VT52 Troubleshooting Guide
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Fault-finding on the VT52 follows chapter 5 of DEC's VT52 DECscope Maintenance Manual (EK-VT52-MM-002): basic checks, two symptom tables, then voltage checks on the Monitor/Power Supply module with component-level tests for each supply rail.[1][2]
Warning: DEC's manual repeats that high voltages are present in the power supply and the CRT. Unplug the terminal, connect a ground lead first, and discharge the CRT and the filter capacitors at the points in DEC's Figure 5-2 before handling the monitor/power supply or CRT parts. The CRT anode connector, at 11,000 V, sits close to connector J2 where the supply voltages are measured. See CRT Discharge Procedure.[3][2]
Basic checks
[edit | edit source]DEC's first questions, before taking anything apart:[1]
- Is the unit switched on and plugged in?
- Is the intensity set too low? The control is on the rear of the terminal; moving it to the right makes the screen brighter.[4]
- Are the baud rate switches in legal positions? Combinations 1A, 1C, 2A, 2C, 3A and 3C of S1 and S2 are illegal.[5]
- Do the terminal's send and receive rates match the host's receive and send rates?
Then put the terminal off-line (S1 position 1) and type at the keyboard to build a list of symptoms.[1][5]
After warm-up the cursor should sit blinking at the home position. DEC allows about half a minute.[4] If it does, the CRT and the Monitor/Power Supply module are working and the fault is elsewhere (Table 5-2 below). If not, turn the intensity to maximum and look for a raster. With a raster, the monitor, power supply and CRT are probably good apart from the video amplifier; DEC notes that on some units the raster cannot be seen even at full brightness.[1]
Power-on faults
[edit | edit source]| Symptom | Possible cause, most likely first | Action |
|---|---|---|
| No cursor | Intensity control turned down | Turn up the intensity at the rear |
| No cursor, no raster | Blown fuse; no mains; anode cap disconnected at the power supply; anode clip disconnected at the CRT bell; monitor/power supply assembly; CRT; RUT module; DP module; character generator | Check the fuse and mains first, then the anode connections with the power off and the CRT discharged |
| No cursor, raster present, arcing heard | Anode cap loose; anode clip loose | Tighten the cap or clip |
| Cursor present, display flickers | Jumper W7 on the RUT module wrong for the mains frequency | Remove W7 for 60 Hz units; fit W7 for 50 Hz units |
Operating faults
[edit | edit source]| Symptom | Possible cause, most likely first | Action |
|---|---|---|
| Focus cannot be adjusted; display fuzzy | Monitor/power supply board | Replace or repair the board |
| Whole raster too short | Vertical size or linearity misadjusted | Adjust R98 and R92 (VT52 CRT Alignment Guide) |
| Raster or character width cannot be adjusted | Monitor/power supply board | Replace or repair the board |
| Whole raster too narrow | Horizontal size misadjusted | Adjust coil T3 |
| Intensity cannot be adjusted | Monitor/power supply board; CRT | Replace the board, then the CRT |
| Characters too short | Vertical size; vertical linearity; monitor/power supply board | Adjust, then replace |
| Illegal characters generated | Character generator; RUT module | Substitute |
| Random characters, or screen fills with random characters at switch-on | Wrong ROM or PROM chips fitted; character generator; DP module; RUT module | Check the chip types against DEC's list, then substitute |
| Wrong character displayed when typed | Keyboard; keyboard character generator | Substitute |
| No key-clicks, cursor present | RUT module | Substitute |
| No key-clicks or cursor, raster present | DP module; keyboard | Substitute |
| No key-clicks, characters displayed | DP module; RUT module | Substitute |
| No key-clicks, wrong but legal characters | DP module; RUT module; keyboard; keyboard character generator | Substitute |
| Whole raster tilted | Yoke rotated on the CRT neck | Realign the yoke |
| Raster too large, characters enlarged and fuzzy | Yoke not fully forward | Push the yoke forward against the CRT bell and tighten its screw |
DEC's PMK04 off-line terminal tester can exercise the terminal and its interface to separate a terminal fault from a line fault.[1]
