Friday, March 6, 2015

Found cause of carriage restore issue with 1132 printer, plus began hand soldering a SAC interface card


Running down some signals to diagnose the printer issues - both the missing column on print and the failure to do a carriage restore.

It is clear that the hammer driver was firing for column 60 and I even swapped driver cards only to see the problem remain at the same spot. Definitely mechanical. Another confirmation occured when I swapped the wires between magnets for columns 56 and 60 - no movement of the symptom.

The carriage restore button works fine, sets on the skip start latch and the restore latch in the adapter logic inside the 1131. The skip start should be activating the interposer magnet, but the output of the inverter was about half a volt while the input was good. Not sure whether it is downstream, e.g. a short or problem in the printer itself, or a failed SLT card.

I tried swapping the card with another that is involved in carriage space, but didn't see the restore operate. Tracing back through all the events that occured, it became clear that the cause is that the interposer contacts, a pair of exposed contacts inside the carriage mechanism, are permanently open circuit. They need to close to confirm to the adapter that the gearbox has shifted into high speed mode for a skip.

This type of contact is often oxidized so badly it takes quite a bit of scraping as well as chemical intervention before they are released. I wish it wasn't in a very inconvenient spot that will be a pain to access, but it is nice to know that there is a simple fault here that can be cleaned up in order to make the high speed skip functional again on the printer.

Carriage mechanism, contacts hidden under upper part of this picture


I tried to use my keypunch interface to punch out a 1401 program that a museum docent, Michael Albaugh, has written to play music over an AM radio held near the mainframe. His current deck wouldn't punch because it used an 'invalid' character per the ascii encoding set up at CHM for punching 1401 cards.

My program adheres strictly to the same encoding just to ensure backward compatibility with decks produced at CHM. Mike has to adjust the deck after which I can create his punched cards.


I began assigning the signals to the pins and cables I will use with the new FPGA board, although I ran into a conflict between various bits of information from the existing cabling and will need to validate a few things before I finish.

I also got impatient to check out the analog quality of the new boards, since my stencil masks won't arrive until tomorrow, which really means end of day tomorrow. I began hand soldering one board, a tedious process but something that feels like progress.

As of the end of the day, I had all the capacitors and all the diodes on the board, plus half the transistors. Another 18 transistors and then scores of resistors to go. Not too bad, although the SOT-23 sized transistors are the most fussy and slowest to solder down. I am checking continuity and lack of short circuits as I finish each component.

Partially populated board as of end of the day today

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