Megger Major: Infinity Adjustment?

[russell_w_b]

I have an ex-MoD 'Megger' Major, 250, 500, 1000V and low-ohms, which I would guess to be about 1970's vintage. It is marked as being manufactured by Evershed and Vignoles at Dover, and bears the catalogue number 70154 and NATO number Z4/6625-99-620-8939.

Trouble is, it doesn't read infinity, but settles at 100 Megohms irrespective of selected range (so it isn't caused by leakage inside). I was wondering if anyone knew how to set the infinity point? Any bridge-'Megger' I have used has a 'set infinity' point (otherwise an innacurate reading would be made on 'bridge'), but this is an ordinary, insulation-test 'Megger', and it's niggling me, although I realise 100 Megohms is purely academic for most of the use it gets.

There's a small pot inside which appears to set the 'zero' ohms (yes, I had a bit tweak at it), and a magnetic shunt across the genny that sets the output voltage (I had a bit tweak at that as well!), but no obvious means of setting the infinity point. In theory, at infinity, the pressure coil should be at the weakest part of the movement magnets, at 90 degrees to the pole-pieces. I don't really fancy breaking sealed screws on the movement without further consultation, hence this post!

All advice welcomed.

[Brian R Pateman]

You may have checked this already (ignore me if you have) but it is worth making sure that the movement is free over the whole of its travel.

These instruments often suffer from being knocked or dropped, which can cause odd problems with freedom of movement without there being any obvious signs of damage.

Regards,

[russell_w_b]

Brian,

The movement seems to be free insofar as I can get the pointer to reach beyond the infinity mark by gentle agitation, and nor does it stick anywhere. I haven't had that close a look at it, but I shall take it to work next week and open it up again. The pointer moves (and reads) satisfactorily within its range, so I don't envisage finding a damaged pivot or what-have-you, but I shall check this out under bench conditions.

As I've adjusted the 'zero' pot, I shall also measure some known resistors to see if they tally with the appropriate scale-markings and set the pot again if necessary. If it's been dropped, it's been well camouflaged, as it's in excellent condition cosmetically!

[russell_w_b]

Originally Posted by Brian R Pateman

...These instruments often suffer from being knocked or dropped, which can cause odd problems with freedom of movement without there being any obvious signs of damage.

I guess you were correct, Brian. I opened her up under the bench-magnifier, and it was evident that the coiled spring-wire connector (which appears quite floppy; no spring tension required in this type of movement) had become entangled j-u-s-t enough to prevent the armature from being driven electrically to the 'infinity' end, but not sufficient that it wouldn't go there by hand.

I managed to untangle it using the pointy end of a small nylon zip-tie, and all is now satisfactory; just so you all know... , so this thread may now be closed - unless someone spots something erroneous in the picture below and can tell me more!

[Brian R Pateman]

Well, it's nice to be right for once! Another use for tiny-tiny cable ties.

I doubt if there's much else wrong with it. From the photo it looks pretty much in order.

I'll leave this open for a little while in case any of the other instrument engineers want to come in.

Regards,

[pmmunro]

Russell,

Does your photograph show the ligature in its tangled condition; it looks that way to me but I'm not yet familiar with these movements?

PMM.

[russell_w_b]

No. I took the pic after I'd freed it. The uppermost turn (green arrow) had dropped beneath the turn below and stuck, preventing an 'infinity' reading but allowing satisfactory measurement otherwise. Because the 'Megger' has a dynamometer movement it doesn't rely on tension from a hair-spring for position, and I don't suppose it's that critical insofar as it only seems to be used to supply voltage and current to the coils.

The ligature does look slightly kinked, and I'd really like to compare it with a known 'good' movement. That said, I get satisfactory readings when measuring a range of resistances, so perhaps it isn't too bad.

[Station X]

If it works leave it alone. It's all too easy to damage hairsprings like this.