Bringing to life the canonical DEC VT100

Original author: Mattis Lind
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Previously, there was a translation about restoring another terminal - LA30 . Italics in my comments


DEC VT100


The VT100, in a sense, is the industry standard among terminals. The set of commands supported by this terminal was borrowed by anyone just getting there. In the DEC line of terminals, the VT100 was the first to be released with a freestanding keyboard. The Intel 8080 microprocessor acts as the heart of the device. The VT103 terminal also had a similar case, in which two TU58 tape drives and a small backplane with QBUS bus were added. In addition, VT125 was also produced, which, in fact, differs from VT100 only in the presence of an additional board to support ReGIS instructions that implement simple computer graphics.



Launch




We decided to first test the power supply, because the terminal last turned on many years ago, I connected a continuously variable transformer ( variac / variac ) to the 110V AC input ( not sure why this was done), and also shunted a small transformer of the primary circuit, through which a bias voltage of + 12V is provided for the control circuit. Then I applied bias voltage to a small laboratory power supply. After connecting the test load to the + 5V output, the incoming alternating current began to grow slowly. The key element was quietly switched, and the PSU issued rated voltage according to the specification. The power resistor of the damper circuit (R27) emitted a faint odor, but, since everything else was in order, I decided to connect the PSU back to the terminal. The R27 resistor in the PSU was 1kOhm, although, judging by the diagram below, it should have been 500 Ohm.



After installing the PSU in the terminal, I started the system. After some time, a dull and unfocused cursor appeared on the screen, and the smell from the resistor only intensified. Since there was no focus, I missed all the potentiometers on the monitor board with the Deoxit D5 contact cleaner.



However, the next launch attempt was not as successful as the first. Almost immediately, a haze came from the horizontal transformer. A bad sign ... Nowadays it is difficult to get lowercase transformers, but I was lucky - a friend-collector from the USA kept several pieces and sent me one of his supplies. At the top left in the photo is my broken original one, on the right is a new replacement.

Transformer bell


How can we check the operability of a line transformer and not burn the transistor of the horizontal output stage? Let's ring him! We drive the signal in the form of a meander through a small capacitance (3.3nF, for example), connected in parallel to the primary winding of the transformer. The signal can be taken, for example, from the calibration output of the oscilloscope. A functional line transformer must attenuate on both the positive and negative edges of the meander. If there is at least one shorted coil in the coil, then this will show the continuity. A shorted turn can be simulated by simply wrapping the transformer ferrite core with wire.



Above is the oscillogram of the working horizontal transformer, and below the transformer with a shorted turn. Such an element needs to be changed!



Screen board


The board is connected to a horizontal transformer and to the neck of the CRT. Unfortunately, the old burned-out transformer caused damage to the transistor of the horizontal output stage - a short circuit occurred between the base and the emitter. The transistor BU407D (Q414 in the diagram) has been replaced by a BU406D with a slightly larger maximum allowable voltage. Short circuit of the transistor caused knocking out a small two-ampere fuse.



As a precaution, I tested all capacitors with an immitance meter. One of the measurements showed a slight underestimation of the characteristics, but, nevertheless, the value fit into a 20 percent tolerance.



Having replaced the transistor of the horizontal output stage, the fuse and the line transformer, I decided to carefully check the operation of the monitor without the risk of breaking any component. Therefore, I connected the laboratory power supply to the horizontal section of the board. I was able to do this by soldering the 1-ohm power resistor R478. He launched the terminal and gradually increased the voltage supplied by the laboratory power supply. At about 5V, the screen showed a weak and narrow picture. When applying 10V and adjusting the height, line linearity and brightness through the corresponding potentiometers, I got a pretty good image. The current strength of the laboratory power supply remained at an acceptable level and I did not feel any strange smells (well, besides the power resistor in the power supply).



Terminal control board



The terminal is controlled by the processor board on which are located: intel 8080 (the processor itself is located under the AVO board, Advanced Video Option, in the photo below), several memory chips, firmware on four ROMs, a character generator ROM, and two TTL logic matrices designed for video processing -exit. In addition to these components, of course, there are many standard TTL-chips.



Our board also has an optional AVO (Advanced Video Option) board with 4 sockets for ROM chips, which could overlap ROM data from the main board. The pin connector on top was designed for the VT125 terminal, and allowed the special module to intercept RS-232 signals before they are processed by the VT100 main board. This module recognized special ReGIS commands that were used to render graphics.

Documentation



The instruction for the user is very helpful in setting up, because it is quite difficult to remember which option in the menu is responsible for what. During the repair, of course, you need a service manual and a set of circuits .

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