Scopewriter Issue Resolved.
mike at willegal.net
Sat May 18 19:45:43 EDT 2019
I finally found time to properly debug the Scopewriter issue that I have been having. This was noticed by many visiting my VCF east exhibit. Basically, with certain strings being displayed, the string would randomly change. I found that the issue was inherent with the Scopewriter itself, and didn’t have anything to do with the SCELBI interface. The Scopewriter uses a TMS3112 shift register a little bit like an Apple 1 or TV Typewriter to refresh display data. It turns out that the hold time of the low level phase of the clock wasn’t meeting the TMS3112 specs causing the recycled data to be occasionally lost. Changing capacitor C2 to a .1uF value increased the amount of time of the clock being held at low level and solved the problem.
It’s interesting how many of the hobby designed products from the mod 70’s have marginal aspects to their designs. Whenever anyone recreates or restores any old electronic device, it often can be a tricky problem to determine whether a problem is introduced or original to the design
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