SCELBI CPU card under construction

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Here it is, without the CPU socket, which will be made up of old fashioned Molex strips of pins. 7400 series parts are soldered directly to the PCB, without sockets. I will be socketing SRAM parts on the SRAM board, when I get to that.

SCELBI CPU under Construction

SCELBI CPU under Construction

I’ve been following the old SCELBI documentation which can be found here:
http://www.olson-ndt.com/Scelbi/Schematics2/

The documentation is actually quite good, which saves me a ton of effort in writing my own. However, I’ll share a few additional tips here.

  • The CPU card contains both 7404 and 74L04 parts. Don’t mix them up like I did. 🙂
  • There are 8 1K resistors
  • Actual boards that I have seen, use 3K instead of 3.3K resistors
  • The 5K trimming resistors have the pin that is offset from the other 2, offset by 10 mills. Bourns offers two spacings, either 20 mills or 10 mills. Somehow, though I knew the difference, I ended up with the wrong version. I was able to bend the pin under to fit, for now, until I am able to replace them with the correct part.
  • The are 8 1N914 diodes and 2 zener diodes. The document only mentions 9 diodes.
  • I’m using modern 6.2 volt and 12 volt, 5 watt zener diodes for the over voltage protection circuit. Actually, I don’t have the 6.2 volt ones on hand, at the moment, so they will be installed later. The leads on the 12 volt zeners are too big for the holes, so I drilled out the hole with a 3/64″ bit mounted in a pin vise. I drilled in a bit in from each side, rather than straight through from one side, so I didn’t risk delaminating the pads. This effectively removes the plating, so I soldered the end that had a trace on the top side on both sides of the PCB. This isn’t noticeable.
  • After soldering, I clean the flux from the board with isopropyl alcohol.

    • I obtained parts from http://unicornelectronics.com, http://www.onlinecomponents.com and http://www.mouser.com. It is possible that Unicorn may be able to supply complete parts kits in the future, as I am going to provide them a complete BOM, once I get the prototype working.