Restored power to M1000 Documation reader, working on the OneDark signal flakiness

 DOWN TWO WRONG ROADS WORKING ON THE MISSING POWER

Because the power disappeared abruptly right after I inserted a card with tacked on signal wires, I assumed that I had somehow shorted the 5V rail and blown the fuse that is inside the reader. My voltmeter confirmed the absence of 5V on the motherboard, which was consistent with this hypothesis.

I opened up the front and back covers of the reader to get access to the power supplies below. First up was a test of the Slo-Blo fuse. It was good! I then jumped to the second wrong road, assuming this meant that the power supply itself had failed. A filter capacitor sits right after the bridge rectifier, feeding DC to the voltage regulator that would produce the 5V used by the logic. A quick check showed 16V and very little ripple; that too was consistent with the guess that the voltage regulator had failed.

Bridge rectifier and filter capacitor input to 5V regulator

I disconnected the resistor board that feeds the LEDs for the photocells, disconnected the regulator from the filter capacitor, and removed the allen screws holding it to the chassis. It was my intention to feed it 16VDC with bench supply while I did troubleshooting to find the failed component. 

5V regulator unit

Because I had access to the main wire that deliver the 5V up to the card cage above, I moved them around to allow me to to move the card cage further out for access to signals. It was then that my eyes spotted the loose wire where it should be attached to the motherboard.

White positive lead is disconnected!

The stranded wire had flexed enough to have snapped all the strands, one by one, until it cleanly detached. After I removed the remnants and resoldered the wire, I believed that 5V would be back and the reader was operational. I reattached the regulator board and wired it in, then confirmed that my power problem was corrected.

ONEDARK SIGNAL CONTINUES TO BE FLAKY

When I had power back, I could see that OneDark was off, as it should be initially. I moved a strip of cardboard in the photocell slot to toggle the signal on and off. However, it behaved very strangely - flipping on then decaying back to off even though the cardboard remained. Looking at the schematic, I do see a 0.01uf capacitor to ground in the OneDark circuit, as well as in the OneLight circuit which toggles accurately matching the holes and empty columns as a card moves through.

Later, it stopped giving me any OneDark at all. The signal is produced by twelve open collector inverters tied together, so that if any photocell is blocked its inverter pulls the shared line to ground. That shared line is passed through a normal inverter to create the OneDark logic signal.

I put the voltmeter on the input to the final inverter and see that the shared line was sitting at 1.65V. This is an invalid logic level and very suspicious. It is hard to see if one of the twelve open collector gates is partially conducting since they are shorted together as a wired-OR gate. I decided to start with the 7404 inverter chip which delivers the final One dark signal, as it was relatively easier to change that multiple 7405 open collector chips 

Alas, no change with a brand new 7404 in place. At this point, I see three possible failed components. Either of the two 7405 hex inverter chips could be bad, or the ceramic disc 0.01uf capacitor could have turned into a resistor. The last failure would explain the voltage level if the capacitor is so leaky that it has a resistance under 250 ohms. Thats a fairly extreme failure for a ceramic disc but worth checking. In fact, it is easier to yank the capacitor, check its resistance and replace it with a known good part than it will be to pull the two inverter chips so I will do that first when I return to the shop.

VARIOUS SLOWDOWNS IN MY WORKSHOP ACTIVITIES

I am forcing myself to spend an hour each time I visit the shop doing sorting and organizing tasks. That way it will slowly but inexorably get more organized and productive over the course of a year. 

Second, the rented house we have for another two months is more than 30 minutes drive each way from the shop. The condo we are buying will be five minutes away but I have a month at least until I can benefit from that.

Third, we have logistics to deal with related to the sale of our California house, the purchase of the beach home, and buying all the furniture we will need while the Labor Day sales are on.