Note: Full resolution versions of images on this page can be accessed by clicking on them.
Prinztronic Mini 7 Specifications:
Date introduced: 1974.
Date discontinued: Not Known.
Main IC: Texas Instruments TMS0807NL
Display: LED - Bubble Lens Type, 9 7-segment digits. 8 Used for Numeric and 1 for status indication.
Functions: Addition, subtraction, multiplication, division, percentage, square root & Register exchange function.
Memory: None.
Digits: 8, Floating Decimal Point.
Keys: 22, clear plastic with reverse printed legends. Clicky Tactile dome switches.
Other controls: Sliding power switch.
Power: 6V, 4x AAA Batteries.
Weight (including batteries): 108g.
Dimensions (width, height, depth in mm): 55, 120, 12.
Original purchase price: £20-30 depending on exact model.
The battle in the early 70s had been to get truly portable, pocket sized calculators to market and to improve on the features they offered. Towards the middle of the decade however that focus had very much shifted on bringing the cost down. Some of the approaches to this resulted in just as much innovation in some fields as was originally seen in getting calculators to market in the first place.
One of the budget lines of calculators in the mid 70s was the Mini range from Prinztronic. This featured a handful of physically similar models with varying features, all featuring a very smart, modern looking two tone plastic case and distinctive clear, reverse printed keypad. A lot of the savings in this have been done on the case and overall construction, while the calculators themselves had a pretty standard feature set for the day.
This isn't a brand that you've likely heard of if you're not from the UK. That's because they're not technically a manufacturer. The Prinz brand actually dates back to the 1950s, being one used by the Dixon's brand of electronics stores (who have more recently also been part of the Curry's and PC World empire). Prinztronic was used on electronic items such as this, and Prinzsound for audio equipment - I've got a pair of headphones from them which are still regularly used. Generally they had a pretty good reputation for producing kit which did exactly what it said on the tin for a reasonable price.
The Prinztronic Mini (the 7 in this case signifies seven functions) is quite a striking looking little calculator I think.
The two-tone blue case and completely un-fussy detailing must have looked quite futuristic at the time. It's a very simple looking design, but without looking over-simplified to the point of looking cheap.
Something I had never fully appreciated with these calculators until mine arrived in the post was quite how small they are.
You wouldn't really want it to be much smaller, the keypad is about the ideal size to be used one handed.
In my head for some reason I had always pictured the keypad being about the size of the whole calculator. For a model from 1974 this really is quite compact, even if it's got a bit more depth than some of the high end models appearing at the time.
One of the ways they achieved this was to make the somewhat unusual decision to use AAA batteries, rather than AAs which were the go-to for small calculators at the time.
While this does help keep the bulk and weight down, it wouldn't likely have earned them any love from the end users. AAA batteries have a considerably lower capacity than AAs, and until relatively recently have tended to be far harder to find. Unlike a lot of calculators of the period as well there is no DC in jack, so you are completely reliant on battery power.
It does make for a pretty light calculator though, tipping the scales at just over 100g with batteries.
They also dispensed with any power supply circuitry on the board itself, instead buying in a DC-DC power supply module from Astec.
While this probably added a little to the component cost, the reduction in components and time saved on the assembly line probably made it absolutely worth it.
A lot of the design features of this calculator centre around how they could bring the purchase price down - one of the most obvious things being the case itself.
There are no screws at all used, it is simply a snap-together all plastic clamshell.
The internals are based around one main PCB and a secondary one which houses a display driver IC.
The usual springs behind the battery terminals have been dispensed with, instead foam pads have been placed behind the terminals to brace against the side of the case. This is a bit of an issue now as the foam used is starting to decompose and turn to powder - which had allowed excess strain to be placed on the main positive battery terminal, causing the contact to split away from the PCB.
At least this was an easy enough repair - though all of this foam will need to be removed and replaced to ensure long term reliability.
The very simple power switch mechanism is another example of cost savings...rather than buying in an actual switch, there's just a metal wiper attached to the physical slider that bridges a gap in the main power supply trace on the PCB.
The keypad is held directly onto the front of the main PCB with heat stakes from the plastic surround. The display shines through a slot in the PCB itself. The black tape is simply a trim to ensure that the PCB traces aren't visible through the display window (which as you can see above is considerably larger than the actual display). That window being larger allowed flexibility as some of the other variants in the Mini range had different display configurations. At least one has an additional status indicator below the main display line.
One of the most striking things about this calculator is that lovely looking clear keypad. This it turns out is actually a clever bit of cost saving as well, as I discovered when I took it to bits - as when I got this calculator the keypad didn't work (except for the 2 key!).
