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Warning, this is a work in progress, mainly cause I am a Dumbie in this area

VT100 Trouble Shooting for Dumbies



This isn't about me, but you should know that I'm not an EE, I've got ME and CS experience with degrees even. But when it comes to electronics I'm a dummie. As such I find much of the information in the VT100 Technical Reference Manual fairly incomprehensible. It assumes I know a lot more than I do, such as how to recognize the zener diodes on the video control board. So with assistance from some much more knowledgable people, I have attempted to create this guide. I would greatly appreciate additional input, please email corrections or additions. However please don't email large attachments, sad to say I am still on dial-up! I would much rather have a link.

Common Problems

If you don't have the Technical Reference Manual, get it (see the Reference List). The Tech Reference has a lot of useful information, including troubleshooting. At power up a fully functional VT100 should flash the keyboard LEDs and produce a beep from the keyboard speaker during the selftest. It takes the video display a little while to warm up (typically 30 seconds), but you should see some form of cursor within a minute in the upper left hand corner of the video display. Once the cursor is visible you should be able to enter setup by pressing the Setup key and toggle back and forth between setup A and B with the '5' key near the center of the top row of keys. If you see the full displays indicated in the User Guide or Technical Reference Manuals you have a functional video system and a significant portion of the terminal controller is also operational. If you are going to try repairs, I suggest you also get the Field Maintenance Print Set, it provides schematics and helps locate components. As of 2008 the Technical Manuals and Print sets are available for the VT100 and VT103, but nothing seems to be currently available for the VT102. If I'm wrong about this please send me the URL!

I test the RS232 by connecting the serial output to a PC and running a terminal emulator. A second terminal would work fine also. Be sure you have the correct gender connections (Tx <-> Rx) and the same baud rate on both. Per below, sometimes only one mode works, ie Tx or Rx. On one of my VT102 systems toggling the SW6 on the terminal controller board corrected a Tx failure!

  • Error code # on video display -
    See the tech reference or user guide:
    #4 is a keyboard failure, be sure its plugged in, if you have another try it.
  • No video -
    I've had several that do this. Could be a power supply problem or the video controller. If the keyboard leds flash and you get the keyboard beep during self test a lot may be working. Check the RS232 to see if the terminal controller is fully functional. Then start checking video system voltages.
  • Cursor appears, but nothing else works -
    Probably a terminal controller problem. The self test and keyboard beep are done in the terminal controller.
  • Nothing works -
    Try the obvious, check the power cord and fuse next to the one off switch. Then check the power supply.

Terminal Controller

This is the board with the keyboard, video, and RS232 connectors on its back side. The print set calls this the 'VT100 Basic Video', On a VT100 the front side has a series of fingers which plug into a connector that carry power from the power supply and signals back to the video controller. I have seen one VT100-NA where there was a key in the connector below the top four fingers and a corresponding slot in the controller pc board, non of the other VT100 controller boards I've seen have this slot (anyone know why?). These boards all have a verticle row of pins near the center that can mate to the option AVO board. They also have an STP socket for option modules. Its the STP which is used by the VT180 and VT103 to interface the terminal with an integral CPU card. See the Technical Reference for more information on these options.

At this time I have little to contribute regarding repair. As mentioned above, if at power up the keyboard LEDs flash and the keyboard beeps a significant portion of your system is operational. You may be able to transmit characters over the serial port even if the screen stays dark.

Video Controller

The VT100 Technical Reference Manual and Print Set suggest that there were two interchanable Video Controller boards used in these. One was manufactured by Ball, the other by Elston. I've had a number of VT100 and VT102 terminals in my hands, but never physically seen a Ball Controller. If you are attempting repair, you need the print set, but functionally they seem to be the same. These controllers seem to have been used in all the VT1xx line.

Power Supply

sorry, no info yet

VT102 Differences

If you just want a RS232 character display there is little functional difference between a VT100 and its cousins, the VT101 and VT102. However if you open the case the internals of the terminal controller and power supply are significant, and not interchangeable. The terminal controllers and power supply on the VT101 and VT102 are similar and probably are interchangeable, however the VT101 lacks the printer port. I believe the upper printer port available on the VT102 was also available as an STP option with VT100 systems put have never seen one. It looks like the video controller, flyback transformer, and video tube are interchangeable between the VT100 - VT103 series.

Externally, you can obviously spot the VT101 and VT102 series by looking at the model #. VT100 series terminals have two video BNC connectors just below the male 'EIA communications connector' (RS232 port) and above the keyboard connector. The VT100 series have a video input (top) and video output (bottom) bnc connector. The VT101 and VT102 lack the video input connector, ie there is only one bnc connector on the back (vido output). Appendix E of the VT102 User Guide gives a more detailed description of the function differences between a VT100 and VT102. You may also note that when viewed from the back the RS232 connectors on the VT100 series are to the left side of the removable cover pannel and the VT101 and VT102 are to the right side of this cover pannel.

Internally, when you pull the cover off the VT10x, the differences are clearer. The power supply is different. It uses a transformer that is not mounted on the PC board, but rather on the chassis. The power supply also mates to the terminal controller board in a different mannor. On the VT101 and VT102 series the terminal controller board has a built in socket that directly plugs into the pins on the power supply. One mounts in by pushing it onto these pins and securing thumb screws. The VT100 the terminal controller has fingers on the front side the mate to a socket and wiring harness that split the signals to the video controller and recieve the lines from the power supply. On slides the VT100 terminal controller into this socket from the back and its aligned by tracks the guide the top and bottom of the card. Its these differences in the mounting and interconnection the make the terminal controllers incompatible. Presumably they generate the same signals as they seem to use the same keyboard, video controller and display.

The VT103 (PDP11 system) and VT180 (CPM system) are based on the VT100 chassis and terminal controller. As described in the companion document, the VT103 does this with the addition of a QBUS backplane and a beefed up power supply. The VT180 doesn't require a full backplane as the connection to the CPU is done entirely through the STP connector.