Burroughs C5155 Calculator Refurbishment

I again was­n’t look­ing to start anoth­er cal­cu­la­tor project when this Burroughs C5155 cal­cu­la­tor showed up in one of my auto­mat­ed search­es. The price includ­ing ship­ping was a bar­gain, so I pur­chased it.

The Burroughs C5155 is a 10 dig­it plus sign, four func­tion cal­cu­la­tor that was built in France in 1973, mak­ing it over 50 years old as of the writ­ing of this blog. It has five fixed dec­i­mal point set­tings and a (K) con­stant set­ting via slide switch­es on the left side of the key­pad.
It was pack­aged very well for ship­ping and arrived in over­all great shape with a few excep­tions.
One of the plas­tic front cov­er screw posts had sheared off pos­si­bly dur­ing trans­port, and the Panaplex dis­play mod­ule came unseat­ed from it’s con­nec­tor and dam­aged two of the dis­play’s elec­tri­cal con­tact pins while bounc­ing around inside the case.
The dis­play was attached at the fac­to­ry using a brown Mylar tape to affix the mod­ule to the con­nec­tor and sup­port struc­ture. Over the past 50 years the glue on the tape had lost it’s adhe­sion allow­ing a small (or large) impact to dis­lodge it from the connector.

The cal­cu­la­tor was quite dirty, but over­all in very good con­di­tion with no cracks, dings, or scratch­es vis­i­ble on the case. The top of the case is held on by two slot­ted head screws in the front cor­ners, and a plas­tic clip tab locat­ed under the rear ID tag loca­tion. Internally the cal­cu­la­tor was fair­ly clean with the excep­tion of the area under the keypad.

The case cleaned up very well with a cit­ric based clean­er, with the excep­tion of dozens of blue ink marks all over the top and bot­tom case halves. I was able to remove the ink marks using iso­propyl alco­hol and allow­ing it to soft­en the out­er lay­er of the dried ink, then blot­ting it away with a paper tow­el, then repeat­ing the process many times until all traces of the ink were removed.

The key­pad is held in by two screws on the key­pad frame. The two slide switch­es are also attached with two screws allow­ing easy removal of the switch­es for cleaning.

The key switch­es used on the C5155 were man­u­fac­tured by Elec-Trol, part num­ber KA1011-0017, and are mag­net­i­cal­ly acti­vat­ed reed switch­es. The cork / rub­ber cush­ions on top of the switched were still in good con­di­tion. The key switch­es attach to the main board with two card edge con­nec­tors, and the slide switch­es are attached with a 8 pin head­er and connector.

The back of the key­pad cir­cuit board has a part num­ber of C 5050 A and a date of 24 July 1973 etched on the board.

A check of all the elec­trolyt­ic capac­i­tors showed that they were all with­in their capac­i­tance tol­er­ances and had rea­son­able but not great ESR read­ings. New axi­al capac­i­tors were ordered and I will replace all of the 50 + year old elec­trolyt­ic capac­i­tors. All of the film capac­i­tors were in great con­di­tion and will remain in the cal­cu­la­tor with the excep­tion of the large 1 uF 250 volt Rifa capac­i­tor used in the 150 volt divider for the display.

The Burroughs C5155 uses a American Micro Systems (AMI) three piece LSI chipset com­prised of ceram­ic 40 pin IC’s which are sock­et­ed. The part num­bers for the IC’s are: 1236–0574, 1236–0582, and 1239–8756. The only oth­er IC on the board is a 18 pin Dionics D1298N cath­ode dri­ver. The 40 pin IC’s were care­ful­ly cleaned with DeoxIt D5 clean­er and reseat­ed into their sockets.

The front right side of the board includes the dis­crete com­po­nents for the ~38.6 kHz two phase clock of which one phase is shown in the pho­to above.

The 8 seg­men­t’s are refreshed at 877 Hz from the 1236–0582 IC using the Dionics dri­ver IC and eight 470k Ohm 5% resistors.

There are two reg­u­lat­ed sup­plies and one un-reg­u­lat­ed sup­ply used in this cal­cu­la­tor. The reg­u­lat­ed sup­plies are ‑15 and ‑27 volts DC, and the un-reg­u­lat­ed sup­ply is +180 volts DC which is used for the Neon gas-dis­charge display.

