 4-114, section 4.9.5. Power supply output specifications; no output voltage adjustments.</ref>
<ref name="mm73">Digital Equipment Corporation, ''VT100 Mini Maintenance Hardware Guide'', EK-VT100-J1-001, September 1978, p. 7-3 ([[:File:VT100 Mini Maintenance Hardware Guide (EK-VT100-J1-001).pdf]]). Interconnection diagram: connector pin voltages, CRT anode 11.5 kV.</ref>
<ref name="mp633">Digital Equipment Corporation, VT100 Field Maintenance Print Set, MP00633, February 1982, H7831 parts list 5412996-0-DBP revision P, sheets A1 to A3 (PDF pp. 23, 24 and 26) ([[:File:VT100 Field Maintenance Print Set (MP00633, Feb 1982).pdf]]). Part types and values.</ref>
<ref name="hick">Randy Hick, "VT100 trouble analysis of video monitor board", text file hosted at [https://bitsavers.org/pdf/dec/terminal/vt100/vt100.symptom.txt bitsavers.org]. Community repair notes.</ref>
</references>

[[Category:Digital Equipment Corporation]]
[[Category:Troubleshooting Guides]]