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Date introduced: 1972.
Date discontinued: Unknown.
Date of this unit: 1973.
Main IC: Texas Instruments TMS0105BNC.
Display: Neon 7-segment, individual tubes. Rodan MG-17G X 9.
Functions: Addition, Subtraction, Multiplication, Division, Constant Facility.
Memory: None.
Digits: 8, fully floating or switchable 2/3 digit decimal point.
Power: 6V, 4xC batteries or external power supply.
Original purchase price: Unknown.
In the early 70s pretty much any company with experience in microelectronics was trying to get a piece of the calculator market, and as such if you look at any catalogues from there time you will probably see a plethora of names that you've never heard of. By the 1980s a huge number of these earlier manufacturers had either vanished entirely or exited the market as it became increasingly difficult to compete against the major players.
Kovac were one of those names it seems. The K-80D featured here appeared in a couple of variants with different key types/colours and was badged under a few different brands. I've not seen any evidence of models produced beyond the P81 of 1975.
The K-80D is pretty typical of a well made calculator of the time. While not bristling with features by the standards of the time it's an extremely well built, solid calculator which wouldn't have disgraced any desk of the time be it that of an engineer or senior executive.
Probably the single most unusual feature of this calculator though is the display. Larger portable calculators like this had pretty much adopted vacuum fluorescent displays by this point so it was quite a surprise to see this using 7-segment neon display tubes (Rodan MG-17Gs in this case). These are pretty much identical in their method of operation to the Panaplax planar neon displays of the same time period, however rather than the multi-digit planar build of the Panaplax units these are individual single digit tubes.
You've probably noticed by now that this unit does have a fault with the display. This is that the top segment in all of the display tubes is inoperative. I have yet to investigate this, however due to how the display is multiplexed am expecting to find either an open circuit drive transistor or an issue with the signal to said transistor. If time permits and I am able to repair this I will of course update the photos here and ensure the repair is documented. In the meantime though by pure chance this is the one segment you can lose on a 7-segment display without really adversely affecting the usability of the display so it's not a huge issue.
The window over the display is tinted a pale grey which vastly improves contrast and pretty much eliminates reflections from the display tubes themselves. The window is very reflective so makes the display a little tricky to photograph (plus digital cameras always struggle to capture the true colour of a neon discharge), but in reality these reflections really don't affect the readability of it.
The display even with its missing segments is extremely clear and easy to read in any normal lighting situation. I've tried to adjust the exposure of the photograph below to as accurately represent how bright the display looks to the eye under normal room lighting.
While quite bulky (fitting four C size batteries into the case sees to that even before you pack in any circuitry!) it's quite a handsome unit I think with a nice two tone case and brushed aluminium faceplate.
The display digits are formed in the "conventional" 7-segment manner as you'd expect to see on any calculator even today. Note of course in the photos below that the uppermost segment is not currently working on any of the digits on this calculator.
The negative indicator always shows at the far left under the OVF/NEG marker.
Due to the individual display tubes there is some very slight vertical misalignment from one digit to the next if you look really carefully. Digit 5 in particular sits a little lower than the rest.
The small rectangular device above and to the right of the display is a battery condition meter.
This would be particularly relevant if you were running the calculator from rechargeable cells as the border between the green and red sections denotes the point where the batteries need to be recharged.
On the subject of batteries, if a 6V power adaptor is used and that rechargeable batteries are fitted to the calculator that it will charge them (whether the calculator power is turned on or off). It should be noted though that this charging circuit was designed for trickle charging NiCad batteries that were commonplace in the 1970s and I wouldn't recommend trying to use it with modern NiMH cells as it is very unlikely that there is any really effective end-of-charge detection in place. The DC input jack is on the upper right hand corner of the calculator (as you're looking at it in front of you as if you were using it).
The batteries are inserted from underneath the calculator. A battery removal ribbon is supplied to help get them out. I can vouch for the fact that if you accidentally insert batteries without ensuring the ribbon is under them, they're a pain to get back out.
There's a paper label in the battery compartment showing you how the batteries should be fitted.
