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 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.

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.

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.

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.


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.


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

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.

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

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.
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!
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)