Monday, September 22, 2014

Busy day, not too much time with 1130, but some diagnostic progress


It was very clear that the keyboard selects and latched down keys are released when the restore button is pushed, but otherwise it is not functioning. The Int Req button should immediately trigger a level 4 interrupt, but nothing is occurring. As well, when I select the keyboard, the Select lamp lights, then push any key, it should interrupt on level 4 and allow an XIO read to grab the character pushed. That does not happen.

Out came the oscilloscope, allowing me to trace the state of various signals until I can sort out where the problem lies. First up, I watched the two latches - interrupt requested via Int Req key, and keypress occurred - but they never responded in any way. Moving backwards through the logic, I found the signal from the Int Req key as it arrives from the keyboard, but that didn't register either. In fact, it seems to have an unusual voltage on the line.

Continuity from keyboard to gate A - good
I moved to the pin where the cable is plugged into the logic compartment, but that showed no sign of life. It was time to check signal continuity on the cable back to the keyboard assembly and if needed, back into the mechanism itself to the switch. Out came the voltmeter for some power-off testing.

Immediately it was obvious that I had good connectivity through to the keyboard and through the key. The last element to check was the +12V that runs through the keyboard contacts and down to the logic.

Connections for Int Req are in that clump of wires at top
Testing connection to the 12V supply into the Int Req switch
Where is the 12V supply to the keyswitches?
Open up everything, get to the keyboard assembly
It comes in to the keyboard, through a contact what is interrupted as the keyboard restore takes place - the bails are pushed back by a bar energized by a solenoid, with a contact switch on that bar. It is the same kind of contact switch as used on the keypunch - and those were so oxidized they wouldn't conduct a current.

Zooming in on all the contacts (bails, latches and connector boards galore)
Keyboard tipped up on its side to see the underside
The contact switch for the keyboard restore bail - our problem
Same situation here - the keyboard restore bail contact switch was oxidized to the point where even with deoxidizer and rubbing of the contacts, it had a resistance of over 30 million ohms in spite of being closed. This will probably require emery sandpaper to cut the oxide down to the conductive metal. I made no progress by the time I had to close down for the evening.

36,130,000 ohms instead of 0

I wrote a nice routine to read a card, store the characters in a buffer and keep count of columns read, errors encountered and so forth. With it, I found I was receiving error checks right away on the card, probably timing issues where the card is not moving fast enough to satisfy the error checking, due to gummed up clutches and excess friction in the mechanism or drag on the cards themselves.

I am deferring work on this while I get the keyboard working. That should be relatively easy and rapid. It will be reassuring to have a complete XIO and interrupt behavior working perfectly, even for the very simple keyboard device, before I am troubleshooting the reader, printer and disk devices. Otherwise there might be a CPU problem that I would assume is an IO device issue, wasting time.


I received the ribbon color shift tape from Lukas Tschudi today, which I began to install on the typewriter mechanism until I ran out of time. I will work on this along with the keyboard when I get back to the system tomorrow.

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