Thursday, January 23, 2014

Keep It Clean

When last we left our daring hero, the Micro Chroma 68 board was just barely showing some signs of life.  The video output was stable, but the garbled mess on the screen was a telltale sign that something was still wrong with the circuit...

Micro Chroma 68 Display
By The Book

The manual for the Micro Chroma 68 board has some simple procedures defined for debugging a non-functioning board.  The first step, of course, is simply to check your work:  check power and ground connections; ensure that parts are installed correctly; and, check for shorted or open connections on the board itself.  I completed those checks, yet the problem persisted.  Fortunately, the next round of testing was more fruitful.

The manual described a few simple and temporary modifications to make to the circuit.  These modifications hold the CPU in reset while forcing the address decode circuitry to select the ROM chip.  This puts predictable values on the address and data lines, allowing for some simple testing with a voltmeter.  These tests showed that the CPU was behaving correctly, but that the data bus held the wrong value.  Further testing showed that while the CPU was driving the address bus correctly, the ROM was seeing the wrong values on its address lines.

Ancient Anti-Static Foam
Baked-On Gunk

More checking continued to show the proper connections on the board between the CPU socket and the ROM socket.  Digging deeper revealed that the disconnect was between the CPU socket and the pins on the CPU itself!  This is a new (or NOS) socket, and it shows no sign of being worn-out.  So, what was happening?

The CPU is one of the chips that were original parts included in the box of the Micro Chroma 68 kit.  The chips were inserted into a block of anti-static foam which served to contain the chips and to protect their pins from damage, and there they sat for roughly 35 years.  When I removed the chips from this foam, I noticed that some bits of the foam stuck to the pins of the chips.  I scraped the visible bits of foam away and gave it no more thought until I witnessed the results of the testing described above.

I acquired some parts cleaner from a local electronics retailer and used it to clean the pins of the chips that were included with the Micro Chroma 68 kit.  After this cleaning and closer visual inspection, I was satisfied that the chips were clean and serviceable.  I re-installed the chips and repeated the tests described above.  Some of the pins now properly reflected the state of the address bus, but to my chagrin some of them were still incorrect.

Strangest Electronics Tool Ever
File It Away

After some thought, I decided on a more aggressive strategy for cleaning the pins of the chips in question.  Using some small hobby files, I filed the pins until they were slightly shiny.  I then hit them again with the parts cleaner, just to remove any remaining filings.  I re-installed the chips onto the board, and another round of testing showed correct values for the address and data lines at both the ROM and the CPU.  Then I removed the test hacks to the reset line and the ROM address decoding, and I powered-up the board.  The picture at the top of this entry shows the result -- the TVBug sign-on message and an apparently working Micro Chroma 68 board!

So, where does that leave things?  I still need a keyboard solution to allow for input -- more on that in the next post, most likely.  I'm also still missing a couple of capacitor values related to the audio cassette interface for storing programs -- hopefully they will arrive soon!  Finally, it would be nice to see at least one program running on the board by the end of the month.  I'm not sure if I can solve all of these issues between now and then, but I might be able to keep things interesting in the meantime.  I hope you will stay tuned!

1 comment:

  1. The Proper and Correct way to do it would be to adapt a CoCo keyboard.