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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.



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