I had assumed that the keys on this had the legends actually printed on the reverse of the keys, however when I took it apart I discovered that the keys themselves are actually all entirely clear, identical to each other and that the legends are actually just printed on a piece of paper that sits between the keys themselves and the metal domes of the key switches. Said metal domes in this case it turned out were rattling around freely behind the keypad because the tape originally holding them in place had disintegrated.
I've seen a few of these calculators on eBay where the paper sheet has been damaged by either damp or leaky batteries...In case you've got one of these, here's a scan of mine - this was scanned at 600dpi so hopefully should be decent enough quality to print. I quickly touched up a couple of the most obvious imperfections. Even if you have a different model with a slightly different layout it should be easy enough to modify for your purposes.
It took a bit of time and patience but I was eventually able to wrangle all the keypad domes back into place and tape them back into place with decent quality tape...so hopefully they'll stay put for another 47 years!
There are small lenses moulded into the back of the keys, which both help magnify the legends slightly but also help the keys to locate in the "dimple" on the key switch domes. You can see them acting like tiny magnifying glasses in this photo.
Before I put the case back together I of course made a point of having a peek at what IC is at the heart of this machine - no huge surprise to find a TI TMS series chip at the heart of a calculator of this vintage. This also allows us to pin down the date of this exact unit to mid 1975 based on the date code.
There is only one other IC used in this calculator, a display driver which I believe takes the raw digit "data" from the TI TMS IC and converts it into a multiplexed stream to actually drive the display. This is a National Semiconductor DS8855N chip.
The display itself is a conventional for the time "bubble" type LED device.
One of the downsides of the design of these displays is that they have a very narrow viewing angle due to the magnifying lenses. Many calculators using these tended to tilt the displays towards the user to help prevent this from causing issues. With the display being mounted flat in this one however you really do have to be looking straight at it to view the display. This isn't an issue if you're holding it in your hand, but using it on a desk can be a bit frustrating as you tend to have to lean forward over it to see the display properly.
The display makeup is absolutely standard, with no surprises in store really.
The ninth "digit" on the left is used purely for negative and error indications - so the full 8 digits are always available. The negative sign always appears there rather than "following" the leading digit.
Overflows are shown by a small "u" at the far left of the display. This shows the result of 99999999+1.
Underflows are handled the same way - though the "u" instead appears as an "o" due to the negative indicator also being shown. This is the result of -99999999-1.
Overflows and underflows can be recovered from by pressing the CE key.
Arithmetic errors are handled by showing the "u" symbol and the display showing zero. This was the result of me attempting to divide by zero.
While it does catch divide by zero, it will quite happily allow you to take the square root of negative numbers.
There don't seem to be any particular oddities to operation that I've seen beyond this - though the fact that it shows the negative indicator on the display during subtraction calculations is a little unusual.
It IS quite slow though - running a square root on all 9s takes about 1/2 a second, and most division commands about 1/4 a second. As there's no display blanking during these calculations you can see the display flickering as it works furiously to work out the answer for you.
After 25 seconds the display blanks into a low power state with just the negative indicator shown.
This can be recovered by pressing the "D" key on the keypad next to the power switch to restore the display.
The thing which made me want this calculator was that gorgeous clear keypad though...
The only real quirk of the layout of that keypad in my opinion is the location of the - key.
I would probably have put something less commonly used there such as the % or square root key, and keep the main arithmetic operators together in a single column above the = key. Just personal preference though and something it's really not hard to get used to. The colour coding really does help with the clarity at a glance to keep the numeric and operator keys separate.
It really is a lovely looking keypad though, I'm a complete sucker for things with that reverse-printed 3D look to them. The whole thing really is nice looking...so time for a few more general photos of the keypad and calculator as a whole.
The glossy finish could very easily have just looked cheap...but it just fits the overall shape of this thing so well that it really doesn't.
As with most calculators until quite recently this came with a little protective slip case.
Given the overall lack of scratches or any other damage on my example it seems to have done a decent job, though the overall condition definitely shows that this was looked after by its original owner.
While the case does feel a little flimsy it's a perfectly usable calculator. The only real black mark against it back in the day probably would have been the cost of batteries. It's such a unique looking thing that I'm really glad to have one in the collection. It will definitely be in the cycle of calculators that sees service on my desktop.
A good friend of mine has a Prinztronic M (which differs from the Mini 7 here in dispensing with the square root function, instead having a memory feature and having a two tone warm grey case rather than blue), and was responsible for me discovering this family of calculators when he shows me that one. Once the current lockdown restrictions have eased I fully intend to get some photos of the two of them together as that really has to happen.
Page originally created: 10th February 2021.
Last updated: 30th April 2023.
