Saturday, March 7, 2015

SAC interface board construction underway, solid progress on 1132 printer and card deck punched for 1401


I spent a few minutes checking the column 60 issue, by issuing writes to only that column which should allow me to check for any motion of the various parts of the printer mechanism. Hopefully I can guess what component is stuck, rather than trying to take apart and adjust everything a second time.

I attempted to get to the Interposer Contact switch in order to de-oxidize the contact surfaces and restore operation. It turned out to be easy if you stick one hand from the read under the back of the raised hood and stick the other hand in from the side,twisted about 60 degrees, holding the burnishing tool. A bit of sawing away to remove oxide and the carriage restore function works exactly as it should!

I exercised the printer a while, which did eliminate the sporadic wrong characters which were simply the levers sticking a bit and being too slow to activate. Once they were worked enough through repeated printing, that issue went away.

The missing column 60 problem is more mysterious. I though I had determined that it had to be mechanical but when I stroke the solenoid arm manually, it consistently prints on that column! I might have a bad solenoid, if I am seeing the line fire electrically and the armature moves manually. Time to measure the resistance of the coil.

I am feeling very good about the printer and its rehabilitation. It appears that we are just one problem - column 60 not printing - away from full operation. I can do a test by manually entering the bootstrap loader for DMS2, with the drive active, and bringing up the monitor. It should print a welcome message on the 1132 printer before it goes into a wait due to the card reader being offline.  I am just slightly hesitant because not only is it a lot of toggling to enter 80 words of boot loader card, but then I have wiped out my printer test program which I would have to toggle back into core word by word.


My friend Mike Albaugh updated his program for the 1401 and the deck is now accepted by my keypunch program. I fired up the keypunch and produced the card images for him this morning. I produced 22 cards which I am holding for Mike.

During the course of the punching I discovered a couple of behaviors I didn't like. Sometimes, when a card is not ready in the punch station and you subsequently ready it, the system sits waiting until I touch a key on the PC - any key - after which it picks up and continues. This must be some weirdness having to do with serial communications - perhaps if the Arduino believes that it has part of a character it hangs until the stop bit arrives?

As well, I did see the system drop a card or two when first trying to punch. By rewinding the file and using Pause and Start Punch, I was able to get everything punched out but I need to understand the incorrect behavior and fix it. Another task to stick on the long list.


I organized my design, determining where each SAC signal from the circuit boards would be wired onto the FPGA and what pin on the Spartan 6 are connected to those signals. I also printed my SAC interface card layout and marked up which resistors are to be installed at each location across the board. With this, I am ready to continue soldering the first board, while I wait for my solder paste stencil that is currently at the local post office waiting for delivery.

Hand soldered board #1 ready to be wired into SAC Interface box
I finished the board this morning, drilled holes for mounting and then began wiring it to the 160 pin connector. With that finished and all the headers installed, I have done as much as I can until I have the stencils tonight and can produce the next three boards. When those are all done, I can build a harness to connect the FPGA board and four circuit interface PCBs I just completed.

Wired to 160 pin connector but waiting cables to FPGA board
The stencil arrived in the early evening and I decided to make one board. I didn't anticipate how slow and tedious is the process of placing 144 teeny parts on the paste before placing the board into the oven. I only put 120 on the board, as the capacitors are tantalum (polarized) but my guide sheet didn't mark the polarity of the pads. I know that most are placed with + above ground, but a few are flipped.

Stencil and board holders arrived, ready for use
The oven worked great and to the eye, everything soldered perfectly. The resistors couldn't be better, but the transistors did set with some skew. If they are properly connected, all is good since these are low power, not needing a good bond to a head conducting pad, but I do have to do testing. Once the board tests okay as it was soldered, I will hand solder the capacitors plus the through-hole components (a dozen diodes and 13 small headers.

Board #2 after reflow oven - needs diodes, capacitors and headers installed
Now that I know about the tedious process and how sensitive the transistors are to initial placement, I will leave more time to set them up on the next board. I will also have checked and marked the capacitor polarity so they can be put into place as well. 

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