Thursday, March 5, 2015

Digging into remaining printer problems plus successful build of reflow oven and arrival of the new FPGA board


I did some chasing down of the signals in the 1131 (processor) that pertain to carriage restore - I saw the odd behavior that the first time after power-on when I pushed the button, I saw both signals change but after that, there was nothing or only the smallest of blips on the incoming signal.

I moved to the 1132 side (printer) to look at the signals, but as I looked at the ALDs I realized I could do some basic voltmeter testing before scoping the lines. The logic for the switches is odd. The switch connects a capacitor to +6V through a resistor when not pressed - the other end of the capacitor is to ground this this charges the capacitors to +6V. When the switch is pressed, it connects the capacitor to a voltage divider whose far end is pulled down to -3V and whose intermediate point is a logic signal going over to the 1131.

When I measured the voltage at the switch for the normally open contact, it registered as -1.2V. I checked the carriage space switch adjacent to it and found that NO terminal to be at -1.9V. This may be normal, with the logic over in the 1131 pulling the -3V up to the measured voltage. Still, I decided to measure the supply voltage at the printer to be sure that the three logic levels of SLT - +3V, +6V and -3V, are all good.

In order to reach the power strip and to put the scope on various SLT card signals inside the printer, I needed to move it out from its current position as the card compartment is at the rear of the printer.

The voltage levels are all solid and I scoped the push button for carriage restore, which is working just fine. On to check the entry of the signal into the 1131 on the other end of the cable - could be cable damage or the problem could be over in the processor.

While I believed I was seeing the printer hammer driven for column 60, the one that is missing, I decided I need to test with the scope on the AND gate that selects for this hammer.

Won't get to any of these additional tests today, but will get on it tomorrow.


Spent a couple of hours outdoors finishing the reflow oven construction. After that, I put it in learning mode, which took many cycles during which it has to heat up then cool back down to near ambient temperature. After it learned how to get the board up to a stable near-melting temperature, it then had to learn how to force the temperature up to the liquidus point fast enough yet not overshoot beyond the target max. When it finally could attain and hold temperatures while the solder reflowed, it then was ready to let things cool down.

Newly updated reflow oven

Radiant reflective tape lining inside of the overn
With the controller trained and ready, I put in a sacrificial board and a few SMT components just to check its behavior with a real reflow job. I found some solder paste from about a year ago, which I hoped would still be good. It appeared to be appropriately melting the solder paste but my actual board test was inconclusive.

I ran out to pick up some new solder paste, a sacrificial board that will better test the kinds of components I will be soldering, returned and ran additional tests. The oven is working exactly as it should.

The FPGA board was delivered today and my paste stencils for the circuit boards are expected to arrive on Saturday. All is proceeding well for the creation of the all-new SAC interface box. If I wasn't waiting for the paste masks in order to reflow the boards, I would probably be manually soldering all the parts on a board.

FPGA board to be used in the SAC

Board has about 100 input-output signals

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