Began assembly and test of the core memory replacement for the 1130

ALL PARTS ARRIVED FROM DIGIKEY AND THEN THE BLANK PCB SHOWED  UP

The last of the orders from Digikey was delivered on Tuesday. All that remained was waiting for UPS to deliver the PCBs which had been fabricated by JLCPCB. It took a few days to get here from China but I had them in my hand by Thursday. 

PIECEWISE ASSEMBLY AND TESTING OF THE BOARD

I decided to construct logical sections of the board one at a time, completing testing of its functionality, rather than soldering together all the components and attempting to debug the entire thing at one time. There are quite a few very tiny surface mount parts, each of which has the potential for unconnected pads, solder bridges or other shorts. Building 'bite size' portions increases the chance I will completely check over each connection and part both visually and with electronic tools. 

POWER SUPPLY CIRCUITRY INSTALLED AND TESTED

Building the power supply was the obvious starting point. This takes 12VDC as an input on a jack and produces 3.3V rails for all the logic chips. The LT8635S device is a mere 4mm x 4mm with 24 pads around the periphery plus four ground pads underneath and four corner tabs to anchor the chip. 

It will be challenging to solder this on as there are barely any external pins - mainly the connections are underneath as is the potential for solder bridging between pads. I will get the chip installed before any other part, so that I have the best chance to test for connectivity and for shorts. Once that appears good, all the resistors, capacitors and the inductor will be arrayed around the chip to complete its circuit. Connector J1 gets put on the board and then I can apply 12V to verify that it is producing 3.3V. 

SOLDER, REMOVE, SOLDER, REPEAT

The pesky LQFN sized chip has only .25mm pin to pin separation so the potential for solder bridges or shorts was very high. Time after time, I would use the hot air tool to melt the solder and try to get the chip positioned perfectly. I would then test for shorts using a continuity tester, find one or more, and remove the chip. 

CHIP SEEMED PROPERLY CONNECTED SO I ADDED THE OTHER COMPONENTS

In finally got the chip so that there was no short on all the pads connected to the part. I finally got to the inductor and realized it didn't match the footprint on the PCB. In fact it was too big, not enough room from the chip to the pad. 

I tried to solder leads to the inductor and somehow get it into circuit just for testing, but it broke. I grabbed an inductor from my parts bin - not the right value but something I could try to test with. As I tacked it onto the pads I did a short test. The output, one side of the inductor, was shorted to ground!

SOMEHOW THE CHIP IS NOW SHORTED OUTPUT TO GROUND

When I first tested it, the chip wasn't shorted. I put down all the components then it went downhill. I had to walk away and work on something else. In the interim I can modify this to accept 3.3V regulated input so that I can proceed with testing the rest of the board. Later I can figure out how to get this power supply working or do something different for power.