How to fix a broken TTI 1905a keyboard cable?  - Page 1
Forum section: electronics / repair

author: theHWcave
author_rank: Regular Contributor
timestamp: November 04, 2022, 06:07:00 pm
content: I have scored this TTI 1905a multimeter cheaply as not working. It seems to work fine in its basic functions. The problem is the ribbon cable that comes from the front panel membrane keyboard and connects in a tight bend to the upper PCB. Several of the traces at the bend are cracked and have no contact anymore. To be clear, the cable and the whole keyboard are one unit so replacing would be the last resort.I considered scratching insulation away and soldering thin copper wires across the cracks but I wonder if there is an easier way.How would you repair such a problem?

author: xmris
author_rank: Contributor
timestamp: November 04, 2022, 06:23:17 pm
content: Should you use a conductive paint like the Bare conductive - pls don't take me wrong I don't advertise it but is something that really works : it's almost a year that tea+milk dropped over my keyboard and as you may guess lines from the membrane were damaged!Is not that an expensive model but I mainly pissed off to throw it out and produce more e-waste.I got a couple of those noname conductive paints from ebay , none of them worked so I ve bit the bullet and got that "bare conductive" I though "probably snail oil" but mate believe me IT IS working - I also used some very thin cable (the one for PCB repairs/rewire) to connect a long lost trace, I ve applied the conductive paint over it.Also this paint helped me to repair trhe old membrane of an LCD alarm clock.YMMV but avoid anything too hot that may damage the whole membrane.

author: theHWcave
author_rank: Regular Contributor
timestamp: November 04, 2022, 07:19:08 pm
content: Conductive paint... that's an idea!How did you use it to fix your keyboard? Did you scrape the insulation away first and then paint over or just apply over the crack and hope it will seep through and reconnect?

author: xmris
author_rank: Contributor
timestamp: November 04, 2022, 07:44:38 pm
content: Nope I just applied the paint over the broken lines, the membrane of the keyboard has non-insulated lines.attached some pic of that, note the repair is on the bottom membrane (you can see the black spots of the paint, those are areas that were damaged) , I ve attached together the two membranes via selotape just to keep them more stable- because I did some bridge with a very thin cable (as some big damage), I glued it using the paint!edit:Now in your case: I would carefully scrap the insulation of the membrane, then apply the conductive paint -wait to dry - then check if there is continuity, if all ok : cover with some kapton just to be sure the conductive paint will stay there, alas in your case the cable is bending there I am not very sure is the conductive paint is too flexible for this, maybe you can apply the paint after the membrane is in place.

author: xmris
author_rank: Contributor
timestamp: November 04, 2022, 08:03:16 pm
content: another idea, if the 'curve' of the bend is more of less permanent, may you try to stabilise it by applying some extra adhesive under it just before starting the hacking - note that I used a toothpick to apply the paint - and btw I ve got the 10ml one , not that expensive and its enough for this task, about £8.

author: theHWcave
author_rank: Regular Contributor
timestamp: November 04, 2022, 10:45:27 pm
content: Yeah, I had a look at their website and the small tube would be more than enough.Quote from: xmris on November 04, 2022, 07:44:38 pmYes that could be a problem. Need to ponder that for a while and maybe do some tests.Haven't decided yet to go with paint or something else, but I recon in any case the first part is somehow getting the isolation off. The whole thing is so fragile...

author: macboy
author_rank: Super Contributor
timestamp: November 05, 2022, 09:00:55 pm
content: I also have a 1905a and struggled with the same issue.Don't scrape the insulation as suggested!The cable is in two layers. One has 3 conductors and one has 8 conductors (3x8 matrix with IIRC 21 keys). You can see in your photo that the left 3 conductors are one layer and the right 8 are the other. If you remove the ribbon from the connector, it is easily separated. The Silver conductive ink conductors are then directly exposed. You can then apply the repair product over them.I attempted this repair, and it didn't quite work, and intended to try again, but then I acquired a couple nice bench meters, and just lost interest. So the 1905a sits on a box in pieces (shameful I know).In my case the keyboard had been working fine, but I had a fault which caused strange readings. That turned out to be a short circuit of one of two larger Tantalum caps on the +-15V rails. I replaced them with regular electrolytic plus a small ceramic in parallel. That worked fine. I guess from the stress of disassembly and reassembly, the keypad cable developed some fault.Fun fact, press "*" then "5.5" to enable a hidden 6.5 digit mode. The usefulness of that last digit is questionable but it is fun to see.