Monday, September 14, 2015

SAC Interface Box still one signal failing, plus console printer and console switch issues


I ran the KB/Console Printer diagnostics although I am not sure exactly how to control them. They start off in the keyboard mode, but it never switches out even though the documentation claims that it should end with the console/keyboard switch moved to console.

I did do work on the resistor panel when wiring in the emulation engine board and was very close to PR1A which is the connection for the console/keyboard switch to indicate it is in Console mode rather then Keyboard mode. If this is not working, it could cause the difficulty I am having with the diagnostic getting past the keyboard entry routine.

In the keyboard echo mode, I found a few keys that aren't producing the correct typed character, which is an issue with the microswitch contacts inside. S, +, (, the cent sign and the vertical bar are the few in error. The common failure with +, (, vertical bar and cent sign is that these are a combination of the 12 row plus other rows, but the 12 row is dropped. S on the other hand is a 0 and 2 row, but is dropping the 2 row.

Through examining the switching matrix in the keyboard, which consists of latches that are unique to each key stem plus bails that some number of key stems can activate when depressed, I figured out what two contacts are oxidized and not contacting reliably. Bail 12 is tasked with emitting a row 12 for any shifted (numeric side) character that requires it. That matches all the failed characters except for the problem with S. The key stem for S activates latch 9 which is only used to emit row 2 when the key is pressed in alpha mode. I will use my burnishing tool on them later.

I ran diagnostics a few more times and did discover that the console/keyboard switch is not operative, so that the machine believes it is always in keyboard mode. This blocks the diagnostic from moving on to other tests especially of the console printer. Task - repair this switch.

Since I still have the defect in the pivot arm for carrier return and index operations, I really need to attend to that before trying to run too many more diagnostics on the console printer. The 1/4" OD hollow tubing arrived, from which I will cut the bushing which the pivot arm will rotate around. Task - repair pivot arm.


I pulled the connectors dealing with the four interrupt requests off from the driver board and the fpga board, tested continuity and verified correct wiring, resistance and polarity for the outgoing lines. Everything looks good, so another power on test is in order, although I didn't actually change anything.

I have continuity from the fpga pin out to the chip input, but the value never goes on even when I request an interrupt on IL4. This may well be an fpga board issue, either a blown output pin or some error in the synthesis tool.

I will make a few changes to the logic and rebuild, so that if this is a random error it will move around. Otherwise, I have to consider pin D12 of the board to be unusable and copy the line over to another of the precious few IO pins left, B27.

I finally realized my documentation was out of date and pin D12 was feeding Int Req Level 5, working just fine. Int Req Level 4 is on pin C12, whose continuity I had not explicitly checked. Still, when I put a voltmeter on the wire to pin B27, it didn't rise to logic level 1 and this is a copy of my level 4 interrupt request so it should rise even if C12 does not.

I shut down for the night, but know what testing I need to do next - diagnose and repair the interrupt level 4 issue.

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