Monroe 40 Calculator Refurbishment

An ear­ly hand­held cal­cu­la­tor from 1974, with a 10 dig­it Panaplex dis­play and recharge­able bat­tery.
As I con­tin­ue search­ing for Monroe 400 series cal­cu­la­tors, I came across this Monroe mod­el 40 cal­cu­la­tor at a rea­son­able price, with a few small issues.
Normally I don’t have an inter­est in hand­held cal­cu­la­tors as most are too new. This one caught my eye with it’s unique styling and 10 dig­it Neon Panaplex II display.

The pre­vi­ous own­er stat­ed that the cal­cu­la­tor worked fine when plugged-in, but the bat­tery pack would not hold a charge. Other than some scuffs, a light­ly scratched dis­play bezel, and 49 years of accu­mu­lat­ed dirt and grime, this cal­cu­la­tor was in over­all good shape.
Before turn­ing it on, I removed the low­er case to inspect for any­thing that may have come loose dur­ing ship­ping, and to get start­ed on clean­ing the case and bat­tery com­part­ment door.

Upon remov­ing the case and bat­tery I quick­ly found what was most like­ly the prob­lem with the bat­tery not hold­ing a charge. The neg­a­tive con­tact for the bat­tery on the pow­er board was heav­i­ly encrust­ed with cor­ro­sion and pre­vent­ing a con­nec­tion with the bat­tery terminal.

After quite a bit of scrub­bing with DeoxIT D5, I was still not get­ting low enough resis­tance mea­sure­ments from the con­tact, so it was time to bring out the 1500 grit wet/dry sand­pa­per. After sev­er­al min­utes of care­ful sand­ing and then a quick fol­low up of con­tact clean­er to remove any residue I had a good low resis­tance contact.

The bat­tery pack is made up of four Ni-Cd bat­ter­ies in series for a nom­i­nal out­put of 4.8 volts. The pack is held togeth­er with a plas­tic skele­ton case with an arrow on top mark­ing ori­en­ta­tion, and two con­tacts on the bot­tom of the pack.

One of the pre­vi­ous own­ers must have been hav­ing issues with the bat­tery as they used a sol­der­ing iron to melt holes in the case where the cell con­tacts would be locat­ed. Using the holes in the case I checked the cell volt­ages and in gen­er­al all cells were below 1 volt.
I decid­ed to con­nect the pack up to my Hewlett Packard / Agilent 6632B pow­er sup­ply to see if I could get the pack to accept a charge. My charge set­tings were 5.8 volts and a cur­rent lim­it of 120 mA.
The pack was accept­ing a charge, but when check­ing the indi­vid­ual cell volt­ages using the holes in the plas­tic hold­er I was get­ting some very odd read­ings, with some cells show­ing twice the expect­ed voltage.

I decid­ed to remove the cells from the hold­er, by scor­ing one side of the plas­tic case at the weld line. After some gen­tly pry­ing I was able to wig­gle out the cells by spread­ing the case apart.
It did­n’t take long to real­ize why I was get­ting my odd read­ings from the cells. A quick check of the actu­al cell ter­mi­nals showed all the cells were charg­ing at close to the same volt­age which was very good.
To get the pos­i­tive and neg­a­tive ter­mi­nals in the cen­ter of the pack the design­ers used some cre­ative cell strap­ping to accom­plish it.

All of the holes on one side of the pack only made con­tact with the long con­nect­ing strip. and two ter­mi­nals on the oth­er side access the con­nect­ing strip of two cells. Using the exposed ter­mi­nals and four holes it is pos­si­ble check each indi­vid­ual cell voltage.

Now that the bat­tery dilem­ma was solved it was time to con­tin­ue clean­ing the cal­cu­la­tor. Most of the case and key­pad cleaned up well, but there was still some yel­low­ing of the beige case that need­ed addi­tion­al work. I had pur­chased a gal­lon of 12% hydro­gen per­ox­ide solu­tion quite a while ago and this was my sixth reuse of the solu­tion. It is still work­ing good but I can tell that it is get­ting towards the end of it’s use­ful life. The case only need­ed around 5 hours in most­ly sun­ny con­di­tions to remove all the yellowing.

