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Old 6th Dec 2017, 1:08 pm   #1
SeanMcK
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Default Megger MIT310 with corroded? battery terminals

I realise it's a bit young to be vintage but...
I have wanted an insulation tester for a while and was offered a dead MIT310, it looked like the problem was corroded battery terminals which had no connection to the negative end of their respective battery, see attached photo.
It turns out that that diagnosis was correct as a jury rigged repair has the meter functioning apparently perfectly.
Does anyone have such a unit? If so could you photograph the battery terminals and post those photos? I can not find such a photo on the web.
The corrosion residue appears to be rust and on close inspection one -ve terminal had what looked like the base (one turn) of a VERY fine spring attached. Therefore I assume the rust stain is from steel springs which have failed through corrosion.
The terminals are only available as part of a new rear case.
I have googled small springs but haven't found anything that looked suitable and was thinking of canabalising the springs from something like the 10 way battery holder shown. It looks feasible for the 3 terminals that link adjacent battery rows but I think the problem is going to be the terminal ringed in red, it is in a bad way and relatively narrow.
The current solution for this terminal is also shown.
As suggestions welcomed.
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Old 6th Dec 2017, 2:18 pm   #2
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Default Re: Megger MIT310 with corroded? battery terminals

Snap.
I had to build a good Megger out of two faulty ones.
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Old 6th Dec 2017, 2:32 pm   #3
SeanMcK
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Default Re: Megger MIT310 with corroded? battery terminals

Jumping Jehoshaphat.
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Old 6th Dec 2017, 2:47 pm   #4
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Default Re: Megger MIT310 with corroded? battery terminals

On yours the springs look like they were so small that they have vanished into thin air.
Is there any room inside the case behind the remains of the existing terminals.
I was thinking of salvaging some of those little gold plated plungers from an old mobile phone battery compartment.
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Old 6th Dec 2017, 4:28 pm   #5
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Default Re: Megger MIT310 with corroded? battery terminals

How about surfing for phosphor bronze strip/s on eBay? Bend to shape and solder in place? The railway modelling stuff looks too small but there must be more meaty stuff available.
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Old 6th Dec 2017, 9:56 pm   #6
SeanMcK
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Default Re: Megger MIT310 with corroded? battery terminals

Refugee, there is a narrow groove in the plastic behind the linking terminals, in line with where the -ve connection should be but only solid plastic behind the other -ve terminal. I don't understand you idea for mobile phone contacts, sorry.
Phil, akin to the 4.5V battery terminals in an Avo 40, 7 etc.? Thinking on that.......hmmm, the gap to be taken up is rather tight, about 1/8th" or less, the bending shaping might be 'interesting'.
The current solution for the linking terminals is 3 6BA brass washers soldered together as a stack and to the terminal with a copper beryllium wave washer soldered to the 'top' of the stack to act as a spring.

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Old 6th Dec 2017, 10:35 pm   #7
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Default Re: Megger MIT310 with corroded? battery terminals

I have salvaged substantial phosphor bronze strips from the boiling cut-out switch mechanism of an old corded Russel Hobbs kettle that was beyond repair.
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Old 6th Dec 2017, 10:38 pm   #8
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Default Re: Megger MIT310 with corroded? battery terminals

See what you mean Sean, if trying to get it to do a 160 degree bend it may well go 'bang'
Maybe something with a bit more 'give' in it.
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Old 7th Dec 2017, 12:47 am   #9
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Default Re: Megger MIT310 with corroded? battery terminals

I have not got an old mobile phone to hand.
Mobile phones often have little cylinder like plungers that make contact end on.
There must be hundreds of lithium battery chargers out there with good springs that have either failed electrically or fallen out of use.
I have taken a photo of my camera charger. They are another type to look out for in your hunt for a fix.
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Old 7th Dec 2017, 2:15 am   #10
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Default Re: Megger MIT310 with corroded? battery terminals

Over center spring contact out of a microswitch?
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Old 7th Dec 2017, 10:41 am   #11
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Default Re: Megger MIT310 with corroded? battery terminals

Contacts from an old remote? We all have those lying around somewhere.

I will not use those Panasonic batteries at all. They are OK if used regularly but left in unused equipment for more than a few weeks and they corrode.
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Old 7th Dec 2017, 11:21 am   #12
SeanMcK
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Default Re: Megger MIT310 with corroded? battery terminals

I love these ideas, great ingenuity and proper repairs rather than the "buy new parts" ideology.
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Old 7th Dec 2017, 10:04 pm   #13
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Default Re: Megger MIT310 with corroded? battery terminals

Be careful taking red/yellow metal springs out of kettles. Some of them may well contain Berilium. I don't think that includes the OLD Russel Hobbs.
Les.
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Old 7th Dec 2017, 10:14 pm   #14
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Default Re: Megger MIT310 with corroded? battery terminals

Springs out of old thermostats are made from a much safer form of Berillium than the heat sink paste found in RF output transistors.
It should be reasonably safe to work with.
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Old 7th Dec 2017, 10:45 pm   #15
SeanMcK
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Default Re: Megger MIT310 with corroded? battery terminals

The things you learn, I take it Copper Beryllium washers are safe enough
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Old 7th Dec 2017, 10:51 pm   #16
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Default Re: Megger MIT310 with corroded? battery terminals

You are right. They are a metal alloy. Beryllium oxide is the poisonous form found in RF output transistors.
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Old 8th Dec 2017, 2:03 am   #17
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Default Re: Megger MIT310 with corroded? battery terminals

In the 1970's I handled numerous stud-mounted RF power transistors used a visible BeO insulation layer between the die and the stud, and my understanding from their documentation was that they were safe to handle while intact. It is only the dust which is hazardous, hence the warnings not to abrade, drill, or file the stuff. Beryllium alloyed with Copper is an alloy rather than a ceramic, and would not have the toxic dust problem.

My experience with Phosphor Bronze has been that it has a definite grain. Bend it along the grain, and no problems. Bend it across the grain, and it will fracture. I cannot claim to be an expert, but I have cut strips from a small length of wide Phosphor Bronze strip that I once salvaged from the scrap bin at Marconi's to make replacement brushes for small electric motors. These involved making right angle bends to match the original brushes, and I found it essential to consider the grain when marking out the metal to avoid fracture at the bends.
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Old 8th Dec 2017, 11:50 am   #18
SeanMcK
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Default Re: Megger MIT310 with corroded? battery terminals

I am leaning towards trying a cantilever type leaf spring for which I think there might be room of the common terminals at least. Whilst thinking about that I remembered that somewhere I have a length of springy wire with a copper coloured surface that, for its diameter, is very stiff. It takes solder and if it can be shaped into coil springs it might be the solution.
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Old 8th Dec 2017, 4:05 pm   #19
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Default Re: Megger MIT310 with corroded? battery terminals

I was faced with this problem recently with one of those "Rugged Radios" used by builders. They use 2 AA cells to maintain the clock and presets. These had corroded and totally dissolved one of the tapered coil contact springs. I slid out the brass strip, cleaned it and soldered what looks like a phosphor bronze coil spring to it. I've no idea where the spring came from. Every time I scrap something, all the springs it contains get slung in to a plastic box and this was one of them. I could have used a spring taken from a battery holder, but preferred not to destroy one.
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Old 17th Dec 2017, 1:51 am   #20
SeanMcK
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Default Re: Megger MIT310 with corroded? battery terminals

Though I am going to try to wind my own springs from the wire I mentioned I had a thought today for leaf spring material, the contacts off those 4.5V batteries themselves.
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