Refurbishing A DEC 340 Monitor

Back in the “good old days” movie theaters ran serials. Every week you’d pay some pocket change and see what happened to Buck Rogers, Superman, or Tex Granger that week. Each episode would, of course, end in a cliffhanger. [Keith Hayes] has started his own serial about restoring a DEC 340 monitor found in a scrap yard in Australia. The 340 — not a VT340 — looks like it could appear in one of those serials, with its huge cabinets and round radar-like display. [Keith] describes the restoration as “his big project of the year” and we are anxious to see how the cliffhangers resolve.

He’s been lucky, and he’s been unlucky. The lucky part is that he has the cabinet with the CRT and the deflection yoke. Those would be very difficult to replace. The unlucky part is that one entire cabinet of electronics is missing.

Keep in mind, this monitor dates from the 1960s when transistors were fairly new. The device is full of germanium transistors and oddball silicon transistors that are unobtainable. A great deal of the circuitry is on “system building block” cards. This was a common approach in those days, to create little PC boards with a few different functions and build your circuit by wiring them together. Almost like a macro-scale FPGA with wire backplanes as the programming.

Even if some of the boards were not missing, there would be some redesign work ahead. The old DEC machine used a logic scheme that shifted between ground and a negative voltage. [Keith] wants to have a more modern interface into the machine so the boards that interface with the outside world will have to change, at least. It sounds like he’s on his way to doing a modern remake of the building block cards for that reason, and to preserve the originals which are likely to be difficult to repair.

The cliffhanger to this first installment is a brief description of what one of the system building block cards looks like. The 1575 holds 8 transistors and 11 diodes. It’s apparently an analog building block made to gate signals from the monitor’s digital to analog converters to other parts of the circuit. You’ll have to tune into the next episode to hear more of his explanation.

If you want to read about how such a thing was actually used, DECUS had a programming manual that you can read online. Seeing the round monitor made us think of the old PDP-1 that lives at the Computer History Museum. We are sure it had lots of practical uses, but we think of it as a display for Spacewar.

16 thoughts on “Refurbishing A DEC 340 Monitor

    1. I actually did one ECL design in my career. I couldn’t find a reference if that was really ECL or if it was some other forgotten logic convention. I know the KL10 up to one of the VAXes was ECL but I think this predates that. So maybe but I wasn’t sure.

        1. You just did the project a big favor. That is the document I am working the most from but the copy I have is a pdf from a different photocopy and not as good as the one you found. I can see things in your copy that I could not see in mine. Thanks

  1. There’s no need at all to have used ECL logic in an application this mundane, this old, and something this undemanding, from an engineering-design viewpoint. As a matter of fact, the very age of this machine does not coincide at all with the appearance of affordable–if ECL can be ever said to be ‘affordable’–ECL logic. ECL was ONLY used in very-demanding, high-performance, and very often military applications, And was often water-cooled, because of its high power consumption.
    At this period of time, all kinds of logic ‘standards’ were being tried–a lot of which used negative voltages.
    Remember (you probably don’t) what put Digital Equipment Corporation–DEC–on THE map? It was a very major paradigm shift in electronic design called “Flip Chip Modules”, which enabled electrical engineers to design digital-logic systems from pre-built logic modules (gates, registers, flip-flops, etc), the discrete-component parallel of the yet-to-come integrated circuits. These “Flip-Chip Modules” used negative voltages. Electrical engineers built entire computers using Flip-Chip modules. So did DEC.

    Remember the PDP-series of computers? Started out bring built from “Flip-Chip Modules”. Using negative voltage levels, Heresy.

    No; there is nothing in this story to even remotely suggest the presence of ECL logic. It wasn’t even available.

    1. The logic levels that the digital circuitry used are indeed negative. Ground and a voltage more negative than -3 volts defined the other state. There were no such things as ‘standard logic levels’ at the time. Negative levels were common because PNP bipolar transistors were easier to make than NPN transistors and PNP transistors use a negative power rail. Designers produced their own logic gates from discrete components and until a market for standard gates came along later with the introduction of integrated circuits the need for standard levels did not exist.
      In the basement of the Living Computer Museum in Seattle there is a rack mount filled with System Design Modules. 19 inch wide racks and perhaps 8 racks in the rack mount. It is about a third of a DEC PDP 7 CPU I think. A hundred pounds of metal easy, but half a dozen TTL dip packages would be able to replace all the logic a few years later.
      Flip-Chip modules were another series of plug in modules. Building computers at the time depended on standard modules to make the complexity of computer design and construction manageable. Things really have not changed but over time the modules used did more and more until an entire computer could be put on a single circuit board. Before integrated circuits modules had to implement even basic gates so a computer would have hundreds of modules and there was no way around it if a company wanted to make a profit building computers.
      Later desktop computers would have a ‘motherboard’ into which other circuit boards were plugged but only a few were needed. It was the same concept but technology allowed more functionality to be implemented in a single module.
      I would not argue against flip chips putting DEC ‘on the map’ but they did not invent the concept. The first computers used tubes and modularity evolved because circuits had to be changed out when tubes went bad. Necessity was the mother of invention. A gate or flip flop would be implemented on a module just like flip chips would later.

    1. One with very little inventory turnover.

      One which might be right on the border between “surplus store” and “junk pile”

      The BEST kind of surplus store…one that pays YOU to take the stuff :-)

  2. Reminds me of the “glory days” of working on old televisions in the mid-late 70’s that had the round 20+ in picture tube.
    I hated those things. They had such a HUGE glass mask, to prevent some kid with a toy smashing them they were
    heavy som’ bucks!

  3. Surplus Sales Of Nebraska had the other tube. Other tubes may be found and of course I’ll nab any I can. Because the tubes are military radar tubes, they had to be changed out periodically. Hence tubes with many hours of good life are still out there, maybe. The hard part is coming up with a magnetic yoke.
    The technology is all discrete bipolar transistors and diodes. The frequencies emitter coupled logic can handle is high but that speed would be impossible using discrete components. Emitter coupled logic operates in an analog fashion in that the circuits never really turn on and off so changing states is fast. That is where ECL speed comes from but that speed could not be realized unless ECL were implemented as integrated circuits, which it is.

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