APF Mark 1 Calculator

The newest addi­tion to my small cal­cu­la­tor col­lec­tion is an APF Mark 1 desk­top unit. I found it on Ebay a few dozen miles from where I live. As usu­al it was untest­ed, as it did not have a pow­er cord, and oth­er than being dirty it looked in good condition.

APF Mark 1 cal­cu­la­tor after repair and cleaning

APF was an American com­pa­ny that built, import­ed, and sold con­sumer elec­tron­ics, includ­ing cal­cu­la­tors. The com­pa­ny was found­ed by Al & Phil Fried­man, and the com­pa­ny name is based on the ini­tials of the two broth­ers.
The APF Mark 1 was built in Hong Kong, and is designed around the (TI) Texas Instruments TMS0101 cal­cu­la­tor chip.

Texas Instruments TMS0101 cal­cu­la­tor chip in the Mark 1

The TMS0101 is part of TI’s TMS0100 series of sin­gle chip cal­cu­la­tors and fea­tures 8 dig­its, 4 func­tions, and a sep­a­rate equal key. Other fea­tures on the Mark 1 include a con­stant mode (K), four fixed dec­i­mal posi­tions, minus sign, and over­flow indicator.

APF Mark 1 Panaplex display

What I real­ly like on this cal­cu­la­tor is the Panaplex style dis­play, made up of 9 indi­vid­ual tubes with 8 dig­its and one tube for over­flow and minus indi­ca­tion. For those new to these dis­plays, they are sim­i­lar to nix­ie tubes in that they uti­lize a sim­i­lar tech­nol­o­gy. It is a gas dis­charge dis­play that uses neon like a nix­ie tube, but instead of illu­mi­nat­ing indi­vid­ual num­bers, the dis­play illu­mi­nates indi­vid­ual seg­ments like a 7 seg­ment LED dis­play would.

Being a cold cath­ode gas dis­charge dis­play, it requires a high tube anode sup­ply volt­age of 200 volts and mod­er­ate ion­iza­tion volt­age of around 170 volts in the Mark 1.

The Mark 1 came in sev­er­al vari­a­tions which also includ­ed a vac­u­um flu­o­res­cent dis­play, and dif­fer­ent key­cap types.

Power sup­ply sec­tion of APF Mark 1 calculator

Using a mul­ti-sec­ondary trans­former the pow­er sup­ply sec­tion of the Mark 1 pro­vides a high volt­age DC sup­ply of 200 to 220 volts, along with a Vss sup­ply of 7.2 volts, and a Vgg sup­ply of ‑7.2 volts. The AC pow­er jack uses a minia­ture rec­tan­gu­lar plug with a key slot in the mid­dle of the bot­tom side of the jack.

APF Mark 1 AC 117V pow­er jack
Bottom sec­tion of Mark 1 with key­board con­nec­tor unplugged

The key­board and switch­es are con­nect­ed to the main board with a card-edge con­nec­tor. Signals D1 to D11, KN for dig­its, KO for oper­a­tors, KP for dec­i­mal posi­tion, and KQ for the con­stant switch pass thru the cable to the key­board assembly.

APF Mark 1 key­board assem­bly and cable / connector
Back low­er por­tion of Mark 1 cir­cuit board

The low­er sec­tion of the cir­cuit board con­tains the TMS0101 IC, key­board assem­bly con­nec­tor, volt­age reg­u­la­tor, and clock generator.

APF Mark 1 cir­cuit board back traces

The cen­tral sec­tion the board is com­prised of the col­umn dri­vers for each of the dis­play tubes, and the upper right cor­ner includes the cir­cuit­ry for the seg­ments a thru g and the dec­i­mal point which are com­mon to each tube.

Bottom of APF Mark 1 mod­el tag and instructions

The bot­tom of the case hous­ing includes a small slot­ted air vent, mod­el tag, and instruc­tion label.

APF Mark 1 oper­at­ing instruc­tion label

This cal­cu­la­tor is an E series, and was most like­ly man­u­fac­tured in 1973. It was built for the U.S. mar­ket with an AC sup­ply of 117 volts at 60 cycles. I did not see any pro­vi­sions for chang­ing the input volt­age and there are only two pri­ma­ry leads on the trans­former.
I did end up replac­ing all of the elec­trolyt­ic capac­i­tors with Nichicon and Rubycon extend­ed life units. This cal­cu­la­tor is now around 50 years old as I write this blog, so I fig­ure it deserved some new capac­i­tors before it was too late. The fil­ter cap on the high volt­age sup­ply was a 10uF at 350 volts axi­al capac­i­tor. The cir­cuit board has traces for mount­ing either axi­al or radi­al lead­ed capac­i­tors, so I opt­ed for a replace­ment radi­al 10uF at 450 volt unit.

Mark 1 with error and minus sign acti­vat­ed on ninth tube

This cal­cu­la­tor now resides on my work­bench for dai­ly use, but I am still look­ing for a 10 to 12 dig­it Panaplex dis­play cal­cu­la­tor with addi­tion­al func­tions that needs some repairs, but hope­ful­ly rea­son­able in price.
Until then I will enjoy the warm neon glow of the Mark 1.

2 Replies to “APF Mark 1 Calculator”

  1. Hello! I found one of these E‑Series cal­cu­la­tors at my local Goodwill yes­ter­day, and want­ed to replace the caps on mine as well. I could­n’t find a schemat­ic online, so I was won­der­ing if you remem­ber what caps I would need to order. Thanks!

    1. Hi,
      There are at least three dif­fer­ent ver­sions of the APF Mark 1 Rev E calculator.
      As far as capac­i­tors go, I would guess that the main dif­fer­ence would be in the dis­play pow­er sec­tion and it’s capacitors.
      For the ver­sion I worked on with the indi­vid­ual Panaplex type dis­play tubes, I used the following:
      Two Rubycon 47uF @ 50 volt capac­i­tors for the units in front of the display.
      Behind the dis­play a 470uF @ 25 volt Nichicon,
      a 10uF @ 450 volt Nichicon,
      and a 220uF @ 25 volt Nichicon capacitor.

      These choic­es were part­ly based on what I had on-hand, but I also like to go up at least one volt­age step when replac­ing vin­tage capacitors.
      The old­er capac­i­tors had high­er work­ing volt­age tol­er­ances than the print­ed volt­age on the capac­i­tor, where­as new­er capac­i­tors are much clos­er to their stat­ed ratings.
      For all the 16 volt capac­i­tors I would rec­om­mend 25 volt radi­al replace­ment units. The only axi­al is the 10uF capacitor.
      Greg (Barbouri)
      APF Mark 1 Rev E removed electrolytic capacitors

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