Burroughs C 3207 Vintage Calculator

Well I just fin­ished work­ing on the APF Mark 1 cal­cu­la­tor so I was­n’t look­ing for anoth­er cal­cu­la­tor project right away, but I stum­bled upon this Burroughs C 3207 cal­cu­la­tor with sev­er­al issues for a price that was too good to pass up. And it’s a vin­tage 14 dig­it Nixie tube cal­cu­la­tor Built in late 1969.

Burroughs C 3207 Nixie tube cal­cu­la­tor after clean­ing and repairs

I have been look­ing for a 12 or more dig­it Panaplex dis­play cal­cu­la­tor for quite a while, and I thought a Nixie tube cal­cu­la­tor would be way out of my bud­get.
I am quite famil­iar with sev­er­al of Burroughs mechan­i­cal cal­cu­la­tors and their series of com­put­ers, but I knew noth­ing about their elec­tron­ic cal­cu­la­tors. So when this unit became avail­able, I real­ly did not know what to look for, or to look out for.

Sellers pic­ture of the Burroughs C 3207

Some of the issues I noticed before I pur­chased it were a non-func­tion­al 11th dig­it, hazy scratched dis­play lens, dam­aged pow­er cord, and over­all wear and clean­li­ness of the unit.
A quick web search before the pur­chase was of lit­tle help, and most of the par­tial infor­ma­tion avail­able, was scat­tered across many sites and dif­fer­ent mod­el num­bers.
Hopefully I can pull some of that togeth­er in this blog post.

Burroughs Logo before clean­ing with back­ground paint missing

A bit of his­to­ry on the Burroughs elec­tron­ic cal­cu­la­tors can be found on the DoPECC “Burroughs Calculators — Overview” web­site. The Burroughs 3200 series of elec­tron­ic cal­cu­la­tors were built by Hayakawa Electric Co. in Japan, lat­er known as Sharp Corporation. The C 3200 series of cal­cu­la­tors seem to be based on the Sharp Compet CS-23 cal­cu­la­tor with some changes to the case, dis­play and key­pad layout.

C 3207 ID plate and dam­aged AC pow­er cord
Bottom of C 3207 with stand, before cleaning
First look inside the C 3207 and dust accumulation

It was time to start some of the obvi­ous repairs and clean­ing. First item on the list was the dam­aged AC pow­er cord. This unit was con­fig­ured for 120 volts at 50/60 cycles, and uses 0.235 Amps and 25 watts accord­ing to the ID plate. The pow­er cord was very frayed and held togeth­er with many wraps of fiber­glass pack­ag­ing tape which also had failed.
I want­ed to retain as much of the orig­i­nal cal­cu­la­tor as pos­si­ble, so I end­ed up clean­ing and reusing the orig­i­nal pow­er cord as the only dam­aged area was where it entered the unit.
After cut­ting off around 6 inch­es of cord, the wires were re-sol­dered to the line fil­ter and new heat-shrink insu­la­tion added.

Repairing AC pow­er cord on C 3207 calculator

The next item on the repair list were the 50+ year old elec­trolyt­ic capac­i­tors, main­ly locat­ed in the pow­er supply.

C 3207 pow­er sup­ply mount­ed in case before capac­i­tor replacement
C 3207 pow­er sup­ply removed from case
C 3207 pow­er sup­ply pass tran­sis­tors and heatsink bracket

The larg­er capac­i­tors were in fair shape with ESR read­ings a bit above nor­mal, but not bad for this age of capac­i­tors. The small­er capac­i­tors did not fair as well, with high ESR val­ue on the 22 uF at 35 volt unit, and with the 1 uF at 300 volt grey capac­i­tor com­plete­ly open circuit.

