I recently had the opportunity to play around with the innards of my Heathkit MicroMatic Keyer. I couldn’t find the manual for it, but fortunately, I was able to locate the schematic on the Net.
Unfortunately, Heathkit schematics only call out the Heathkit part number and not the generic part number. For example, U7 of the keyer is labelled on the schematic as 443-897, and without the parts list, you’d never know what it was. You could guess the function of the chip from the symbol, but that only goes so far.
To the rescue is Technology Systems’ Heathkit Parts Cross Reference”. Using this page, I was able to determine that the 443-897 is actually a 74LS174. Cool!
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