Sunday, January 12, 2014

Rapid Advance

The Micro Chroma 68 build is making good progress!  My schedule aligned on Saturday such that I had several hours which I could fill with the smell of melted metal.  So far the only casualties are a spool of solder and the worn-out tip of my soldering iron... :-)

Progress on Micro Chroma 68 Construction

Progress Report

With most of the required parts in hand, it was easy to make progress on construction.  In particular, I had been waiting on the various IC sockets to arrive.  It is standard practice when assembling a PCB to install the sockets first.  Sockets have lots of pins, and it is easiest to get them installed properly when there is nothing else on the board to prevent them from laying flat while being soldered.  Other parts that lay flat (like resistors and diodes) are typically next in line, followed by capacitors and special items like crystals, connectors, and switches.

I now have the IC sockets, resistors, diodes, and capacitors installed in the Micro Chroma 68 board, with exceptions for the RF tank circuit (discussed below) and the handful of parts which I still don't have.  This includes a handful of oddly valued capacitors, and one socket that I overlooked when ordering the rest.  It should be simple to install those parts as they get here.  As long as they arrive on time, those parts should pose no barrier to completing construction.

Imperfect Fit for C5 and R2

I have been pleased at the availability of some of the less common parts required for the build.  Between surplus electronics providers and various auction sellers, I have been able to locate usable options for every required part so far.  Unfortunately, it can be difficult to find the exactly correct footprint for every part.  In particular, I had to do some creative bending of the leads on both the potentiometers and the adjustable capacitor in order to fit the holes on the PCB.  This isn't uncommon or unexpected -- I'm just glad that the pins were long enough to reach!  Of course, doing this probably gives the vapors to all of those "period correct" Apple I replica kit builders... ;-)

Engineering Opportunity

One part that I made no attempt to find is the variable inductor that forms part of the tank circuit for the RF modulator.  This part is unnecessary because I don't intend to have RF output for the video.  Such a circuit was useful back in the days when the only way to use an unmodified TV as a monitor was by building a tiny TV transmitter into your device.  But nowadays simple composite video is a widely available option, and it will generally provide a much higher quality display as well.

Fortunately, the Motorola MC1372 part used in the RF modulator circuit can be configured to produce just the composite video without the RF modulation.  As detailed in the datasheet for that part, this mostly requires replacement of the RF tank circuit with a single diode.  The devil is in the details, of course.  But I intend to work-out those details and find a way to hack the PCB in order to produce composite video instead of the RF output.

Other than the video output hack, the board is almost ready for some limited testing.  Unfortunately, one of the missing capacitors is part of the clock circuit.  So while I can do some limited testing for shorts/opens and the like, I can't do much to test the functioning of the CPU or VDG or anything else in the system that needs the clock to function.  The waiting game continues, so like me you will just have to stay tuned...

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