VR14 and VR20 Troubleshooting Procedures Jun72

This document, published by Digital Equipment Corporation in June 1972, provides detailed troubleshooting procedures for VR14 and VR20 display units. Its primary purpose is to identify and correct the underlying causes of common component failures, as merely replacing faulty parts often leads to recurring issues.

The guide categorizes troubleshooting into three main procedures based on symptoms:

1. Procedure A: Overheated Power Transistors (Blown Fuse & Shorted Transistors)

   • Symptom: Blown fuse, and 2N4399/2N5302 transistors shorted.
   • Common Causes: Insufficient thermal compound, high power supply voltages, high-frequency oscillations, fan failure, or prolonged beam deflection off-screen.
   • Solutions: Proper component replacement (using specific transistors and thermal compound), checking and correcting power supply voltages (e.g., specific ECOs for G836 regulator and A225 D/A converter boards), ensuring adequate fan operation, and a critical physical rework of the deflection heat sink to improve heat dissipation (swapping X and Y heat sinks). For the VR14, it also suggests reducing high voltage by changing the transformer tap.

2. Procedure B: CRT Arcing (W682/W683 Intensity Board Failure, Blown Components)

   • Symptom: Intensity board failure, blown power transistors and fuses, often with ±22V at a ±40V level.
   • Cause: CRT arcing causing overvoltage surges.
   • Solutions: Replacing fuses, power transistors, 709 operational amplifiers on the G836 board, and potentially the CRT itself if there's a history of arcing failures.

3. Procedure C: Blown Fuse Only (No Other Component Failure)

   • Symptom: Only a DC fuse blows, due to current overload, faulty connections, or a power latch-up problem.
   • Solutions: Replacing the fuse, checking for faulty cable connections by wiggling, diagnosing intermittent excessive input deflection signals or D/A converter issues (including oscillations), and correcting power latch-up conditions (e.g., ensuring R19 on the A225 board is 200 ohms, allowing sufficient power-down time between restarts). It also mentions an ECO for PDP-12 D/A converters to prevent issues from power supply latch-up.

The document emphasizes using specific components and proper techniques, such as applying sufficient thermal compound, to ensure lasting repairs.