It's notable that the springs in the battery compartment of this calculator seem to be of quite a bit better quality than a lot of them out there.
On the underside of the calculator there is a label which contains a brief rundown on how to use it and also has the serial number stamped in the bottom. It's notable that on my unit the protective film from the factory is still present.
It's actually worth reading this as it doesn't behave entirely as you might expect, as this calculator uses Reverse Polish Notation (RPN) for input of arithmetic operations. Well...It kinda-sort-almost does. Some operations behave as you'd expect for an RPN calculator, some are entered in a more conventional algebraic fashion, just using the "+=" key to show the result as there's no dedicated equals key (the first clue to it being an RPN unit). If you're not familiar with the concept of RPN, I've linked to the Wikipedia page on the topic just above, it's well worth a read. It feels really strange to use an RPN calculator initially but I personally found that it pretty quickly became second nature for most operations.
In operation there are a few different indicators used on the display to show error conditions.
The first is if you type more than 8 digits into the calculator it lights C in the OVF/NEG indicator to alert you that you've run out of digits. This doesn't have any effect on calculations and purely appears to be to give users a heads up. Note that this appears as an L on my display due to the dodgy top segment drive. I've input 1234567890 here - everything beyond 8 has of course been disregarded by the calculator.
Overflows are indicated by a small U showing in the OVF/NEG indicator. The display here is showing the result for 99999999+1.
Underflow is handled the same way, though the - marker also being lit means that rather than a U, the indicator shows o instead.
The entry here was -99999999-1.
Arithmetic errors are shown similar to overflows, but with the display showing zero rather than a decimal shifted result. The usual 1/0= results in this being shown.
It appears to be a reasonably quick calculator for the time, with just a hint of display shimmer visible for a tiny fraction of a second when doing the all 9's divided by 1. There is no display blanking while the calculator works, and due to the slight variation between digits you do sometimes see the results appear on digits in a random order.
A demonstration of the calculator doing the classic all 9s calculation can be seen at this YouTube Link (0:33).
I wouldn't call this a handheld calculator...It does *just about* fit in your hand...but you're not going to be sticking it in a pocket (it weighs nearly 1kg with batteries anyway)!
I think the term "portable desktop" calculator fits the form factor of this one.
It feels exceptionally solid. There is absolutely zero flex or give in the case if you squeeze or twist it, and the keypad doesn't move in the slightest while in use.
The keypad actually is worthy of a mention in itself as it's rather pleasant. The keys have a really long travel for the size (looks to be 6mm) and have just the right sort of degree of resistance. The layout is pretty conventional and logical. I'd have no worries about having this as a "daily driver" on the desk. If I had to criticise the keypad in any way whatsoever it's that it might be a little noisy in a busy office.
The CD key clears your most recent entry, whereas CA clears the entire calculator. The switches to the left select the decimal point mode and enable the constant mode (directions on how to use that are in the instructions on the back of the case shown above).
The case is held together with four machine screws threaded into brass inserts. The leads to the battery meter are *just* long enough to allow you to lay the lid next to the case.
You can't really see much to be honest aside from the display with the keypad in place as all the good stuff is hidden underneath it.
Six more screws hold the keypad in, then we see what makes this thing tick.
The vast majority of the discreet components in there are the display driver hardware.
I suspect that our missing display segment issue is due to problems with one of the transistors nearest to the camera in the above photo. I haven't had the board out yet either, so it's not impossible that it's just something simple like a dry joint...though that's probably wishful thinking.
The actual calculator is basically just this one chip, a Texas Instruments TMS0105BMC. TI pretty much invented the "calculator on a chip" idea in the early 70s, and their range of products ended up in countless calculators both of their own manufacture and sold on to others.
Also gives us a nice clear indication of the production date too, given the 1973 date code on the IC.
It wasn't long after this that the race for the bottom as far as the price of calculators were concerned - especially for desktop ones like this for which portability wasn't so much of an issue. This really seems to predate that though and is a really nicely made unit.
Hopefully there will be an update to follow at some point when I get the display properly sorted out.
Page created: 8th January 2021.
Page last updated: 30th April 2023.
