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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 15th Jan 2019, 6:20 pm   #1
Johnny_b
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Default Wee Megger - HELP

Hello.. I have had this Megger for many years( about 30 ) but the other day I needed it. I thought it did work but after looking inside I think it was tampered with long ago

I have looked at some pictures of the inertia of a model like this but mine has an extra coil. ( 3 instead of 2 )Does anyone have an internal picture of this model to I can check to see if it has been connected correctly.

I have attached a picture but not sure it if shows,,

Thanks

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Old 15th Jan 2019, 7:56 pm   #2
The Philpott
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Default Re: Wee Megger - HELP

How did it behave when you went to use it?

Is that capacitor a replacement? The make Dubilier is correct but i have only dismantled one Megger so not sure if these ones had cylindrical capacitors in them from new.

Dave
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Old 16th Jan 2019, 9:30 am   #3
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Default Re: Wee Megger - HELP

Hello, When I wind the handle the needle does nothing but is free to float back and forth if I tilt the megger. I have tried the generator on its own and it goes easy up to 100 volts. The Capacitor is the original.

If I unsolder the meter and use an Avo to test it the needle does end stop and if I reverse the meter probes the needle ends stops the other way.

I can't fine out the resistance value of the coils. Not quite sure what the Capacitor does as I can find a circuit for this particular version.
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Old 16th Jan 2019, 11:09 am   #4
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Default Re: Wee Megger - HELP

Aren't there two diodes and two capacitors as part of the DC generator?

They are not normally shown on the diagrams that are used to illustrate the way in which these meters work.

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Old 16th Jan 2019, 12:35 pm   #5
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Default Re: Wee Megger - HELP

... at least that is the case in later meters (AC generator and rectifier).

Earlier ones had a DC generator.

Nick.
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Old 16th Jan 2019, 12:55 pm   #6
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Default Re: Wee Megger - HELP

Hi, no there are no diodes and just the one Capacitor. If I disconnect the Generator from all connections, then I get AC volts of about 100 volts and DC of less, so I am assuming the Generator works.

I have totally dismantled the generator and there is nothing hidden inside, apart from the 50 Plus needle roller bearing that fell out all over the worktop,,

Had to hold then in place with Vaseline to get it back together. There is only one Patent number on the case 400728 and made by general Electric
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Old 16th Jan 2019, 1:14 pm   #7
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Default Re: Wee Megger - HELP

The capacitor 0.1mfd 1500v wkg should be connected in series with 47k resistor across output of DC generator to form a snubber circuit smoothing the output. The other two resistors probably100k each.
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Old 16th Jan 2019, 1:45 pm   #8
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Default Re: Wee Megger - HELP

The capacitor may well have been of good quality and high voltage rating, but it's still highly likely to be leaky, and for that leakage to disproportionately worsen as applied voltage increases. Although it's in series with 47/50k, the generator has a high output impedance and even slight leakage will significantly limit the cranking-speed voltage, meaning that test results will be misleadingly optimistic.

It also means that cranking will be unnecessarily hard, not to mention stressful on the gear-train and bearings.
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Old 16th Jan 2019, 2:07 pm   #9
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Default Re: Wee Megger - HELP

Apology if I am completely wrong, but it was the three wire wound resistor bobbins that may be misleading me.

This is just an insulation tester, not the later combined insulation and continuity tester? The combined tester does appear to have three wire wound resistors in the circuit diagram (and would have two scales on the faceplate).

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Old 16th Jan 2019, 4:48 pm   #10
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Default Re: Wee Megger - HELP

Hello, This is what the inside looks like without the Meter.
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Old 16th Jan 2019, 6:02 pm   #11
Johnny_b
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Default Re: Wee Megger - HELP

Just as a thought... Using a digital volt meter, what do you think I should get a reading of on the AC scale and also the DC scale...

Whilst I have had this many years, when I was an electrician, I can't remember if it was actually working when I inherited it or not. I did find a strange item inside the meter ( bit of plastic which looks like a roller ) when I took the top off a few days ago.

Clearly it does not belong to the meter, but I assumed that it had always been there. Maybe someone has looked before and re-connected it up wrong.
I can't find a Plan View of this exact type to check....

Oh dear.....
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Old 16th Jan 2019, 6:38 pm   #12
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Default Re: Wee Megger - HELP

there will be 3 coils, the control coil is in two parts and is connected across the generator the deflection coil is in series with resistance to be measured. See diagram (the wee megger has no guard terminal) there is more here:

http://www.richardsradios.co.uk/megger.html
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Old 16th Jan 2019, 8:18 pm   #13
The Philpott
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Default Re: Wee Megger - HELP

It looks like your Megger is DC so....the (DC) voltage seen by your multimeter upon cranking should be quite near the voltage stated on the Megger. (250v or 500v for example.)

A significant shortfall would indicate the capacitor is failing.

As i said, i have only had 1 of these apart and Richard (and others) know a great deal more. I was lucky enough to simply be able to slot in a modern capacitor and immediately saw an improvement to around 450v on a Megger which claimed 500v when new.

Fortunately a dead wee megger of this vintage can be replaced quite cheaply.(or a parts donor sought) The fantasists who ask 100 or more are comprehensively undercut by those who ask the 'correct' price- usually by moving the decimal point one place.

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Old 16th Jan 2019, 8:22 pm   #14
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Default Re: Wee Megger - HELP

The little roller may be one of the two buttons which are used to depress the contacts for test leads. The DC output will not be pure and have an AC component so measure with an analogue meter.
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Old 17th Jan 2019, 9:34 am   #15
Johnny_b
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Default Re: Wee Megger - HELP

Hello, I much appreciate all your help and explanation from all you guys...

I think that at some stage someone ( who worked for the same company many years ago ) must have taken it apart.

Attached is the picture of the roller. it is nothing like the little button that are depressed to hold the test leads.

Also the Capacitor

Whist I am familiar with Capacitors ( Electrolytic, Tantalum, Disc ceramic, etc ,etc. I don't know what this one it. The word "Dubilier" can be confusing as when I was repairing TV we had Triplers ( Caps with Diode inside ). Any Guidance about this would be helpful.

I did notice that Dubilier was also a Brand name

I will check out the wiring to see if it is connected as it should be so thanks for that bit of info.

Looking at the Picture from that link at the inside of a very similar meter it is connected differently. Even more confusing...

Thanks again
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Old 17th Jan 2019, 10:52 am   #16
The Philpott
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Default Re: Wee Megger - HELP

Dubilier is indeed the manufacturer.
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Old 17th Jan 2019, 2:27 pm   #17
Johnny_b
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Default Re: Wee Megger - HELP

Hi, have you any idea as to what type it is. Can't find any Info on the Dubilier site. Thanks
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Old 17th Jan 2019, 8:20 pm   #18
The Philpott
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Default Re: Wee Megger - HELP

As i only have the one Megger and wanted it to be as reliable as possible, i replaced the capacitor with a modern polypropylene one with as high a voltage rating as i could afford/justify. I ended up with an oblong component from ebay for 7.50 which i araldited in as it was designed for board mounting only.

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Old 17th Jan 2019, 8:41 pm   #19
The Philpott
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Default Re: Wee Megger - HELP

Rather a tight fit. The ebay listing no exists for this particular capacitor.
I can't quite remember why the resistor was changed, but the old one may have been a carbon one which had drifted.

Dave
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Old 18th Jan 2019, 9:27 am   #20
Johnny_b
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Default Re: Wee Megger - HELP

Hello, Oh nice one. Many thanks for the info..Regards
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