I received this very nice gift from an old friend about a year ago. It is a Monroe 1651 engineering calculator from 1971. Sometimes called a scientific calculator, sales were targeted to engineers as well as mathematicians. It is in magnificent condition considering its age and even came with the original dust cover. Only the power cord had degraded with age but that was easy repaired. I have waited a long time to power it up but wanted to have a variac to bring it up slowly to 120v AC the first time. I didn’t want to risk flipping it on a full 120v AC and having one little problem fry irreplaceable components. In this video I only give an introduction to the 1651 and a full teardown. The power up and and functionality tests is now available in (part 2).

<Video intro & teardown>

Here are some helpful links to information I sourced during my research.
This 1665 is a very close functional calculator to my 1651 and was a wealth of information.
List of Monroe calculators from this time range and some old ads.
This is a listing to another 1651 calculator like my 1651.
Here is a listing for a 1655 which is just like my 1651 but is programmable and uses a punch card reader! COOL!
I found this video of a much abused 1655 in the wild uploaded on YouTube at channel “qwaqwa1960”:

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