Spring tab for button cell (LR44)? - Page 1
Forum section: electronics / repair

author: artv
author_rank: Contributor
timestamp: May 31, 2020, 07:24:26 am
content: HiI'm trying to repair a Mitutoyo digital caliper (500-321). I can't unfortunately get a replacement reading unit assembly so am trying toreplace the tab accepting the SR44 button cell battery. I can't find such a tab on ebay - would anybody be able to point me in the right direction?I found a similar unit on ebay - so you can see what the tab should look like. I have tried manufacturing one using a little tab from picture framing - but it's hard to solder. It does work in a fashion but I would like to find the correct tab (or a possible workaround?).Many thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: May 31, 2020, 09:19:35 am
content: If the OEM wont cooperate, its incredibly difficult to find exact replacement battery contacts unless you happen to live somewhere like Shenzhen. Anyone with a stamping press can make them and the setup cost for custom ones is fairly low, so the main hurdle is simply identifying who's making a compatible oneIf a battery contact hasn't corroded away too much, its usually possible to restore satisfactory operation and reliability by cleaning off all the corrosion with a fibreglass pencil brush or fine grit wet&dry paper, then tinning the contact with solder and brushing off as much as possible to leave a thin tinned surface to resist any future corrosion. N.B. some disassembly is likely to be required to aid removing the corrosion and avoid heat damage to plastic parts.Otherwise, you can probably make a replacement tab from phosphor bronze shim stock. Unlike ordinary brass, phosphor bronze is resilient enough to be used for springs. Tin the contact faces and tail to be soldered to the PCB before fitting.

author: artv
author_rank: Contributor
timestamp: May 31, 2020, 12:37:59 pm
content: Thank you - that's really helpful! I was surprised I could find the tabs on eBay - just thought I was searching for the wrong thing.Do you have any ideas where to get phosphor bronze shims in (I'm in UK) - have searched but not found anywhere.Unfortunately I don't have the old contact - if I'd realised how easy it was to take apart before I could have done a decent repair before it was too late!Cheers.

author: dacman
author_rank: Frequent Contributor
timestamp: June 01, 2020, 01:23:44 am
content: A Keystone 2997 is for an SR44 but you would need to modify that hole to fit it.

author: texaspyro
author_rank: Super Contributor
timestamp: June 01, 2020, 04:07:22 am
content: Maybe get one of those dirt cheap Chinese calipers (like $10 at Harbor Freight) and salvage the contact from there.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 01, 2020, 04:19:42 am
content: HiProbably the best you can do is brass shim stock, everything else has been exported to China.George

author: Ian.M
author_rank: Super Contributor
timestamp: June 01, 2020, 05:34:20 am
content: Railway modellers who scratch-build locos use narrow strips of phosphor bronze shim for wheel contacts, so try good specialist model-making supply shops - if they don't stock it they msay still be able to order it in for you. Wider strips and sheets are a *LOT* more difficult to find and not cheap. UK based specialist engineering suppliers *may* be able to supply sheets at a price that will make you cry. McMaster-Carr in the US stocks it at more reasonable prices but that's not a lot of help to you. Otherwise, as George said, you may have to settle for brass shim.

author: artv
author_rank: Contributor
timestamp: June 01, 2020, 07:28:54 am
content: Thanks some great suggestions. Funnily enough have a cheap set of calipers - guess what - the terminal is beyond repair on those too. Also that terminal is smaller and only connects to one contact on circuit board - the Mitutoyo one spans across connecting to 2 contacts?The little framing tab is actually doing the job at the moment surprisingly - but was hard to solder though and don't think it will last long. Will probably initially try some brass stock or possibly raid some children's toys! Not sure how I could modify the Keystone 2997 - but could also try that.Thanks again.