Sunday, March 8, 2015

Attempt to fix bad solenoid for column 60 of the 1132 printer, plus completing construction of boards for SAC interface


I measured the coil for column 60 and found it to be an open circuit. I removed the bolts holding the magnet assembly in place and will move it out of the printer and begin looking more closely at the coil. Best case it is a broken external wire. Worst case, I assumed, I swap it for column 120 then rewind the bad coil.

With the magnet assembly pulled out and open to more detailed inspection, I began by verifying that it was on open circuit but that the wires appeared to be connected. I then wiggled the wires various ways in the hope that the break is somehow in the external lead which can be bypassed, rather than inside the winding of the coil itself. It does not appear to be this case.

Magnet unit removed from printer and ready to test out the solenoid of column 60

I removed one pair of armatures to check the left coil which is an open circuit

Wires removed and checked to see if the leads are the problem

Unhooking wires from what I thought was a group of 12 magnets

This is actually a single bar with 24 solenoids left to right
My hope that I could pop the magnet from column 120 and swap it with column 60 does not seem very practical. Magnets are assembled on bars of 24 across, not individually mountable. I can't even swap the group that includes 60 with the group that includes 120, since those two groups are on a single bar. Any swap of a bar with another only moves the bad magnet in a narrow range between columns 56 and 60.

The coil itself is pretty firmly attached to the external metal core which concentrates the field under the actuator arm. I see what appear to be two thin spikes that were driven down between the coil and the core around which it sits, one on each side, to force the coil firmly in place. I tried prying the coil upwards gently but it didn't budge. I can't tell if there is an adhesive in place or it is simply the wedges holding it so tight.

The next step is to attempt to extract one wedge and gauge whether the coil will loosen and be amenable to extraction. There isn't a good angle to get a tool in to grasp the wedge, alas. The removal of the bad coil might be a fully destructive process for that coil, leaving me remnants to use as a template for a replacement coil that would need to be manufactured anew. That isn't a great option.

I thought about building something similar in concept to gear pullers that are used to remove gears or pulleys from the ends of shafts. However, there really isn't enough clearance. I might be drifting towards the semi-destructive removal approach, pulling it out however I can with an attempt to keep it as intact as possible.

In the interim, I may put the printer back together and use it until I have the repaired coil, trying to ignore the gap at column 60.


I marked up the capacitor polarity on my diagram first thing in the morning, then checked the continuity and freedom from shorts for the board I put through the reflow oven yesterday. A bit later, I opened the garage and soldered on the capacitors, diodes and the headers. The second board is now ready to be wired into the SAC interface.

Board two finished with manual installation of diodes and capacitors
Once it got a bit less chilly outdoors and the fog burned off, I set up to create the third circuit board using the reflow oven. The steps involved in building one board are:

  1. put the circuit board between the holders and tape the kapton paste mask in place
  2. apply and spread solder paste over the mask to adhere to the metal pads on the board
  3. remove kapton paste mask and board holders
  4. place 36 transistors onto their pads, held by paste, in good orientation
  5. put 24 polarized tantalum capacitors in place
  6. put 12 100 ohm resistors in place
  7. put 12 500 ohm resistors in place
  8. put 12 700 ohm resistors in place
  9. put 12 1000 ohm resistors in place
  10. put 12 1500 ohm resistors in place
  11. put 12 4700 ohm resistors in place
  12. put 12 12,000 ohm resistors in place
  13. carefully lay circuit board in reflow oven
  14. reflow the solder paste to bond the components in place
  15. test all connections for appropriate resistances
  16. install 12 diodes through holes, solder by hand and cut off excess leads
  17. install 3 pin header for power connection and solder by hand
  18. solder 12 2-pin headers for FPGA board input-output
  19. cut four notches for placement on the standups that hold the cards in the enclosure

The longest part of the process occurs in steps 4 through 12, manipulating teeny parts into place in proper orientation, held down by the dab of solder paste on the PCB pads. The next longest task is number 15, verifying that the board is in good condition. While the oven requires about 20 minutes to cycle a board, the third longest activity, I can be doing other things while that proceeds.

A fourth board that contains just five of the 24 possible circuits is the remaining object needed to wire up the interface box. I decided to install all the power filtering capacitors on the board to give good clean power behavior. It was also done by the reflow oven just to get consistent quality and a cleaner appearance similar to the majority of the other boards.

Boards 3 and 4 almost completed, just needed headers solder in place
My new supply of headers arrived in the afternoon which meant I could finish all the boards Boards 2 3 and 4 are waiting on step 19, as I hope to cut the notches all at once, but I am wiring board 2 into the enclosure. By the time I wrapped up work today, I had 15 of the 24 signals from the 160 pin plug attached to it. That brings the total up to 39 of the 77 active signals on that plug, essentially half done. 


  1. Apparently the unit was never intended to be field-servicible, only replaceable, as the maintenance manual only describes removal and replacement.

  2. No question about that, but since parts department doesn't exist any more, I don't have much choice and have to repair it.

    I had hoped that the left and right halves were separate, in which case the right hand coil which is currently column 60 could have been relocated by swapping the halves so that it was now column 120.

    The bar with 24 solenoids was an orderable part thus field replaceable, although not field serviceable. I can disassemble it to the long bar, which I probably will do, because that at least gives me better access to the coil.

  3. I was able to remove the coil, open it and find that continuity where I can install new leads. See the second blog post for March 8 for details and pictures.