A 1968 vin­tage nichicon capac­i­tor with logo

I end­ed up replac­ing all the axi­al elec­trolyt­ic capac­i­tors in the pow­er sup­ply with new Nichicon and Vishay units of slight­ly high­er volt­age rat­ings, that I had in stock. The 0.1 uF at 450 volt non-polar­ized capac­i­tor was also replaced.
This pow­er sup­ply pro­vides ‑24 volts, 200 volts, and 0 volt GND to the small card edge con­nec­tor on the dis­play assem­bly.
There is an addi­tion­al 1 uF at 150 volt radi­al elec­trolyt­ic capac­i­tor on the dis­play board that I did­n’t have a replace­ment for, that will have to wait until my next Digikey order. It test­ed in fair con­di­tion, but I don’t want to wait too long before replac­ing it.

New capac­i­tors fit­ted on the C 3207 pow­er supply
C 3207 Power sup­ply wire pad con­nec­tions view

Now that the pow­er issues have been addressed it was time for some deep clean­ing. If I had to guess, this is only the sec­ond time that this case has been opened. The first time being a quick tem­po­rary repair of the AC pow­er cable by pulling the cable fur­ther inside and re-clamp­ing it. I am also fair­ly sure no clean­ing was per­formed dur­ing that repair. There were pack­ets of dust and debris 3/4″ deep in the front cor­ners of the case.

Overall the main and dis­play cir­cuit boards were fair­ly clean, with most of the lint and debris around the out­er edges, dis­play bezel, key­pad, and screened vents.

C 3207 dis­play board assem­bly with Nixie tubes
C 3207 dis­play board annun­ci­a­tor side view
C 3207 dis­play board tran­sis­tors under side view
Back side of the dis­play board in the C 3207 with Power con­nec­tor low­er left

The cal­cu­la­tor is com­prised of four major assem­blies con­sist­ing of the dis­play assem­bly, main log­ic board, key­pads, pow­er sup­ply and wiring har­ness.
The dis­play assem­bly was easy to remove by un-screw­ing a met­al clamp between the two card edge con­nec­tors on the back of the main board. After remov­ing the left con­nec­tor on the main board and small­er pow­er con­nec­tor on the dis­play board, the assem­bly is attached with four plas­tic push-pull lock­ing pins, and lifts straight up.

C 3207 main board in case after removal of dis­play assembly
Back side of the main log­ic board in the C 3207

The main board is held in by the pre­vi­ous­ly removed met­al clip and the rub­ber lined met­al pow­er sup­ply brack­et at the front of the case. I found that it is much eas­i­er to remove the key­pad wiring har­ness con­nec­tor on the back of the main board just to get it out of the way before slid­ing the board back and lift­ing it up to remove.

C 3207 back of pow­er sup­ply and attach­ment bracket

The next step for dis­as­sem­bly is to remove the pow­er sup­ply and wiring which I orig­i­nal­ly per­formed out of sequence. The trans­former is attached with four large screws, but to access two of the screws, the fuse block needs to be removed first. The odd thing I noticed about the fuse when remov­ing it to gain access to the block mount­ing screw was that it had wire leads sol­dered to the ter­mi­nals of the fuse block. Something I had nev­er encoun­tered before.
With the trans­former now loose the pow­er sup­ply brack­et is held on with 5 screws, one of which secures the pass tran­sis­tor bracket.

C 3207 pow­er sup­ply and key­pad wiring har­ness and odd fuse attachment

All that is left now is to un-screw the key­pad brack­et from the top case. The brack­et holds the indi­vid­ual key mod­ules, dec­i­mal point selec­tor switch and pow­er switch.

C 3207 top case with key­pad and switch­es removed

The tint­ed plas­tic dis­play bezel and red plas­tic thou­sand indi­ca­tors are held in by a met­al brack­et and three screws. The bezel on this unit was heav­i­ly scratched and hazy from many years of improp­er cleaning.

C 3207 dis­play bezel before polishing

I used some Novus #2 plas­tic pol­ish, and a hour of hand pol­ish­ing to semi-restore the bezel to near new con­di­tion. There were a few deep scratch­es that were most­ly removed, but still vis­i­ble upon close inspec­tion. Luckily the pret­ty glow of the Nixie tubes dis­tract most casu­al view­ers from look­ing too close.

