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DEC VT100

We have a few functional VT100s and a stack of broken ones awaiting fixes. Some of them are connected to a BeagleBone running Linux, some of them are connected to a PDP-11/34.

We acquired literally a palette full of VT100, VT180, VT130, etc terminals. Most of them were marked with various failures (bad tube, bad power supply, etc). One worked great as is, and we've repaired some of the others.

Repairs

Capacitor C439 on the "5014130 D-P1 / VT100 Monitor" board is the most common cause of failure. We also found F401, a 2.5A @ 125V fuse to be bad on a few. This 1990 sci.electronics post suggests these repairs:

  • As always, CAREFULLY discharge the second anode of the CRT with a grounded screwdriver. BE CAREFUL, 10KV can kill you.
  • Replace C439 with a 75 to 100 Microfarad capacitor rated from 50 to 100 Volts. The original part may be as low as a 6 volt part.
  • Replace R481, 100 Ohms, 1/2 watt or greater.
  • Replace R110, 3.3K Ohms, 1/2 watt or greater.
  • Resolder the pins on the two white plastic connectors leading to the yoke and the flyback transformer. The solder joints frequently overheat and get microscopic cracks around them.

By the way, the symptom you get before C439 fails completely is a white stripe on the left side of the screen. When the cap fails, it may explode in a ball of smoke or just poot out a bit.

ROMs

VT100 ROM schematic.png

There are four 8316E ROM chips for the 8080 CPU on the logic board. Since they are custom mask ROMs, Digital was able to have a different chip-select bit pattern and avoided having separate NOT gates on the inputs. Note that the schematic appears to be incorrect -- E56 is the low order, and E40 is the high order address bits. VT100.bin is a ROM dump. If you like this sort of thing, vt100romhax might also be of interest.

ROM chip CS1 (20) A11/CS2 (18) A12/CS3 (21) Mapped address
E40 H H H 0x1800
E45 H L H 0x1000
E52 H H L 0x0800
E56 H L L 0x0000
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