Sunday, June 11, 2017

Building the IBM 1130 light panel upgrade


My PCBs arrived along with the remaining components, while I was on my trip to NY. I found that I hadn't specified the right size hole to mount my turret connectors directly on the board, but I thought I had a workaround that would retain the turret connectors. It did not pan out, so I will be soldering the power wires directly to the board. 

Small board (one of two)
Large board (only one required)
I will begin to build one of the boards to test it on the 1130. First step is to solder down the surface mount resistors, as they are the smallest and closest to the board. Second step is to solder the surface mount triacs in place. Third is to mount the lamp sockets on the bottom side. Fourth is to mount the signal pins on the top side. Fifth is to mount the turret connectors. 

I am concerned about shorts in my soldered lamp holders, since the bulbs have bare wire leads. This vulnerability affected the original IBM boards and will affect mine too, destroying the Triac immediately. I have two ways to address this. 

First, I will work out an insulation scheme that protects the bulb leads and prevents possibility of a short. Second, I will put in a current limiting resistor to the AC line while I am checking out the light circuits one by one, so that I will only have one of three cases for any light circuit:
  1. The lamp lights correctly and all is good
  2. The lamp does not light due to a bad bulb or open circuit, replace and repeat test
  3. The lamp does not light because holder is shorted but the resistor protects the Triac from catastrophe
By Sunday evening I had the small panel for the far right side completed and a number of lamp holders guaranteed to be short free. First, I fitted the board into place to confirm how it sits inside the 1130 pedestal box on the face of the honeycomb. That was a perfect fit.

Trial fit of one small board against honeycomb
As you can see from the board above, not every position is used on the small boards. The first board above only has 27 bulbs out of the 48 possible positions, thus I only installed components on those 27 spots. The middle board, also a small one, has 33 lamp positions utilized The final, large board has every position implemented, a total of 96 lamps.

I began construction of the second small board, installing all the resistors, triacs and lamp sockets by dinnertime. All that was left were the 33 signal pins and the three turret connectors. Soon those were installed as well and I could move on to the final large board. A very long process, soldering 387 components, so didn't finish this evening.

Tomorrow, I will hook them up to test power with the limiter resistor and check each light circuit. Since my hot resistance of the bulb is around 50 ohms, my limiting resistor to protect against shorts can't be more than about 10 ohms if I hope to see the filaments light.

I am still waiting for my 200 light bulbs coming from China, which I will then have to solder onto the holders to plug into the sockets on my boards.

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