Sunday, January 19, 2014

A Few Things Amiss

This weekend brought me a bit more time for working on my Micro Chroma 68 build.  A few more parts have arrived, and I found an alternative for an important one that was still missing.  The board is showing some signs of life, but perhaps just barely so...

Garbled Output on the Micro Chroma 68

I had collected most of the parts required for this build over time, and I ordered the remaining parts during the first week of January.  Unfortunately, there are still parts that have not arrived.  I received two of the missing capacitor values this week, but I am still missing three more.  Two of the missing capacitors are only used in the audio cassette storage interface, so they aren't required for board bring-up.  Unfortunately, the missing parts also include one of the capacitors in the clock circuit.  So, the board has not been testable until recently.

The missing clock circuit part is a 50 pF capacitor.  While poking around in my parts stash, I found a bag of 47 pF capacitors.  The Micro Chroma 68 kit instructions specifically mention the use of a 47 pF capacitor as a substitute for that part.  Since that capacitor is mated with a variable capacitor on the other end of the crystal, the clock can still be tuned to the proper frequency.  I installed the capacitor, tuned the clock using the frequency counter feature of my oscilloscope, and moved onto getting a video display!

Perfboard MC1372 Circuit

The Micro Chroma 68 board is designed to provide an RF signal for feeding a TV through its antenna connection.  As I mentioned in an earlier post, I attempted to hack the circuit to disable the RF signal generation and to just output a composite video signal.  Alas, I don't seem to have understood the MC1372 datasheet as the device was not generating any video output at all.

I went back to the drawing board and built a new MC1372 circuit based on a schematic in the MC6847 datasheet.  The new circuit was similar to the hack I had attempted, but different in a few details.  I built it on a piece of perfboard, and used some wires to plug it into the MC1372 socket on the Micro Chroma 68 motherboard.  Thankfully, this circuit worked to provide the video output in the picture at the top of this entry.


Of course, it doesn't take a lot of expertise to deduce that the screen output pictured above isn't exactly what the board should be producing.  I would have expected to see a sign-on message from the Micro Chroma 68's built-in monitor software, TVBug.  The garbled mess on the screen certainly suggests that something is wrong.

Letting the board sit there and run sometimes results in some screen characters getting changed, and fiddling with the reset button or the break key has some effects that offer tantalizing clues that at least something is working on the board.  Removing the video memory results in a more uniform pattern on the screen, suggesting that the 6847 is able to read the video memory.  So, there are some signs of life on the board even if the current state is far from perfect.

It isn't clear to me what the problems are with my Micro Chroma 68, but these clues suggest that the fix could be as simple as replacing a bad chip or fixing a short-circuit or a cold solder joint.  Finding the answers will require a combination of deductive reasoning based on observations and methodical inspection and testing of the board itself.  There is no telling how long that might take, but please do stay tuned! :-)


  1. Made time for a little poking around... ;-) Also, with a little tuning of the clock circuit I managed to get the pale background of the letters back to the 6847's normal, healthy, nuclear green...

    Using a troubleshooting procedure from the manual, I determined that a couple of the address lines aren't getting driven by the CPU -- cause of that still TBD. But, at least I'm on the trail!

  2. Found your blog due to you mentioning it in the irc chat we had.. pretty cool project.

  3. Do you have a wiring diagram of the mc1372-board? Or is it the standard-circuit as described in the datasheet?

    1. It is more-or-less the circuit from the datasheet. I substituted 2n3904/2n3906 transistors since I had them available, and I may have fudged a few resistor values here and there.

    2. I am attempting to do the same with a vintage game using the same type output. If possible, I would like to see the final schematic for this modification. The video problem looks like a memory problem. Do you have DRAM chips in the unit. I have seen similar displays with bad DRAM.


      Bottom of page 12...

      As for the video problem, I sorted that out long ago by removing the corrosion from the pins on the old chips.