DEC VT240 Terminal

Here are some pictures of a Digital Equipment Corporation (DEC) VT240 terminal with a VR201-C amber monochrome monitor. It’s just about to be sent away as part of a trade deal. I’m posting the pictures here to show them to the collector who I’m trading with, and then I’m leaving them here because there’s always room for more pictures of cool old computer equipment on the Internet. I took the pictures on the tailgate of my truck because it was the only free spot I could find to set the terminal down. :)

The monitor didn’t work when I received the terminal. Studying the schematic diagram I found online revealed that there’s a little soldered-in fuse that was masquerading as a 1/4 ohm resistor. It was blown. I replaced it with a 5x20mm fuse holder tucked inside the monitor case, but the new fuse blew immediately. I began debugging the terminal by bypassing the fuse holder, applying power from a current-limited bench supply, then looking for hot spots with an infrared non-contact thermometer. This is a somewhat gentler method than the time-honored technique of shorting out the fuses, applying power, and looking for smoke. I wasn’t sure if it would work, but it quickly led me to the problem.

The hottest area was a set of four diodes in series that create a bit of a voltage drop to a portion of the circuit. That allowed me to narrow down the problem area in the schematic diagram. Then I looked for the second-hottest spot, and found it in a cluster of components in a tight area. I pulled out a thermocouple probe that I can connect to my multimeter, and found a capacitor giving off the heat. There’s an obvious problem right there! It turned out to be a poly film cap that had shorted out, and replacing it brought the monitor to life.

If I didn’t have the IR thermometer available, I could have just looked for the brown part of the PCB near the cap, and the blackened cap. But my way was more fun. :)

The screen has some burns, and some signs of possible “cataracts” that form on some old CRTs when the front lens starts to separate from the tube after its clear glue decomposes. It’s still quite usable, though. The plastic cases also have some yellowing, which is pretty common on computer hardware from this era.