Monroe 1651 Nixie Tube Engineering Calculator from 1971 Part 1 - Intro & Teardown

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 testing will have to wait until next weeks show, sorry.

See high-res photos at my blog posting:

The power up and and functionality tests is now available in (part 2).

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:

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":

[RimstarOrg]

It's nice seeing this old tech. They sure did care about quality. Thanks for sharing this with us.

[ToddFun]

@nidal hamdan, Sure thing. It is what I like to do, glad you enjoyed. Part 2 coming later this weekend.

[ToddFun]

@Patricia Travers, The diode did test just fine at the end of the video so you maybe right. And I did scrape off the black coating. It did look like silver so I also guess it should be fine. I didn't know they plated leads with silver that would later tarnish like that. Good to know.

[ElectronicTonic]

That teardown was ATOM'LYTIC! I don't know if someone already commented on it, but those two black cylinders on the power supply board look like tantalum caps. I've taken apart old computer tech that had a tantalum bypass cap on every IC. The caps were cylinder shape with round or conical on one end.

[mjlorton]

LOL! Well...I certainly noted what you were saying about time to produce and edit videos in this one. Your production quality is great so you certainly know how much time and effort is involved...and you are doing this part time! Hats off to you, I hope it continues to grow and becomes a viable source of income.
Cheers, Martin.

[ToddFun]

@RadioHamGuy, It is a moment I have waited for a long time myself. You're not going to believe this but the original owner of this calculator is still alive. Turns out the person that gave this to me got it from the person that had bought it in 1971 and he was able to contact the owner. He is now 77 and I will try to include the details at the end of of part 2. WOW! how cool is that!

[ToddFun]

@Levente, They are called neon lamps with 99.5% neon and 0.5% argon and have a low striking voltage. They work on AC or DC but normally will have a current limiting resistor because ones they strike they can conduct a lot of current. Common in dim nightlights and for illumination for switches.

[RadioHamGuy]

That is really great to find the original owner on something like that, I never thought that would happen. Or that kind of thing seldom does, so you are lucky there. I bet the owner is happy to know it is still out there also! I will be watching the next video on it for the details.

[ToddFun]

I was just 4 but even then was fascinated by calculators in the Sears catalogs. I remember getting excited when the Sears catalog would show up and I couldn't wait to checkout the hand held calculators. Strange I never collected calculators but I'm getting a couple now just to teardown being people give them away or I get them for a dollar or two at Goodwill or yard sales.

[ToddFun]

@Eric, You are correct. Another person did email me and told me they were tantalum caps. Thanks for posting here. I will annotate the video too. Thanks.

[ToddFun]

@Russ Blakeman Neat. I'm happy they still work.

[russblakeman]

26th week of 71 I was starting high school, a freshman in Chicago. Later in 72 our drafting class got a different model Monroe calculator with nixie display. All of those Sprague caps including the Atom 'Lytic I remember buying new about that same time, when Radio Shack actually carried decent components and places like Lafayette and Olson Electronics were competitors. My 1972 Heathkit SB-102 transceiver has a lot of components similar to the ones in the Monroe.

[ToddFun]

@Polaventris, I was quite surprised by the care and organization of the cabling. I had seen such work on old railroad equipment when I was young but never consumer electronics.

[ToddFun]

@rapsod1911, tubes will leak naturally around the leads over years but it is very small with some tubes and quite a bit with others.

[RadioHamGuy]

Built like a tank, probably very expensive for that technology back in the day. I remember when pocket calculators first came out, now you can get them at the dollar stores, lol.
Nice video! Can't wait to see it light up.

[ToddFun]

@Indyaner, Thanks. I posted some high-res closeups at ToddFun. Just click any photo 2 times to get to the high-res link.

[ToddFun]

@arheron1776, I was in the process of doing just that but realized the plug end going to the calculator was in great condition. I cut the cord with 3 inches left, stagger the cut the wires, soldered and added heavy layers of shrink wrap to build it up to match a salvaged IEC computer cable. The repair looks good and should last its remaining years.

[brentwaldrop]

Enjoyed your presentation. I was with Monroe at the time of the introduction of the 1600 series. I was tagged to travel He US and Canada to teach the 750 man sales force how to operate, program and sell the series.

[AlainHubert]

@23:59 these black cylinders are Tantalum capacitors (I've got the same type in my 1975 Minimoog synthesizer), positive side is the curved one. Values on yours are 6.8 µF 35V.
BTW, I was 6 years old in 1971. ;o)
Thanks for a trip down electronics history lane.

[mjlorton]

Brilliant seeing equipment like this from the past. Thanks for posting this Todd.