Monroe electromechanical calculators

[Tony Duell]

> 
> 
> I brought one of these home from Bletchley last weekend - turns out
> they're quite addictive little things :-)  It's a Monromatic 8N-3-213
> which was marked as broken - I thought I'd see if there was anything
> obviously wrong inside, plus of course I was curious as to what the
> innards of such a machine looked like.
> 
> Question - does anyone have service information for these machines
> (yeah, right!) or useful tips on things to check over?

No psecific service information, but my first comment is that 99% of 
problems with mechanical calculators are due to gummy lubricants. Worn or 
broken parts are uncommon.

Serondly, do not spray it with Wanton Destruction 40, or anything else 
for that matter. You will do a lot more harm than good...

> 
> So far I've found an acorn jamming up the works (!!) and lots of dirt,

An Acorn... What, a System 1, or an Atom, or a Beeb or an Arch, or what...

(sorry, couldn't resist...)

> but the mechanism's reasonably oily still and in good condition. Some of
> the keys seem to jam occasionally or not latch - I'm hoping that's down
> to dirt rather than wear (it seems to be improving the more I tap away
> at things to be honest).
> 
> After checking that everything was free to turn without jamming I
> applied power...
> 
> There's some earth leakage which would be useful to track down -
> possibly a break-down in cable insulation? I've not worked out what the

Most of the time, that's due to tracking across the impregnated fibre 
insulators used in things like the automatic on/off swtich. 

> large rotating thingy on the outside end of the motor shaft does yet
> though, as this seems to be connected to an insulated plate (and throws
> off quite a few sparks when running)

Almost certainly a governor. Do not be tempted to bypass it, many of 
these machines used series wound motors which will get up to a ridiculous 
speed if not controlled and do considerable damage. 

Trakcing on the insulators in the governor is another source of earth 
leakage.

You generally have to disconnsect the wiring between the power connector, 
motor, automatic switch anf governor and test each bit separately with a 
megger (how do you know there is a leak, BTW?). Hopefully you can repalce 
the offending insulator. If not, you could run the machine off a good 
isolating transformer, and make sure the chassis is properly earthed.

> 
> Can't say I've ever seen such a complicated thing. Electronic circuits
> don't even come close in complexity :-) I don't think I'd be up for a

I've not seen this one, but from the mechanical calculators I have been 
inside, there's typically the same mechanism repeated <n> times (digit 
entry, accumulator register, etc, and some control linkages, which are 
typically outside the side frames. There are a lot of parts, but the 
complexity is not that high.

> complete stripdown and rebuild; I'd never get it all back together.
> 
> ps. Is there a better place to ask than here? Not sure whether
> electromechanical calculators are on topic or not!
> 
> pps. Anyone care to guess a date for this machine? I'd make a wild guess
> at mid-50's but really don't know.

Sounds about right. Maybe a little later (early 1960s). 

-tony