The pow­er / HV board is con­nect­ed to the main board with two long screws which also sup­port the two case brack­ets, and a 30 pin inter-board con­nec­tor. The end of the pow­er / HV board has the pow­er switch which is acti­vat­ed by the slid­ing pow­er / mode switch lever which is mount­ed to the upper case.

There are two semi-rigid foam spac­ers between the two boards, which I assume are meant to keep the con­nec­tor ends of the boards from con­tact­ing. I added some kap­ton tape to the main board just incase the foam dete­ri­o­rates in the future. Most like­ly not nec­es­sary but I had it near­by so why not.

The main board is attached to the top case with 5 screws, with the two out­er­most screws being longer in length. The mode slid­er switch pads are vis­i­ble at the bot­tom of the board, along with the key­pad con­tacts and cen­ter com­mon contacts.

The Burroughs Panaplex II Neon gas-dis­charge dis­play is attached to a sock­et and held in place with a hot-melt glue on each side of the socket.

Tucked away under the Panaplex II dis­play is the MOS-LSI General Instrument 175B cal­cu­la­tor on a chip IC, along with a dis­play dri­ver and oth­er com­po­nents. To the left of the 175B is the glass vac­u­um seal for the dis­play with a cutout hole in the main board to iso­late and pro­tect it.

The key­pad is made up of sev­er­al lay­ers stacked on the key­pad frame. First a thin translu­cent plas­tic sheet, next the met­al com­mon con­tact sheet, and then a grey plas­tic spac­er above the main board con­tact pads. The key­pad sand­wich is held togeth­er by the three mount­ing screws.

The key­pad includes sep­a­rate Total, Sub-total, +/= and -/= keys. Along with per­cent and stan­dard ASMD func­tion keys. The zero and + keys are enlarged for ease of use.

The upper case has a tint­ed bezel that is held in place with heat flat­tened plas­tic posts. Brass inserts are used for all screw con­nec­tions. The pow­er / mode slid­er is attached to two met­al rods attached to the case. I used some Novus #2 plas­tic pol­ish and cot­ton tipped appli­ca­tors for buff­ing out the scratch­es on the out­side of the bezel. This took quite a while, but I was pleased with the final results. To fin­ish it up I applied some Plexus plas­tic clean­er, pro­tec­tant, and pol­ish which made the bezel look almost new.

The pow­er / mode slid­er switch has 10 posi­tions. The modes include Off, full float­ing point oper­a­tion, adding machine mode, or a choice of 0 to 6 fixed dec­i­mal places. 

I checked the only elec­trolyt­ic capac­i­tor with my Peak Atlas ESR70 meter and it was in excel­lent con­di­tion. The capac­i­tor is part of the high volt­age sup­ply for the Panaplex II dis­play. The volt­age across the capac­i­tor was 202 volts with a sin­gle zero on the dis­play after clear. The volt­age dropped to 187 volts with 10 eight’s fill­ing the dis­play with very low AC rip­ple.
The DC pow­er input pins mea­sured 6.5 volts on the pin near­est the con­nec­tor in-place switch, and 4.3 volts on the pin oppo­site the switch, with the cen­ter pin as com­mon. This does­n’t quite match up with the exter­nal pow­er sup­ply spec­i­fi­ca­tion of 6.8 and 5 volts, but the cal­cu­la­tor is work­ing fine at these levels.

Overall I real­ly like the design of the Monroe 40 cal­cu­la­tor.
Litton Monroe built a well engi­neered and com­pact unit con­sid­er­ing the com­po­nents avail­able at the time of design and con­struc­tion. The pow­er / mode switch at the front of the cal­cu­la­tor is nov­el and a good use of lim­it­ed space on this size of cal­cu­la­tor.
Though my favorite fea­ture is the beau­ti­ful Neon orange glow of the Panaplex dis­play, which is what orig­i­nal­ly spiked my inter­est in acquir­ing this calculator.