Monroe 425 Panaplex display calculator

It was­n’t the exact cal­cu­la­tor that I was look­ing for, but the pur­chase was well worth the price paid for it. The mod­el 425 from Litton Monroe is a 12 dig­it Panaplex dis­play with mem­o­ry calculator.

Luckily the cal­cu­la­tor arrived for the most part undam­aged. The sell­er did a real­ly poor job with pack­ing the unit and the front pan­el was popped out of the back case when it arrived.
I was able to loosen the four case screws and the front pan­el went back to it’s nor­mal loca­tion with a bit of persuasion.

Monroe 425 fresh­ly removed from bot­tom case

After mak­ing sure that noth­ing was per­ma­nent­ly dam­aged from ship­ping I opened the case to see what was inside. The only thing that seemed out of place was induc­tor coil L3, which was hang­ing over the edge of the cir­cuit board by it’s leads. There was quite a bit of lint and foam debris float­ing around, but I some­what expect­ed that from the age of this calculator.

Top of main board assem­bly after foam removal and cleaning

The open cell foam behind the key­pad had most­ly dis­in­te­grat­ed due to it’s age and had to be removed along with it’s own accu­mu­la­tion of lint. I end­ed up com­plete­ly remov­ing the key­pad assem­bly to get at the fine foam dust that had set­tled between the key modules.

Monroe 425 key­pad frame assem­bly after clean­ing with key modules

I used some DeoxIT D5 con­tact clean­er to clean up the key con­tacts and cir­cuit board con­tact traces, along with the card edge con­nec­tor for the front pan­el switches.

Monroe 425 main cir­cuit board with key­pad removed
Back side of Monroe 425 main cir­cuit board

The main cal­cu­la­tor IC is a General Instruments 177D and is sup­port­ed with an ITT 50007 and a Motorola 50008 IC. All of the IC’s are dat­ed around mid 1974, which leads me to believe that this unit was built in late 1974. The Neon gas-dis­charge 12 dig­it Panaplex II dis­play is sol­dered direct­ly to the main board along with the short pow­er con­nec­tor wiring harness.

Main cir­cuit board com­po­nents behind the Panaplex display

There is a mem­o­ry in-use LED attached to the right side of the dis­play, and a sep­a­rate pur­ple wire that attach­es to the pow­er sup­ply board (pin E9) with a clip. The rec­tan­gu­lar cutout in the board is where the AC pow­er switch wiring pass­es thru and attach­es the pow­er sup­ply board.

Monroe 425 pow­er car­ri­er frame with trans­former and AC connector
Back side of pow­er assem­bly mount­ing frame

There is a clear translu­cent frame where the pow­er sup­ply board, AC con­nec­tor, trans­former, and fil­ter capac­i­tor are attached. The frame also includes the four thread­ed clips where the bot­tom case is attached along with mount­ing holes for attach­ing the frame to the front panel.

Monroe 425 right side frame, trans­former and pow­er sup­ply after opening

Coil L3 is only attached by it’s leads and seemed out of place upon ini­tial open­ing. I was able to care­ful­ly rotate it about 110 degrees which also tight­ened up it’s loose wind­ings. It was then tacked in place with some ChipQuik elec­tron­ic grade silicone.

Monroe 425 pow­er sup­ply out­put con­nec­tor J1

Voltages on the pow­er con­nec­tor include:
-11.95 volts
-184 volts
space
-92.2 volts
-5.41 volts
-15.2 volts
Common

Electrolytic capac­i­tors include a 2,000 uF at 35 VDC chas­sis mount­ed capac­i­tor, and a 4 uF at 250 VDC board mount­ed capac­i­tor. The 4 uF unit will be replaced with a 10 uF 250 V axi­al, and the chas­sis capac­i­tor will get a 2,200 uF at 40 volt replacement.

Monroe425 PS with new film, elec­trolyt­ic, and tan­ta­lum capac­i­tors installed

There are also three tan­ta­lum capac­i­tors on the pow­er sup­ply board which I replaced with units that were one step up in their volt­age rat­ing. One of the film “but­ter­fly” capac­i­tors in the high volt­age sec­tion had most of it’s col­or code paint peel­ing off, so I replaced both with some 0.1 uF at 400 volt poly­ester met­al­lized capacitors.

Monroe425 PS back side of pow­er sup­ply board

Buried deep under the wiring I found some jumpers for select­ing the 115 or 230 volt AC line volt­age set­ting. The mea­sured AC sec­ondary volt­age at the full wave rec­ti­fi­er diodes was 18.7 VAC at a pri­ma­ry input volt­age of 120.0 VAC.

Monroe 425 AC line volt­age selec­tion jumpers

The trans­former pri­ma­ry is made up of two wind­ings with a red / brown col­ored lead wind­ing, and a red-white / brown-white lead winding.

Total pow­er draw for this cal­cu­la­tor with mem­o­ry LED on and twelve 8’s on the dis­play is 3.2 watts, and with dis­play and mem­o­ry cleared around 2.2 watts. This is quite a bit less than the ID plate rat­ed cur­rent of 0.15 amps. My max­i­mum cur­rent read­ing was 0.055 amps aver­age at max­i­mum load.

Back of the Monroe 425 front pan­el with AC and func­tion switches
Monroe 425 front pan­el dis­play bezel

The pow­er frame assem­bly and main cir­cuit board assem­bly attach to the front pan­el via brass thread­ed inserts and screws. This is one stur­di­ly built cal­cu­la­tor, and I am impressed with the engi­neer­ing that went into the design and con­struc­tion of this model.

Monroe 425 AC pow­er cord mold­ed end

The mold­ed end of the pow­er cable even has the Monroe name embossed into it. The cal­cu­la­tor end of the cord is a non-stan­dard con­nec­tor with three round pins and the cen­ter pin being ground. The num­ber 168 is mold­ed into the end of the plug. While the pin con­fig­u­ra­tion is sim­i­lar to a type 163 and Japanese cal­cu­la­tor con­nec­tor, the Monroe plug has two bevels mak­ing it a very odd plug.

Monroe 425 cal­cu­la­tor after cleanup with a bit of Pi

The bot­tom case had some yel­low­ing on one side from sun­light expo­sure, so I decid­ed to try a retrobrite pro­ce­dure to see if it could be brought back to the orig­i­nal col­or. After 20 hours of treat­ment I would gues­ti­mate that it improved the case to 90% of the orig­i­nal col­or. Now it is only notice­able if the unit is apart, allow­ing a com­par­i­son between the inside of the case to the out­er area. I did use some “Plexus” plas­tic pro­tec­tant to help pre­vent any future dis­col­oration.
It looks like this cal­cu­la­tor is head­ing for my wife’s work desk, as she thinks it is cute and has nice styling. Hopefully it will work well for her for a long time.

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