Voltage checks
[edit | edit source]Tip the terminal back and remove the base (five quarter-turn fasteners, turned 90° anticlockwise with a ¼ inch nut driver) to reach connectors J1 and J2 on the Monitor/Power Supply module.[7][2]
| Connector and pin | Voltage |
|---|---|
| J1-1 | +5 V |
| J1-2, 4, 6 | Ground |
| J1-10 | +15 V |
| J2-2 | −12 V |
| J2-3 | −5 V |
| J2-4 | +5 V sense |
| J2-5, J2-6 | +5 V |
| J2-7 | Ground, remote sense |
| J2-8, 9, 10 | Ground |
If the module's outputs are right, check them where they arrive: for example RUT/DP J1-17 (+15 V), J1-18 (−12 V) and J1-19 (+5 V), and DP J5-9 (−5 V). DEC's Table 5-4 lists every distribution point.[8] If an output is wrong, remove the heat spreader and use DEC's component tests below.[2]
Component tests
[edit | edit source]These are DEC's tests for the linear supply on the Monitor/Power Supply module. Each forces one stage and reads the result.[2][8][9]
| Condition | Action | If the rail reads low or 0 | If it reads normal or high |
|---|---|---|---|
| No voltage at all | Ground the collector of Q2 and measure +15 V | Check for open R1, Q11 or R6; shorted Q1; short from Q11 collector to ground | Check for open Q2, D6, D5 or R27; check R10 and R28 |
| +15 V low | Ground the collector of Q2 | Check C35, R5, R6 and R1; short between Q2 and Q1 | Check D8, D7, R12, R29, R1, R13 and R27; check E1 |
| +15 V high | Fix any −12 V fault first, then short the base of Q2 to ground | Measure E1 pin 6: below 2 V, replace E1; above 2 V, check R9, R12 and R13 | Check for shorted Q2 or Q11 |
| +5 V at 0 V | Connect the base of Q9 to the collector of Q6 | Check for open Q8, Q9 or R35; short to ground at Q8 collector | Check for open Q6, R42 or R41; shorted Q5 or faulty E3 |
| +5 V high | Short the base of Q9 to its emitter | Measure E3 pin 6: at 0 V, Q6 is shorted; at 8 V, check R33, R36, R40 and E3 | Check for shorted Q8 or Q9 |
| +5 V low | Short the collector of Q6 to its emitter | Check for open Q8 or Q9; check R34 | Check for open Q6 or R40; check E3, Q5, R35, D12, R37, R38 and R41 |
| −12 V at 0 V | Short the collector of Q4 to ground | Check for shorted Q3; open R52, Q12, R19 or R20; short from Q12 collector to ground | Check for open Q4 or D20; shorted D10 |
| −12 V high | Short the base of Q4 to its emitter | (reads 0) Check E2, R15, R17 and R14 | Check for shorted Q4 or Q12 |
| −12 V low | Short the collector of Q4 to ground and measure | Check R19, R21, D20, R25, R52, R22 and Q3 |
The vt100.net transcription of this manual omits the +15 V tests of section 5.5.2 and prints the +5 V tests under that heading; the scanned manual has all four sections.[10][2]
Sections 5.6 and 5.7 of the manual give oscilloscope pictures of the timing signals and of eleven monitor test points on a working terminal, for comparison with a faulty one.[11]
Related pages
[edit | edit source]References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 DEC, EK-VT52-MM-002, sections 5.2.1 and 5.2.2, pp. 5-1 to 5-2. Identifying the faulty subassembly, basic checks, PMK04 tester.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 DEC, EK-VT52-MM-002, p. 5-24, section 5.5, Table 5-3, sections 5.5.1 and 5.5.2.
- ↑ 3.0 3.1 DEC, EK-VT52-MM-002, pp. 5-3 to 5-7, Table 5-1, "Power On Troubleshooting Procedure", and Figure 5-2.
- ↑ 4.0 4.1 Digital Equipment Corporation, VT52 DECscope Maintenance Manual, EK-VT52-MM-002, July 1978, section 2.4.1 (File:VT52 DECscope Maintenance Manual (EK-VT52-MM-002).pdf). Flashing cursor after half a minute; intensity control position.
- ↑ 5.0 5.1 DEC, EK-VT52-MM-002, section 3.1.1, Table 3-1, p. 3-2. Operator controls, baud rate switches and illegal positions.
- ↑ DEC, EK-VT52-MM-002, pp. 5-9 to 5-12, Table 5-2, "Operational Troubleshooting Procedure".
- ↑ DEC, EK-VT52-MM-002, section 5.3.1, pp. 5-12 to 5-13. Base removal.
- ↑ 8.0 8.1 8.2 DEC, EK-VT52-MM-002, p. 5-25, Table 5-4 and sections 5.5.2 (continued) and 5.5.3.
- ↑ 9.0 9.1 DEC, EK-VT52-MM-002, p. 5-26, sections 5.5.3 (continued) and 5.5.4.
- ↑ VT52 DECscope Maintenance Manual, chapter 5, HTML transcription by Paul Flo Williams, vt100.net/docs/vt52-mm/chapter5.html.
- ↑ DEC, EK-VT52-MM-002, sections 5.6 and 5.7, pp. 5-26 to 5-32. Timing and monitor waveforms.