There are 8 com­po­nents and a jumper wire attached to the bot­tom side of the main cir­cuit board. There is also a ground­ed sin­gle sided cop­per clad shield board attached to two stand­of­f’s on the bot­tom of the main cir­cuit board. There were orig­i­nal­ly two strips of adhe­sive attached foam which had dete­ri­o­rat­ed to a crumbly dust, which were removed and replaced with some non-con­duc­tive felt strips.

I was able to use a micro sized nee­dle nose ply­er to care­ful­ly re-bend the dam­aged con­tacts back into their orig­i­nal posi­tions with­out break­ing them.
The dis­play mod­ule was secured with some elec­tron­ics grade sil­i­cone to attach the clips on the dis­play hold­er, and a few oth­er areas to insure that the dis­play mod­ule did­n’t work it’s way loose again.
The tan col­ored residue from the orig­i­nal tape that failed is still vis­i­ble on the back of the dis­play and holder.

The cal­cu­la­tor uses a 11 dig­it Panaplex II Neon gas-dis­charge dis­play mod­ule man­u­fac­tured by Burroughs.
10 of the dig­its are used for the numer­ic dis­play and the 11th right most dig­it is used for minus and error indi­ca­tion. The com­ma seg­ments are not uti­lized on this cal­cu­la­tor.
The dis­play mod­el num­ber is BR 11450, and the mod­ule con­tains a small amount of Kr-85 (Krypton) which is a radioiso­tope of kryp­ton gas with a half-life of 10.7 years.
Kr-85 is a beta emit­ter and is used to reduce the strik­ing volt­age and assist in ion­iza­tion, mak­ing it eas­i­er to oper­ate reli­ably even in low-light or dark con­di­tions.
After 50 years only 3.9% of the orig­i­nal Kr-85 remains, and the rest has decayed into sta­ble ⁸⁵Rb (Rubidium).

Before chang­ing out the capac­i­tors I mea­sured the pow­er sup­ply volt­ages so that I had a base­line to com­pare to the new capacitors.

There are taps on the pri­ma­ry side of the trans­former for 100, 110, 115, 120, and 127 volts AC input. The three sec­ondary wind­ings mea­sure 22, 30, and 139 volts AC. The fuse is a 80 mA Slow-Blow type.
Most of the Burroughs C5000 series cal­cu­la­tors that I have seen online are 220 / 240 VAC units mak­ing this 120 VAC cal­cu­la­tor ver­sion some­what of a rarity.

The back frame cleaned up very well with only a few small spots of cor­ro­sion. There is a six ter­mi­nal ground­ing block to insure every­thing is well ground­ed. The black card-edge con­nec­tor attach­es the three trans­former sec­ondary’s and a ground to the main board. There are also sev­er­al stamped pro­tru­sions on the sides of the frame for keep­ing the edges of the cir­cuit board in place.

The ID plate is attached to the bot­tom part of the case and indi­cates that it is part of the C5000 series of cal­cu­la­tors. Unlike many of the C5000 series cal­cu­la­tors, I noticed that the style field also has a silkscreened “A” as a default for the field.

I spent quite a bit of time try­ing to get all the key­caps to a con­sis­tent white col­or, but did not suc­ceed. The key­caps all seem to be orig­i­nal to this cal­cu­la­tor, but some have a light yel­low / brown tint that is con­sis­tent through­out the cap even where there would not be any UV light expo­sure. I tried my usu­al hydro­gen per­ox­ide treat­ment, and also heat­ing the solu­tion to 130 degrees F. I also tried some abra­sive plas­tic pol­ish to remove the sur­face lay­er in an unseen area with no change in the results. My only guess is that some key­caps were made with a dif­fer­ent batch of plas­tic that had dif­fer­ent age­ing prop­er­ties over time.

The Burroughs C5155 is a nice cal­cu­la­tor with a beau­ti­ful dis­play and pleas­ing case, but visu­al­ly and oper­a­tional­ly does­n’t stand out from the crowd of cal­cu­la­tors avail­able at the time of it’s con­struc­tion. It is a well built cal­cu­la­tor using qual­i­ty parts, and was designed with ser­vice in mind.
From my lim­it­ed research, there does­n’t seem to be many of these around, so I would say that it is a semi-rare mod­el that was worth restoring.