Projects

DEC VT131 Terminal - September 8, 2026

This VT131 was donated to me by my friend Steve. The terminal came to me in non-working conditon- the keyboard was missing a few keycaps, and the actual terminal had some faults where it would output the wrong characters and have corruption on the display. I pulled the terminal apart and started with swapping the two large RAM ICs, but that did not fix the issue. I then spent many hours comparing the VT100 schematics (they are available online, but VT131 schematics are not) to the VT131 logic board. I found that most of the logic is quite similar, even though the VT131 has extra logic in some places and simplified logic in others. The VT131 was supposed to be a low cost variant of the VT100, so it makes sense that much of the circuitry was the same or similar. I ended up removing and socketing 12 chips in the character generation circuit, and eventually found out that one of the 3 2111 RAM ICs near the character generator was bad. I could see on the scope that one of the 4 data outputs on that chip was bad, and this was fixed when the new part was put in place. I also found with the scope that the filter capacitor on the -23V DC rail from the power supply was bad, so I replaced that as well, and now the PSU is working great. Now the logic board is fully repaired, and I have learned much more about how the VT100 works. This VT131 will be used with the PDP-11 systems here, as well as with a modem to access BBSs and such. Updated 09/22/2026.