[Menadue] had a vintage Compucorp 326 calculator with an aging problem. Specifically, the flex cable that connects the button pad had turned corroded over time. However, thanks to the modern PCB industrial complex, replacing the obscure part was relatively straightforward!
The basic idea was simple enough: measure the original flex cable, and recreate it with the flat-flex PCB options available at many modern PCB houses that cater to small orders and hobbyists. [Menadue] had some headaches, having slightly misjudged the pitch of the individual edge-connector contacts. However, he figured that if lined up just right, it was close enough to still work. With the new flex installed, the calculator sprung into life…only several keys weren’t working. Making a new version with the correct pitch made all the difference, however, and the calculator was restored to full functionality.
It goes to show that as long as your design skills are up to scratch, you can replace damaged flex-cables in old hardware with brand new replacements. There’s a ton of other cool stuff you can do with flex PCBs, too.
I have a similar problem. But it´s a whole keyboard, integrated into a venerable DOS era computer, with a nice matrix LCD. The whole works, except the membrane kb whose layers cracked. It is highly unobtainium of course. At about 20 square inches surfaces, with two layers + a separation layer, the cost of a new flex PCB is quite prohibitive, so i am a bit desperate what to do. Maybe somebody has an idea ?
I made a replacement flex PCB for a Sinclair Cambridge Z88. It is about A4 size and one layer, but the PCB cost $55 for 5 from JLCPCB. Not too bad, I think. It works too.
You can repair broken flex PCBs. Trace by trace. There are lots of videos on youtube how to repair flex PCBs and ribbon cables.
This is how i started. But the substrate is not polyimide, most likely PVC, and every of my repair was creating new cracks.
We had a fellow with a similar problem show up at a local repair cafe.
He was blind, and had a pocket “Parrot” device for managing telephone numbers and contacts. He could use it by feel since all the buttons were marked with shapes nubs to make them easily detectable. It was made in the late 1990s for use by blind folks. He could “look up” numbers and have them dialed by touch tone at the press of a button.
His “Parrot” quit working. The flex between the keypad and the main board was some kind of glued on carbon ink trace crap. The flex was only a couple of centimeters long.
I first fixed the old CD player he had. Again, a special version for the blind with buttons to feel and audible feedback (spoke words.) By the time that was working again, I had figured out how to fix the Parrot.
I had him come back the next time the repair cafe was open. I brought a broken Android headset, and used short lengths of the wire from the head set to replace the flex. The contact pads on the main board and the keypad board had solder pads as well as the glue-on pads for the carbon trace flex.
I made about 20 little short pieces of the (very) flexible wire from the broken headset, then soldered them in, one by one, being careful to match up the proper pads.
It worked on the first try.
Soldering this type of wire (thin copper braided with nylon) is a pain, once the nylon is burnt away, the copper left is dirty and hard to solder, and there is so little material and the wires are soo thin that it is very fragile, how did you manage ?
The best solution i’ve found is a matching tiny ferrule, crimped over the nylon-copper braid, and soldered with just the minimal heat not to melt too much the nylon.
Like this:
https://josepheoff.github.io/posts/howtosolder-17-headset
It works pretty well.
Oh. Use solder with lead. Lead free solder eats the fine strands really fast when soldering.
That what i was doing before i started with the ferrule trick. With mitigated success: it works well with thicker cable like shown on that link, no so much with thinner ones.
The molded on plastic on plugs is easily removable with the plug in a vise and 2 hands to knife and wire cutter extract the core, heat helps too. Make a stiff wire ground sticking out to form a wire grip, solder to sleeve on plug. Tuff as hell compared with those radio shack type plugs.
That’s no calculator. It has a CPU, RAM and ROM – it’s one firmware upgrade away from being the world’s first laptop. The makers however decided they wanted a programmable scientific calculator, so they dumbed down the user interface by not providing branching, conditional or otherwise.
I have the 322. Mine has the keybounce problem. If I had infinite time, I would reverse-engineer the CPU and old firmware and write a monitor program like the KIM-1. Bam! Laptop from 1974.
The 326 does have conditional jumps and subroutines
http://www.curtamania.com/curta/database/brand/monroe/Monroe%20326/more/Monroe%20326%20User%20Manual.pdf
The 322 is a bit of a different animal.
Whew nice! Oh kay. Yeah, I’m lazy and didn’t check.
I was only aware of my machine, another model with no programming capabilities and a model that had another switch for two banks of 80 step programs with no conditional branching and kinda expected this one to be like that.
And I had mine apart and know it’s got CPU, RAM and ROM so while it could be Turing complete, it isn’t.
I wonder how different are they internally – maybe the 326 is the firmware upgrade I’m looking for – but with all those extra features and prepared programs I’d guess the hardware is probably different enough to not be compatible.
According to Wikipedia it came out in 1974, so a year or two after my 322.
With that, I’d say the 326 would have been the world’s first laptop computer, not the Epson HX-20.
(I have one of those too btw.)
Broken Item #1) I take it apart to fix it. There is something custom built that I have no ability to replace. I throw the item away. A few years pass. New tools and processes are now available. It is possible for a hobbyist to easily recreate or pay someone to recreate that broken part. I wish I didn’t throw the item away.
Broken Item #2) I take it apart to fix it. There is something custom built that I have no ability to replace. I save it because I remember the previous item.
….
Broken Item #9,999,852) My now sentient house just ate the cast of Horders.
makes me wish for somehing like the high-density zebra strips i used to pull out of watch LCDs…you don’t have to match the pitch because it has such a high density of independent connections that there will be several per pin anyways
I haven’t looked at the requirements for flex PCBs but it sounds like something like that should be possible.
I found an e-dart board with the two layer flex circuits ending at a push-in pair of sockets. The ends of the traces have been scraped eroded away, it was stored in a very damp place for too long. The ends only are bad the rest of the score contacts are good. I need to watch some of those videos on repairs.
My daily headphones have in their hacked together form tough as hell telephone handset cord with it’s tinsel-textile “wire”. Soldering is done quick with flux paste on the tinsel first and through a strain relief first, no flexing after that dab soldering. The end with the TRS plug is piano wire making a soldered grip on the sleeve of a plastic molded plug minus the plastic mold on. At right angle a grip for the wire a centimeter away is crimped on the telephone cable and covered with epoxy paste. Flux paste is your friend when soldering has to be spot on in a second no more.
Nice flex, bro
Being there I have a Sony cybershot with a flex pcb for main buttons and is damaged beyond repair so I scanned it and was about to design over it until i realised that I have never designed flex circuits before! so I need a caliper to take good measures of the thickness and compare with those offered by the PCB manufacturers, also I need to watch tutorials and we have the blue stiffener thing… and the calculation of the radius to flex… for me that translate in many many hours sitting in from of my screen,so I guess I have lost part of my original interest in electronics over the years. Not sure if I’m the only one here, but I remmeber when I was like: Hey I saw a laser atomic flux condenser! let make one! I need to buy this and this and… and today I’m more like why should I spend all those hours sitting there? who knows maybe it is just I’m getting old :)