Worked out LED equivalent circuit and finished deconstruction of the old FPGA 1130 extender box

TAKING SAMPLES WITH A 100 OHM RESISTOR AT VARYING VOLTAGES

In order to figure out what was happening with my LEDs I used my power supply through a 100 ohm limiter resistor and measured the current consumption. I did samples at 4V, 4.5V, 5V and 6V as a means of discovering the approximate diode drop of these LEDs. If you remember, the VOMs were unable to measure the voltage drop at all. 

I had values ranging from 16ma to 35ma for the current at these different voltage points. A simplistic application of Ohms law will yield different resistances for each sample, but we know that is due to the fixed voltage drop of the LED. The remaining voltage after the drop is what is limited by the 100 ohm resistor to produce the detected current. 

EFFECTIVE CIRCUIT IS 2.2V DROP AND NO IN-LINE LIMITER RESISTOR

I am doubly glad that I had used the 50 ohm resistor during my initial testing. When I varied voltage drops to get as close to a fixed resistance for all the sample currents, it came out to just a touch over the value of the resistor I used for the sampling - 100 ohm. Had I not used a resistor at all, there would have been essentially no limit on current and the LED would have died. 

With that fixed resistance for all the samples, the voltage drop of the LED would have to be roughly 2.2V. It is probably above the voltage pushed by my VOM in diode drop mode, which explains the lack of readings. This produces a reasonable set of values that explain all the results I saw and allow me to figure out what resistor values I want when I am feeding 24VAC to one of the LEDs.

CALCULATED AND TESTED PROPER RESISTOR FOR THE 24VAC LINE TO LED

I chose 90 ohm for the load resistors on the LEDs fed with 5V and 820 ohms for the one that is driven by 24VAC. That produces the same currents in all and therefore the same perceived brightness. I used a through-hole 820ohm on the bench to verify this before I ordered the surface mount resistors for my project.

FINISHED DECONSTRUCTION AND JUNKED THE OLD EXTENDER CASE

With all the leads to the 160 pin socket beeped out and properly labeled, I could hack apart the rest of the rats nest of cabling and toss out the old PCBs. I removed everything from the case, salvaged a few items I can re-use in future projects, then dumped all the rest into the trash bin. 

PCB REMAINS FROZEN AT 7% OVER IN CHINA

With just 15 hours to go for the 96 hour build time commitment, the board remains stuck at step 1, inner layer trace formation. It is currently morning in China, just before the beginning of a 'day shift', so we might see a burst of activity in the coming hours to complete it all on time. Just as possible, it might take longer than the quoted four days to produce at this pace. 

DISCOVERED THE MISSING SPRING FOR THE 1053 SELECTRIC PRINTER TAB LATCH

My 1053 console printer, an I/O Selectric, will not latch up a tab. When tab is activated, the carrier moves only as long as the cycle that processes the tab command, when it should have latched the carrier and allowed it to continue moving rightward until it 'slams' into a tab pin that was set. The bump into the pin releases a latch and locks the escapement to the current position.

The tiny spring holds the latch in position, so that when the tab operation pushes the escapement out of the rack it will stay that way until hitting a tab pin somewhere to the right. No spring, no latching. 

I discovered this by taking out my USB microscope camera and peering into the CDC 2741 style I/O selectric terminal I own and then into the 1053. Once I knew what was missing, I found it on the Parts Catalog diagram and just need to locate a 1166516 spring from my collection of Selectric parts.