HP Forum Archive 19

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Conductive Epoxy Message #1 Posted by Kevin Kitts on 14 Oct 2010, 10:28 a.m. I found a couple of old posts from 2000 on using conductive epoxy to repair old HP 41 calculator battery contacts. I have a 41cv from around 1981 that had a single (the leftmost) battery contact with corrosion and operation became intermittent and then finally failed. I applied some of the recommended conductive epoxy and now it seems to work perfectly. I am just wondering if either of the two original authors that discussed this are still around - and if the fixes they made in 2000 are still working after 10 years. In other words - what is the longevity of these kinds of repairs. Thanks All! Here are the old posts: HP-41C battery contact fix Message #1 Posted by Erik Wahlin on 2 Dec 2000, 5:07 p.m. I just discovered a relatively easy way of fixing the HP-41C series calculator battery contacts. These are the gold or other gold color metal plated contacts on the battery contact block. Battery corrosion and mechanical wear from the batteries tend to ruin these contacts. You can apply a conductive epoxy to the damaged areas and this seems to work. I don't know yet if this type of epoxy oxidizes in time which would cause it to fail later. The epoxy I used is made by Circuit Works (part # CW2400). It is available through Allied electronics and is not cheap at about $18 for the two tube set (7 grams). The tubes have Chemtronics (www.chemtronics.com) printed on them.(http://www.chemtronics.com/catalog/catalog.cgi?action=list_products&category=7) This product is normally used for circuit board trace repair. Please use at your own risk. Has anyone else out there ever used this type epoxy for battery contact repairs? Re: HP-41C battery contact fix Message #2 Posted by Katie on 3 Dec 2000, 1:28 a.m., in response to message #1 by Erik Wahlin Hi Erik, Good idea! I've used the same stuff for several difficult to repair electrical contacts (e.g., attaching leads to solar cells and fixing rear window defroster contacts), but never for a contact that's likely to see some wear and tear. However, the stuff does seem to last for many years and does not break down easily, not even in outdoor applications. I'll bet that it holds up fine. -Katie

Re: Conductive Epoxy Message #2 Posted by Katie Wasserman on 14 Oct 2010, 11:30 a.m., in response to message #1 by Kevin Kitts It held up for a few years in my 41 but eventually failed because of the flexing -- I need to come up with a longer-term alternative. On rigid PCB's it seems to last forever and the repairs I've made years ago are all still good. -Katie

Re: Conductive Epoxy Message #3 Posted by uhmgawa on 14 Oct 2010, 6:22 p.m., in response to message #2 by Katie Wasserman Quote: It held up for a few years in my 41 but eventually failed because of the flexing -- I need to come up with a longer-term alternative. On rigid PCB's it seems to last forever and the repairs I've made years ago are all still good. I've been concerned about this on legacy voyager units given the unusual length of time a set of batteries can reliably live in one of these. The primary point of failure is the seal of the LR44 cell in contact with the negative power terminal. Leakage from this seal is quite close to this gold plated PCB finger and seems to be the first location for corrosion to begin. One possibility for a permanent restoration of this terminal would be wrapping it in thin copper foil and tack soldering it to the remaining terminal plate conductor. If that is corroded beyond resoldering the foil wrap can be epoxyed to the PCB and electrically connected to the fair sized negative via in the PCB just south of the finger contact. A quick solder tinning is better than the leaving the exposed copper to oxidize. Corrosion of the positive spring terminal seems less likely as the LR44 cells have no case seal at that end. But even here you could find/make another terminal as the original contact spring is only compression mounted between the cell well wall and the positive PCB stub. It can be compressed with tweezers and lifted right out. Prevention is probably a far better route considering the number of years batteries can be living in a voyager. I keep meaning to pull the cell stacks out and wrap them in a layer of thin poly tape, leaving about 2cm overage at each end of resulting cylinder. Fold it over against the positive-most terminal only. Fill the remaining end's overage with a few drops of hot wax from a candle and heat the shebang with a hairdryer to seal the tape and set the wax layer (open wax end pointing up). Put the masterpiece in the voyager and forget about it for 10-20 years or sooner in the rare event you actually deplete the battery before then.

Re: Conductive Epoxy Message #4 Posted by Katie Wasserman on 14 Oct 2010, 7:57 p.m., in response to message #3 by uhmgawa LR44 cells are alkaline and I would not recommend them in a Voyager for the potential leakage problem you cite but more because of the much higher self discharge rate that they have compared to the 357 cells. 357 (also SR44SW) cells are silver oxide which have almost zero self discharge, almost never leak and if they do seem to have a less corrosive chemistry.

Re: Conductive Epoxy Message #5 Posted by uhmgawa on 14 Oct 2010, 11:04 p.m., in response to message #4 by Katie Wasserman Quote: LR44 cells are alkaline and I would not recommend them in a Voyager for the potential leakage problem you cite but more because of the much higher self discharge rate that they have compared to the 357 cells. 357 (also SR44SW) cells are silver oxide which have almost zero self discharge, almost never leak and if they do seem to have a less corrosive chemistry. I tend to have an abundance of LR44s on hand to feed digital calipers where the instrument current consumption overshadows the self discharge rate of either version. But certainly silver oxide chemistry cells do have a flatter discharge curve. Unfortunately they're quite a bit more rare and somehow 16x the cost of alkaline counterparts. Still given the voyager's power consumption that by itself isn't a compelling argument. HP specifies voyagers for either LR or SR cell usage derating LR service life relative to SR at 25..50%, where that range corresponds to static memory backup..active system execution. Somewhere in between is the state of powered-on-idle where the cpu and R2D2s are essentially unclocked sans the separate low frequency LCD refresh clock. This is probably more representative of average "on" usage but I don't know if that's what HP was characterizing vs. continuous instruction execution. I've had an SR44 eat a negative spring-style contact in an early voyager so I'm not too trusting of either chemistry though empirically I'd say alkaline cells have in general done worse to my electronic devices but they're also far more prevalent. Yet given the fragility of the Au plated PCB contact finger I don't think one could be faulted for using a supplemental measure to contain cell leakage. Edited: 15 Oct 2010, 1:29 a.m.

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