Eiko Unitrex 1200M Calculator Refurbishment

Another bar­gain price pur­chase of an Eico Unitrex 1200M cal­cu­la­tor. This mod­el also uses the same CT5005 LSI cal­cu­la­tor on a chip IC as the APF Mark XII Calculator, but is note­wor­thy because of the rela­tion­ship of Eico with Cal-Tex Semiconductor, Inc., the man­u­fac­ture of the IC chip. The 1200M was the first cal­cu­la­tor to use a sin­gle LSI IC with inte­grat­ed mem­o­ry. It also has a beau­ti­ful 12 dig­it Neon gas-dis­charge display.

Amazingly the cal­cu­la­tor arrived in good con­di­tion, con­sid­er­ing that it was just jammed into an almost too small card­board box with­out any pack­ing mate­r­i­al. The cal­cu­la­tor seemed to have been kept in a very moist envi­ron­ment as the case screws showed signs of cor­ro­sion, along with the met­al key­board mount­ing plate which had a sig­nif­i­cant amount of sur­face rust.

I used some Evapo-Rust solu­tion for 24 hours to remove the rust from the key­pad mount­ing brack­et. This also removed the black coat­ing from the brack­et, which isn’t vis­i­ble from the out­side any­way. I used a clear Lacquer coat­ing to pro­tect the brack­et from fur­ther cor­ro­sion. The indi­vid­ual keys use a mechan­i­cal switch, which is sol­dered to the phe­no­lic sin­gle sided cir­cuit board. There are 14 sol­dered wires in the har­ness from the cir­cuit board, attached to an edge card con­nec­tor on the main board.

The Unitrex 1200M also was avail­able in a dark grey col­ored bezel. In the ad below the cal­cu­la­tor sold for ¥44,800 in Japan back in late 1972. The equiv­a­lent val­ue today would be around $979.11 US dollars.

As men­tioned ear­li­er the 1200M uses the Cal-Tex CT5005 LSI IC. In this imple­men­ta­tion the cir­cuit uses a 6259 hybrid seg­ment dri­ver mod­ule along with a 5312 hybrid clock mod­ule for gen­er­at­ing the two phase clock required for the CT5005. The cir­cuit uses a sin­gle log­ic pow­er pow­er sup­ply of 19.3 volts, for the VDD and VGG pow­er pins. The col­umn dri­ver cir­cuit is com­prised of three tran­sis­tor mod­ules of 4 tran­sis­tors each to dri­ve the 12 columns of the Neon gas-dis­charge dis­play. I real­ly like the clean log­i­cal lay­out of this board com­pared to many of the oth­ers that I have worked with.
The data sheet for the CT5005 lists a max­i­mum clock fre­quen­cy of 30 kHz, but this unit is oper­at­ing at a fre­quen­cy of 35.54 kHz and seems to work just fine at this extend­ed fre­quen­cy which is over twice the speed of the 16.8 kHz APF Mark XII cal­cu­la­tor.

I replaced all 5 of the orig­i­nal elec­trolyt­ic capac­i­tors with new Vishay and Nichicon extend­ed-life capac­i­tors of the same val­ues. The capac­i­tors were a lit­tle dif­fi­cult to remove as the leads were bent over onto the sol­der pads which makes for great mechan­i­cal and elec­tri­cal longevi­ty, but a bit more dif­fi­cult for repair. In my opin­ion a good tradeoff.

The two sec­ondary’s of the trans­former feed rec­ti­fi­er mod­ules for the log­ic 19.3 volt, and HV 186 volt sup­plies. There are two induc­tors a switch and a fuse on the pri­ma­ry side of the trans­former.
The dis­play mod­ule is not a Panaplex, but looks to be a JRC Septanix mod­ule sim­i­lar to the 237A1 mod­ule in the Canon Canola L100A calculator.

The AC pow­er inlet is inter­est­ing as it has three pins, but the pow­er cord only has two con­duc­tors. Internally the cal­cu­la­tor cen­ter pin is not con­nect­ed.
The pow­er cord is a Daiwa brand­ed unit with a non-polar­ized twin blade con­nec­tor marked DP‑1 Japan, which is con­nect­ed with a 7 foot 2 con­duc­tor cord to the polar­ized three pin DP-42 end rat­ed at 125 V 7 A.
The met­al chas­sis is not ground­ed, and the back case screw which attach­es to the met­al case is non-con­duc­tive plastic.

The case has a small screened vent above the trans­former, and a small unscreened vent on the bot­tom side of the case in front of the ID plate.

There are three case screws hold­ing the two case sec­tions togeth­er. The sin­gle clear plas­tic screw on the back of the case, and two small met­al screws locat­ed at the bot­tom front of the case.

I per­formed a short Retrobright treat­ment of the case to remove some slight yel­low­ing, which real­ly helped with the look of the case. The case is a styl­ized curved assem­bly with a blue insert which sur­rounds the key­pad and dis­play.
The keys work good enough, but are not near­ly as nice as the type with mag­net­ic reed switch­es. There is not any dis­played indi­ca­tion of stored data in the mem­o­ry reg­is­ter, and it is up to the user to men­tal­ly keep track of the mem­o­ry use. The 1200M is a fixed dec­i­mal point cal­cu­la­tor with set­tings for 0 to 5 dec­i­mal places, there is no float­ing point set­ting.
This cal­cu­la­tor is from the era of of reduc­ing the size and cost of cal­cu­la­tors to com­pete in the ever more com­pet­i­tive mar­ket. Unfortunately this was also the end of the Neon gas-dis­charge, Nixie, and Panaplex dis­plays which were more expen­sive, and required more cir­cuit­ry to oper­ate than the new LED dis­play tech­nol­o­gy with it’s reduced pow­er require­ments and size.