Saturday, June 13, 2015

No luck with two different sources offering parts from 1053 consoles; some work on Dekatron/Nixie clock and on Pertec drive


This was a morning of doors slamming shut on the various alphabet of alternate plans for the broken margin release latch for the 1053. I have had extremely generous offers from two separate restorers of 1130/1800 systems, but in each case, the mechanism they had available did not include the high speed carrier return feature, so the part I need was not present.

In the usual Selectric mechanism, the margin rack (which has the movable left and right margin components that determine the begin and end of a print line) is spring loaded to the right by a coiled spring installed at the left side of the margin rack. There is a bracket on the right side which trips a link when the margin rack is pushed leftward into the spring. This bracket is black, short and accommodates only a small movement of the margin rack.

On the high speed return equipped Selectrics, the margin rack moves further. There is an air cylinder low on the left side, which connects to a lever which pivots left and right at the top. It has a spring pulling the lever to the right. The lever fits into a space on the margin rack. The vertical lever from the air cylinder is what pushes the margin rack to the right. The rack moves leftward when the carrier strikes the left margin component on the rack during a return.

The margin rack locks in the leftward position by the latch I am trying to replace. When a carrier return operation is initiated, a rod pulls on the part, thus its name of margin release latch, and that unlatches the margin rack so the spring from the air cylinder lever can push the rack to the right.

Normal Selectrics don't latch the margin rack to the left and therefore don't have any mechanism to release it to the right at the start of a carrier return. I have another IO selectric already, but it has only the normal margin rack mechanisms, no high speed carrier return. The two printers that my peers checked both use the normal rack mechanism.

With several doors shut, there are only a few options remaining:

  1. Find a way to repair my current latch
  2. Find a way to modify the latch or other parts of my machine to function properly
  3. Find a collector with a 1053 having the feature who also is willing to swap my broken part for the good one they own (yes, this is not a very realistic option but included for completeness)
  4. Get a machine shop to build a replacement part, after building an accurate plan
  5. 3D print this in stainless steel after capturing the 3D shape using cameras and software

The margin release latch is really two parts riveted together. The bigger part mounts to the margin rack and has the broken end that trips the CR unlatch pivot. The smaller part pivots on a pin pop-riveted to the bigger part. This smaller part is spring loaded so that it causes the margin rack to latch at its leftmost position, but a rod in the other end of the small part will pull it away and release the rack to move rightward.

I have to make the bigger part, drill out the pivot pin to remove the smaller part, then find a way to rivet or secure a new pivot pin that will hold the small part on the big part. In some ways, options 1 or 2 are much easier because of the complicated dual part nature (option 3 the absolute easiest). Option 4 could include the drilling and riveting of a new pin, whereas option 5 requires me to accomplish those last steps.

I am looking to find microwelders in the area, who might have equipment fine enough to repair the tab. As it is a weekend here, I can't proceed directly on this but I did some research for numbers to call on Monday.


The regulated +10V supply is nearly a dead short with nothing attached to the servo board. I am reading about .6 ohms which is close to the resistance of the probe contact and traces. The +10V supply is delivered to all the op amps on the board, which I suspect is where the problem is occurring.

While I had earlier thought that the markings on U6, a 741 op amp, where the result of a manual soldering, I switched to a working assumption that this chip is indeed dead as a doornail, shorting the +10 supply to ground. To test this, I cut this chip out of the circuit and remeasured the resistance.
Alas, no change.

It will be very tedious to have to find and lift two dozen or so connections of the +10V line until I find the place that is shorting it to ground. Other than the op-amp supply, I don't see any places where single component failures would short the rail down to ground.

I have built a comprehensive list of all the components touched directly by the +10V line on the servo board, which I will use to very closely examine them in the search for the shorting component.


I wired up the Dekatron tube and did a temporary wiring of one of the nixie tubes to allow me to check the operation of the newly assembled control board. The Dekatron is a tube with ten 'dot' positions in a circle around the top of the tube, which were used to display a decimal digit on the tube by lighting the particular dot on that circle. It is a gas discharge tube, like a more complex version of a neon lamp. Nixie tubes have cathodes formed in the shape of each of the ten digits, plus an anode, allowing the tube to light up with the numeral visible.

With some sophisticated control, a Dekatron tube can display a glow on each of the thirty positions around the circle. As originally used, each dot that would be lit had two steering grids on each side, used to steer or attract the gas discharge to make it move as the digit value increased. Thus, twenty of the spots were used for very short durations as the gas discharge moved from one of the ten main positions to the next.

With clever logic and the right sequence of voltage changes, any and all of the thirty positions can be lit, not just the original ten main positions. This clock uses this to display a 'pendulum' pattern as the clock ticks away seconds. Six nixie tubes display the digits for hours, minutes and seconds. Finally, four simple neon bulbs are used to display the colon (:) between hours/minutes and minutes/seconds.

The Dekatron does its pendulum dance, the one nixie tube lights its numeral, and the neon 'colons' dance on and off. My pushbuttons for hour and minute put the clock into setting mode and advance the digits. The one function that is not known to be working correctly is the sound from the speaker, which should be issuing chime sounds and acting as an alarm. Since I can't see all six clock digits until I wire up all the Nixie tubes, I don't know if we got to a chiming interval nor can I see what time I have set for the alarm.

I should design a nice retro look cabinet for the clock, whcn I get some spare time, and after I permanently wire up the six Nixie tubes and finish the testing. This is a nice diversion and will be a cool conversation piece.

1 comment:

  1. Consider adding a GenRad Bug Hound to your test tool collection - this will let you trace the short in-circuit without disturbing the assembled components.

    You should be able to do a similar check with a high accuracy ohm-meter (4 or 5 decimal places).