Pretty neon glow of the 14 Nixie tubes. No zero suppression
C 3207 dirty key­pad and switch assembly

The dirt­i­est part of the cal­cu­la­tor was the key­pad assem­bly. There was a mix of lint, dirt, food, skin flakes, hair, and oil; my very def­i­n­i­tion of gross. I would esti­mate that at least 50% of my time clean­ing was spent on that assem­bly. The keys use a mov­ing mag­net to acti­vate a reed switch for each key, except the “F” key which uses a microswitch.
Even though there are no con­tacts in the key­pads, I end­ed up using DeoxIT D5 for clean­ing the mechan­i­cal key assem­blies, and it did an excel­lent job.
AC pow­er is rout­ed through two shield­ed sin­gle con­duc­tor cables to the top mount­ed pow­er switch.

C 3207 Keypad and switch­es after clean­ing and re-assembly

After all the clean­ing was com­plet­ed, the quick­er process of reassem­bly com­menced. There were no issues with putting the cal­cu­la­tor back togeth­er, as it is most­ly the reverse of dis­as­sem­bly. I did find it eas­i­er to recon­nect the key­pad wire har­ness con­nec­tor before ful­ly installing the main board, which is the oppo­site of dis­as­sem­bly.
It was time for the first pow­er up of the cal­cu­la­tor, and after all my usu­al pre-checks, I used my Watts Up Pro pow­er mon­i­tor to ver­i­fy nor­mal cur­rent draw for the cal­cu­la­tor.
No major issues with pow­er and the Watts Up meter indi­cat­ed 25 VA which close­ly matched the ID plate wattage spec­i­fi­ca­tion.
There was a small amount of ini­tial con­cern as the dis­play indi­cat­ed a ran­dom assort­ment of num­bers, some with mul­ti­ple dig­its on in the same tube, but this is nor­mal as the reg­is­ters pow­er up un-ini­tial­ized and requires the “C” clear key to clear the reg­is­ters. The same goes for the mem­o­ry func­tion and requires the “I*” key to be pressed.
In clear­ing the mem­o­ry, the mem­o­ry reg­is­ter is trans­ferred to the dis­play reg­is­ter before actu­al­ly clear­ing the mem­o­ry so if there was un-ini­tial­ized garbage in the mem­o­ry, there is now new garbage in the dis­play reg­is­ter again requir­ing anoth­er press of the “C” key.
The quick­est ini­tial­iza­tion sequence is to first press the Memory Clear key, and then the Clear key after pow­er up.
The 11th dig­it Nixie tube was now work­ing with no issues, so I have no clue as to what I did to fix it, or if there was orig­i­nal­ly a prob­lem with it at all.

Keypad of the Burroughs C 3207 calculator

While the Burroughs C 3200 series cal­cu­la­tors seem to be based on the H.E.C. / Sharp 22C and 23C designs, the key­pad lay­out and nam­ing that Burroughs spec­i­fied is sig­nif­i­cant­ly dif­fer­ent. Here is a run­down of what the keys are and their function.

The CD Clear Display key clears the cur­rent dis­play reg­is­ter.
Used for clear­ing fig­ures mis­tak­en­ly set.

The K Constant key is used for car­ry­ing out cal­cu­la­tions by use of a con­stant. Push to lock the key. Push again to unlock the key.

The RC Recall key, exchanges the con­tents of the X reg­is­ter (operand being dis­played) with those of the W reg­is­ter (sec­ond operand).
Useful in cas­es where the divi­sor and div­i­dend in a oper­a­tion need to be interchanged.

The F Floating point key is spe­cif­ic to the Burroughs cal­cu­la­tor, it sets a float­ing point mode of up to 6 dec­i­mal places.
Push to lock the key. Push again to unlock the key.

