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Old 17th Apr 2018, 2:10 pm   #1
The Philpott
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Default Wee Megger (duplex) 500v

I have a hand-cranked duplex megger with 1 ohm and 1 meg at centre scale. It probably dates from around 1948.

Yesterday when I tried to use the high scale, the needle sat firmly over at infinity and would not budge. After a few taps it started to shift, but the output (high range) struggled to get to 200V on a 20kohm/V meter, and could only monetarily reach 250V by overspeeding the mag. It was, at least, gratifying to note that the same result came up using a 4kohm/V meter.

The output on low range is not detectable..

The needle was still not quite moving as it should, and having removed the case front I found that a 4.7kohm resistor was just barely making contact with the rectangular dubilier capacitor. The resistor was reading about 5kohm anyway so I replaced it with a new 4.7k and ensured the capacitor was now connected up. This fix resolved the lazy needle and it showed reasonable accuracy on cranking.

The low voltage output, however, remains the same.

(Note that access was non-existent for this fix until the scale plate was removed; and it would be very easy to ding the needle doing this.)

I shan't be doing any more repairs on this just yet as it is my only Megger, and a degree of function appears to remain. If I got stuck in I could end up with no megger instead of half a megger..)

That capacitor looks like a pig to remove, (it has the flange fixing down in the recesses of the instrument, firmly installed) however I would like to know if there are any other likely candidates causing the low output..

Thanks
Dave
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Old 17th Apr 2018, 4:37 pm   #2
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Default Re: Wee Megger (duplex) 500v

Brushes, soft magnets, and if I remember, that capacitor. It's a while since I had mine apart.

David
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Old 18th Apr 2018, 11:20 am   #3
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Default Re: Wee Megger (duplex) 500v

The output of the generator is pretty high impedance (though not high enough to prevent a pretty severe twinge!), so it only needs slight leakage from the capacitor to drag the off-load output well below the nominal 500V. The leakage characteristic is also pretty non-linear, i.e. an ohm-meter using 1.5V or 9V or even 22.5V is no good for checking the capacitor, it may read sky-high resistance at low voltage but go much lower at three-figure voltages. Whilst the action of the circuit is self-compensating, it does mean that it's misleading to check, say, the primary side of mains appliances as the open-circuit output will be nowhere near the specified voltage. Unfortunately, the bracketed Dubilier capacitor is a special that also has resistance bobbins secured to it and into the case with long brass screws that are vulnerable to shearing (how do I know that....) Try disconnecting the original and putting a modern plastic film type in place (it prevents pointer shimmy, rather than anything more fundamental) and see if the output voltage gets nearer nominal.
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Old 18th Apr 2018, 12:44 pm   #4
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Default Re: Wee Megger (duplex) 500v

Thanks Turret, I referred to your (closed) thread and this was steering me to the same conclusion. So it looks like I need a 100nF 1600v polypropylene cap- to be hot glued or araldited in.

That Dubilier cap is probably coming out today- I think they have dates on them which will be telling.
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Old 18th Apr 2018, 1:21 pm   #5
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Default Re: Wee Megger (duplex) 500v

Just to confuse the whole milli/micro/nano/pico thought process, the early Avo Model 7 accepted script for micro is a capital M with the left hand tail sloping away to the left at it's base. I thought i had a reject scale-plate, then realised they are all like this at that period..
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Old 18th Apr 2018, 6:58 pm   #6
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Default Re: Wee Megger (duplex) 500v

0.1uF capacitor, which has no capacity to speak of (it hasn't got an obvious date, unless it's encoded)

It could be slid out with relative ease, one of the screws you mentioned will ideally need an extra fibre washer under it's head, as it's now slightly too long for it's job.
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Old 18th Apr 2018, 9:10 pm   #7
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Default Re: Wee Megger (duplex) 500v

...Or i could snap the flange off the old cap. and slide it back underneath..

Recommendations for DCV rating of the new one..? - there is everything from 700v to 2000v available online.
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Old 18th Apr 2018, 9:42 pm   #8
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Default Re: Wee Megger (duplex) 500v

Sounds like you're going through a similar thought process to me.... I interpreted "1500V DC test" on the old capacitor as meaning "1000V DC working", reasoning that this allowed for the 500V DC generator plus waveform lumpiness, leads dropping off mid-crank, a bit of overspeed and all that.
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Old 18th Apr 2018, 10:24 pm   #9
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Default Re: Wee Megger (duplex) 500v

Chose one slightly smaller than the original, 1.6kVDC supposedly. It won out as it has tails rather than just stubs for board mounting. This time yesterday i wasn't going to tackle it- then of course you start thinking about it and the screwdrivers come out.

The AVO 7 was hopeless for testing this, it wouldn't even set to infinity prior to the test, so that big green capacitor might be no good as well. There certainly wasn't a splat when the terminals were shorted afterwards.
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Old 27th Apr 2018, 8:58 pm   #10
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Default Re: Wee Megger (duplex) 500v

An improvement- which cost around 2p per volt!-

400v on reasonable cranking speed,
500v balls out.

Flange used from old cap. as a spacer under the bobbins, as I didn't fancy shortening the fixing screws and risking damaging the threads in the casing. They are quite fine. Blob of araldite and silicon sleeving. Thanks for advice.

Dave
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Old 28th Apr 2018, 1:03 pm   #11
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Default Re: Wee Megger (duplex) 500v

Footnotes:

-The accuracy of the resistance reading has measurably improved.

-The fitting of an undersize capacitor can mean that the scale plate doesn't automatically sit flat as you slide it in. This increases the chances of getting grease on it from the gearwheel, or bending the needle. The end stops for the needle also need to be watched as this operation is carried out; they are springy and need to be twizzelled back and forth slightly to shoehorn the curved part of the scale plate in.

-Watch for the needle wandering about of it's own volition!

Last edited by The Philpott; 28th Apr 2018 at 1:09 pm.
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