Vintage Calculator project - Page 1
Forum section: electronics / projects

author: eNinja
author_rank: Contributor
timestamp: August 17, 2016, 08:59:59 pm
content: Hi EveryoneIve been working on a small project for a while where ive been restoring an old Commodore 9R-23 calculator. Its from the 70s and used to used a rechargeable battery of some kind, which catastrophically leaked hence damaging a lot of it.After some time fixing traces and repairing/replacing various components i had it working pretty well for a while, but it started having intermittent issues with ghosting on the display then eventually stopped coming on.I know the Vacuum Tube display is probably fine since it sometimes displays perfectly. The microcontroller (MOS Tech 7545-003) seems to work perfect as well since i tested all functions and they worked.I suspected the problem was the display power circuit below the display, since the display has shut off recently and flickers and if i fidget with that circuit it sometimes starts to display again. Though i think its more serious than some dry solder joints.Anyway, id really like to have this as a working system and am pondering what would be the best route going forward:1. Replace the entire guts and try and get some newer hardware to display on the VFD so i can keep the retro look, though im not sure how i would connect the keyboard to new hardware or get new hardware to display on the screen.2. Try and repair the original power circuit and hope that fixes it, but i havent been able to figure out how that bit works. The board was in bad shape so maybe trying to fix it more is a dead end.Either way, both options will require new power circuitry for the display.My goal for this project is just to have a reliable retro commodore calculator. Im not really interested in keeping it original in terms of hardware, i just want it to look original from the outside.Any suggestions on how to continue with this project or make it more fun?

author: Pjotr
author_rank: Frequent Contributor
timestamp: August 17, 2016, 09:32:11 pm
content: Quote from: eNinja on August 17, 2016, 08:59:59 pmLooks the display is in good working order. So repairing the power circuit is imho your best bet. It is most likely a Royer converter. Those days quite common for these purposes. It is a quite simple 2-transistor circuit. You mentioned intermittent behaviour, probably a broken PCB trace?

author: eNinja
author_rank: Contributor
timestamp: August 17, 2016, 09:42:53 pm
content: Quote from: Pjotr on August 17, 2016, 09:32:11 pmI think these screens are supposed to run on AC so it would make sense that its a royer convertor. I was hoping that since it was running of small batteries that it would be driving the screen fully in DC but probably not.Ive checked all of the traces outside of that power mini board, so if theres a break its on that mini board somewhere.Maybe ill have a go at fixing that circuit and if it fails ill switch to plan B of trying to use different hardware to display on that screen. Failing that i could run a LED or LCD display and wire up all the buttons as individual buttons and hook them up to a generic controller of some kind. That would allow me to do things other than calculator functions on it.

author: Pjotr
author_rank: Frequent Contributor
timestamp: August 17, 2016, 09:53:25 pm
content: Those VFD's usually ran from some 2V AC for the filament and 20V - 30V DC anode voltage (digit segments are the anodes). Digits are muxed by controlling a grid that is overlaid the digit. Putting in another type of display will require a major modification of the display driver.

author: eNinja
author_rank: Contributor
timestamp: August 17, 2016, 10:39:39 pm
content: Measuring across the VFD im getting under 1V AC, which would explain why the display isnt lighting up if you say it should be 2V (some sources say 3.3v).Measuring the DC input to the convertor im getting 4.2V which makes since considering im using 4.5V power, not sure if the board is supposed to get 4.5V input or if it was supposed to be stepped up before.I found a 50V capacitor on board im guessing thats for the anode.Tested all the electrolytics with a Peak Electronics ESR meter. ESR and Capacitance are spot on but these are 40 year old caps so maybe i should just change them anyway ?

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 20, 2016, 12:30:49 am
content: The filament can be anywhere from .9 to 3V depending on the display and the anodes can be anywhere from 12 to 30V. You need a datasheet to know the correct voltages.

author: eNinja
author_rank: Contributor
timestamp: August 23, 2016, 08:09:13 pm
content: I de-soldered the little power daughterboard and I found the problem straight away, broken Zener diode...no labelling whatsoever. Looking at the actual circuit (since theres no schematics) i had to guess what rating it might be, i figured it was probably in the range of 12-13V, though i could only get a 14V 500mW diode at that size...figured what the heck since its dead anyway.Added it in, and the display is working again ! and its clear with no distortion or ghosting, but slightly dimmer (probably less voltage going to the display).Though even moving to smaller 3xAAA batteries i still cant get the batteries to fit in the housing, i have no idea how they got a 4.5V battery pack as small as its original one, it must have been utterly useless.I wanted to use 3x AAAA cells, that might fit but rechargeables would end up being 3.6V, i could go with 4xAAAA but again that probably isnt going to fit inside the housing either.I really wanted to make it have a rechargeable internal battery that is charged via USB (USB is 5V so would have been perfect to charge a small 4.5V pack right ?)So im using an external 3xAAA battery pack with alkaline batteries to get the 4.5V input, it works but its ugly

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 23, 2016, 09:33:51 pm
content: You can get 4.5V rechargeable battery packs, IDK if they are available in that exact size. But you could always put two 3.7V lithium cells (with internal protection) in parallel and regulate the output to 4.5V.

author: CJay
author_rank: Super Contributor
timestamp: August 24, 2016, 06:23:50 am
content: You can get cylindrical rechargeable 75400 120mAH LiIon cells for very cheap, they're very common in the disposable e-cigs you can buy for £1 or less.Smaller than a AAA cell

author: eNinja
author_rank: Contributor
timestamp: August 24, 2016, 07:40:55 am
content: I thought about using li-on, they are easy to get but ive avoided using them since im always afraid of them being overcharged or overheating in my circuits. Ni-MH is much safer.Plus as you said, id need two lithium and need to regulate them down to 4.5-4.6V too.Anybody willing to point me to the right way to do this if i want to try it ? (example online). Sorry if that sounds a bit n00b, but i really don't want to cause an electrical fireId need to build a robust charging circuit, that maybe has an LED so i know its charging, some kind of overcharge/overheat protection, and preferably charge via USB header. Then i need to make sure the voltage output is kept below 4.7V max with no chance of spikes.The calculators board has some proprietry header on it that just goes into + and - leads on the board, i could just desolder it and add a micro usb header or some other generic charge port.

author: CJay
author_rank: Super Contributor
timestamp: August 24, 2016, 09:58:50 am
content: Personally, I'd use a boost converter and a single cell, one of the mobile phone emergency charger packs would be ideal, you could gut the controller from it and use that to boost/charge a single cell.Have you a picture and dimensions of the original battery pack?

author: eNinja
author_rank: Contributor
timestamp: August 24, 2016, 10:20:46 am
content: No, i tried finding pictures but no luck. The original battery was already removed when i got it, only the leftover chemicals from it where still there.Based on the footprint it was about 20% smaller than a PP3-size 9-volt.I managed to find a 5 cell Ni-MH battery pack from a toy remote control car, its 6.0V 260mAh but i could remove one of the 1.2V cells making it 4.8V which would be very close. These cells are tiny so it would definately fit.I think i would just need to wire it up to the internal leads, then remove the old charging port and put something useable and give it a 5V input source ? Theres a sufficient resistor on the voltage input line before the battery terminal so im guessing no change other than the physical port is needed to get it to charge safely ? Though id like to put an LED on it so i can tell if its charging. Would the LED just be in series with the input ?It should need around 25ma to slow charge the battery safely.

author: EdwardB
author_rank: Newbie
timestamp: June 12, 2021, 04:14:04 pm
content: I have one in better condition can't get it to work tried giving it 5v from a power supply that yielded no results. The original batteries are 2xAA