Digitronic‑M Calculator Repair

During my wan­der­ings on eBay I came across this 12 dig­it dis­play Digitronic‑M cal­cu­la­tor. It was an auc­tion list­ing start­ing at $0.99 with a real­ly low ship­ping cost, and the sell­er list­ed it as “works great!”. I put in a very low bid on it, not expect­ing to win it, but it turned out I was the only bid­der. That was the good part of the transaction.

I was start­ing to get a lit­tle wor­ried about the trans­ac­tion, as the sell­er took quite a bit of time after list­ing the track­ing num­ber before it actu­al­ly was shipped.

The cal­cu­la­tor did­n’t arrive in con­di­tion I was hop­ing for. First it was jammed into a card­board box that was too small for it to fit, and then no padding / pack­ing mate­r­i­al was includ­ed with it.
It almost seems like the sell­er was upset that the cal­cu­la­tor sold for the start­ing price, and just did­n’t care about the pos­si­bil­i­ty of dam­age in shipping.

After sub­mit­ting a return request and wait­ing for sev­er­al days for a response, I end­ed up get­ting a full refund for ship­ping and my $0.99 pur­chase price. The sell­er did not want the bro­ken cal­cu­la­tor back. So what do I do now with a beat-up, free cal­cu­la­tor?
Fix it of course!

Now that I did­n’t have to send the cal­cu­la­tor back, it was time to open it up to see what kind of dam­age was to be found inside.
Upon try­ing to open the case I was pre­sent­ed with a sheared off plas­tic screw on the back of the case. The case is sim­i­lar to the Eiko Unitrex 1200M Calculator I recent­ly fin­ished work­ing on with two met­al front screws, and the non-con­duc­tive sin­gle rear screw. The remains of the screw was extract­ed, and I now had access inside.
The first item to jump out at me was lit­er­al­ly the fuse. Somehow the fuse hold­er had bro­ken on one side, allow­ing the fuse to come loose. I used high tem­per­a­ture sil­ver sol­der to repair the hold­er and the fuse fit snug­ly again.

One of the items that was rat­tling around in the box was the dis­play’s slid­ing thou­sandths place point­er. I quick­ly found the rea­son it fell out. The dis­play bezel was hang­ing loose, attached by one heat weld­ed post instead of the orig­i­nal 6.
After clean­ing and rein­stalling the point­er, I was able to re-melt the plas­tic post welds by heat­ing a flat blade screw­driv­er using my hot-air tool to re-smush the plas­tic welds.
The Neon gas-dis­charge dis­play looks to have seen many hours of use with the least sig­nif­i­cant dig­it show­ing some signs of black­en­ing. In nor­mal oper­a­tion sit­ting idle with a cleared dis­play, a zero is con­stant­ly dis­played when the cal­cu­la­tor is turned on.
Some may have noticed on the cir­cuit board below the dis­play “UNITREX 1201M”.

The Digitronic‑M was built by Eico using the Unitrex 1201M inter­nals. While the case was designed to use the 1201M inter­nals, it is a com­plete­ly dif­fer­ent design from that of the Unitrex version.

The key­pad lay­out along with the switch­es are iden­ti­cal to the Unitrex 1201M but the Digitronic‑M has dif­fer­ent col­ored keys. Instead of using a large col­ored insert for the key­pad and dis­play the Digitronic‑M has a small­er insert for only the dis­play and switch­es.
The front of the case has a flat pan­el with the Digitronic‑M badge and is the same col­or as the dis­play insert. The flat pan­el was one of the pieces bro­ken off dur­ing ship­ping, and was reat­tached using a small amount of 5‑minute epoxy to the upper case half.

To clean the case I need­ed to de-sol­der the switch wires as the switch hold­er frame was heat weld­ed to the upper case with no access to the switch mount­ing screws. I debat­ed whether to break the welds and remove the switch­es or to sub­ject the switch­es to the Retrobright solu­tion used for clean­ing the case.

KerPlunk, … into the solu­tion the switch­es went. The switch con­tacts were fair­ly well lubri­cat­ed which gave them a degree of pro­tec­tion, but I did not want to leave them in the solu­tion any longer than nec­es­sary.
Once I pulled the cas­es out of the solu­tion and rinsed, I quick­ly blew the switch­es dry and then cleaned with DeoxIT D5 spray, with an addi­tion lubri­ca­tion of the detent stops using a white lithi­um grease.

The main board uses a set of Mostek LSI (Large Scale Inte­grat­ed) IC’s with sev­er­al sup­port hybrid cir­cuits. The 5317 mod­ule is for clock gen­er­a­tion, the 6259 mod­ule is for dri­ving the seg­ments, and the two 6225 mod­ules are the 12 col­umn dri­vers. The two Mostek IC’s are a MK 5013P and a MK 5014P cal­cu­la­tor set. Date codes for most items are from mid-1973.

Except for the fuse hold­er the main board was in excel­lent shape, with one fac­to­ry repair to replace a dam­aged trace.

Just like the Unitrex 1200M this unit has a three pin pow­er jack on the back of the chas­sis. The cen­ter pin is not con­nect­ed inter­nal­ly, and uses a two con­duc­tor pow­er cable.

I had no issues pow­er­ing up the cal­cu­la­tor after the repairs and clean­ing. Pretty good for a cal­cu­la­tor that has seen many hours of use, and a very rough ship­ping expe­ri­ence.
I have new capac­i­tors on order for this 50 year old vin­tage cal­cu­la­tor and will get them swapped now that I know that every­thing is in work­ing order.
I end­ed up lik­ing the Digitronic‑M cal­cu­la­tor a lot more than I was orig­i­nal­ly expect­ing. It has an unique enough case design, that while sim­i­lar to many of the oth­er 12-dig­it with mem­o­ry cal­cu­la­tor’s of the era, it’s dif­fer­ences are aes­thet­i­cal­ly pleas­ing to me. The only lim­i­ta­tion for me is the lack of a float­ing point mode, which lim­its it to 6 dec­i­mal point places.
So far I have only seen pic­tures of two oth­er Digitronic‑M cal­cu­la­tors, so this does­n’t seem to be a high pro­duc­tion vol­ume unit, and is some­what rare.

If I ever decide to find it a new home, I promise to treat it with the care it deserves and prop­er­ly pack it for it’s travels.