[Coco] CoCo 3 at true 2Mhz
sales at gimechip.com
Mon Nov 8 11:14:56 EST 2010
While scouring around SockMaster and Paul Barton's pages, I considered their suggestions of replacing the 28.63636Mhz Crystal (28.475Mhz PAL version) with a 32Mhz Crystal. This results in true 1Mhz/2Mhz operation, but also affects the video sync signals so the cm-8 can no longer sync to the video. What I was hoping was this: I have two Roy Justus VGA adapters that I bought used. I was hoping at least one of them would sync to the signal. So what I did was socket the Crystal on the CoCo Mobo to make experimenting easier. I plugged in a 32Mhz crystal. Now, both of my Justus VGA adapters sync and create a beautiful stable picture, however, it is up and off the left top side of the screen a bit. In a little while I am going to open one of the converters and see if making the adjustments outlined in the docs can center the picture or maybe I can center it with my VGA monitor controls. It looks hopeful.
Now, the primary reason for socketing the crystal was not for the 32Mhz crystal hack (this idea came later after reading above mentioned pages). What I had initially intended to do was swap between 28.475Mhz and 28.63636Mhz crystals to test mine and my sons hardware projects for compatibility with both NTSC and PAL clocked machines. I have at this time laid out a very small PCB that will house 4 small surface mount crystals and allow jumper selection of 28.475,28.63636,32 and 40 Mhz crystals. This is all purely for experimenting.
I have also replaced the CoCo 3's pass transistor with an ECG377. Paul Barton had suggested, but never got around to testing, that an NTE377 (ECG377) could be used and might make the machine run cooler. Honestly, I didn't think it would run cooler, but I wanted to test it anyway. The transistor works fine as a replacement - but it still gets hot.
More later as testing progresses....
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