Re: In Desperate Need of Monroe 820A Service Information! HELP!

From: Rick Bensene
Email: The Old Calculator Web Museum


The Monroe 820A seems to be a very rare machine, and there's virtually no information out there that I've been able to find. Litton/Monroe didn't make or design their own electronic calculators with the exception of the EPIC 2000 and 3000 calculators. The machines Monroe marketed were made by Canon or Computer Design Corporation (and perhaps some others). The 820A is definitely not a Computer Design Corp. design. The general layout does seem very Canon-like, but it's very difficult to tell without more detailed images of the machine. The image of the 820A posted in the WANTED section of my website is from a Monroe advertisement. So far, no actual examples of the machine have surfaced to my knowledge. It isn't even clear if the 820A actually used a CRT...it might actually use some early form of 7-segment planar gas-discharge display. There were quite a number of calculators that utilized CRT displays, including Hewlett Packard's 9100A and 9100B, Friden's 130 and 116x-series machines, SCM/Marchant's Cogito 240SR, and a few others. Patent information for the Friden 130 and HP 9100 calculators is quite detailed. You can find the patent information on the US Patent and Trademark Office website. From the theory of operation and drawing sections in the patents, you should be able to get a pretty good idea how these machines generated their displays. Most of the display systems utilize modulated sawtooth-shaped waveforms to form the digits. A simple diode ROM would contain the segment renditions for each of the characters (0-9, +, -, decimal point) to be displayed. It's unlikely that it will be possible to find a schematic of the Monroe 820A. Since it isn't even clear who actually made the machine for sale by Monroe, there's really no place to even start looking. Plus, the machine is over 30 years old...and in most cases, the manufacturers have long-since thrown out any materials relating to such old products. Good luck with your project. Rick Bensene The Old Calculator Web Museum http://www.geocities.com/oldcalculators