Conductive Epoxy

[Kevin Kitts]

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

[Katie Wasserman]

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

[uhmgawa]

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.

[Katie Wasserman]

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.

[uhmgawa]

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.