The C Clear key, clears and ini­tial­izes all the con­tents except the mem­o­ry M register.

The Red Minus key, orders sub­trac­tion. It starts the cal­cu­la­tion after chang­ing the sign of the operator.

The += Plus Equal key, derives sum, prod­uct, and quotient.

The Division key orders divi­sion. The key lamp turns on when the key is pressed. The key lamp clears upon com­ple­tion of the oper­a­tion. If the Constant key is acti­vat­ed the lamp remains on until the key is deactivated.

The Multiplication key orders mul­ti­pli­ca­tion. The key lamp turns on when the key is pressed. The key lamp clears upon com­ple­tion of the oper­a­tion. If the Constant key is acti­vat­ed the lamp remains on until the key is deactivated.

The I * Clear mem­o­ry key, recalls the con­tents of the mem­o­ry to the dis­play, and then clears the con­tents in the mem­o­ry M reg­is­ter only. This also clears the mem­o­ry indi­ca­tor lamp.

The <> Memory recall key, sum­mons the stored con­tents in the mem­o­ry to the dis­play. There is no change to the stored con­tents in the mem­o­ry M reg­is­ter, or mem­o­ry lamp.

The Memory minus key, sub­tracts dis­played fig­ures from the con­tents in the mem­o­ry. (there is no change in the dis­play.) The first mem­o­ry entry sets the mem­o­ry lamp.

The Memory plus key, adds dis­played fig­ures to the stored con­tents in the mem­o­ry. (there is no change in the dis­play.) The first mem­o­ry entry sets the mem­o­ry lamp.

The Tabulation slide switch (0 ~ 6), Specifies fixed dec­i­mal places.

Temporary mod­i­fi­ca­tion for illu­mi­na­tion the mul­ti­ply key

I did end up mak­ing a tem­po­rary mod­i­fi­ca­tion to the Multiply key, because it was not light­ing the indi­ca­tor lamp inside the key. I ver­i­fied that there was a 22 volt change on the key indi­ca­tor ter­mi­nals when the key was first depressed.
The solu­tion is a red LED with a 2K ohm resis­tor in series, then attached across the indi­ca­tor ter­mi­nals. This hangs over the back side of the mul­ti­ply key hous­ing and projects towards the cen­ter of the key­cap, light­ing it up.

I would like to find out if the indi­ca­tor bulb is replace­able. It is most like­ly a 24 volt incan­des­cent bulb. There is no access to the indi­ca­tor from the back side of the hous­ing, and my ini­tial attempts in remov­ing the key­cap with­out apply­ing too much force were futile.

Update: The key­cap indi­ca­tor has been par­tial­ly repaired C 3207 key­cap indi­ca­tor repair

C 3207 rear 45 pin inter­face connector

Located under the rear AC pow­er cov­er is a 45 pin inter­face con­nec­tor that is con­nect­ed to the key­pad wiring har­ness and the main cir­cuit board. Along with the key­pad, some oth­er sig­nals on the con­nec­tor are; clock phas­es, Bit and Digit tim­ing, VDD, GND, IDLE, nPXI, divide / mul­ti­ply key indi­ca­tors, along with many oth­er sig­nals.
The H.E.C. / Sharp 22C ver­sion of this cal­cu­la­tor had an exter­nal acces­so­ry, the “Memorizer 60” which was an auto­mat­ic pro­gram­mer that plugged into this con­nec­tor and allowed key sequences to be learned and then played back. Most like­ly it would also work with this Burroughs cal­cu­la­tor ver­sion.

I real­ly would like to find a copy of the oper­a­tions man­u­al, and schemat­ics for this mod­el, but I will enjoy using it in the meantime.

Vintage Burroughs C 3207 elec­tron­ic cal­cu­la­tor with mem­o­ry 1969

If you noticed any­thing I should have includ­ed about the mod­el 3207, let me know in the com­ments below. Thanks!

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