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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 24th Mar 2017, 8:03 pm   #1
Boater Sam
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Default Megger 500v needs intructions, leads and fixing

I have a Megger in the mottled Bakelite case that I have never used.
I need an instruction leaflet and some leads, the side holes for the leads look unusual.
The needle is drifting around in the middle,should it?
Has anyone managed to fix one of these?
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Old 24th Mar 2017, 8:10 pm   #2
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Default Re: Megger 500v needs intructions, leads and fixing

The needle SHOULD drift; it is not sprung as far as i remember. These meters are quite well made so chances are it's ok.
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Old 24th Mar 2017, 8:36 pm   #3
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Default Re: Megger 500v needs intructions, leads and fixing

On mine i think the terminals are female, ie the male bakelite-topped screw goes into it.
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Old 24th Mar 2017, 8:38 pm   #4
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Default Re: Megger 500v needs intructions, leads and fixing

As 'The Philpott' says, yes they do drift, there are balanced connections to the moving coil to make sure there is no torque on the moving system, that's from the manual.
On my test leads I've just crimped blade connectors to fit in the holes.

Some interesting reading below.

http://www.g1jbg.co.uk/pdf/MeggerBK.pdf

http://www.richardsradios.co.uk/megger.html

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Old 24th Mar 2017, 9:49 pm   #5
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Default Re: Megger 500v needs intructions, leads and fixing

The side holes will take old-style "banana" plugs of 1/8" diameter- those of a sensitive disposition should look away, but I wanted to use mine with the array of 4mm plug test leads I've acquired over the years, it's straightforward to simply drill out the receptacles to 4mm, there's plenty meat in the brass blocks to accommodate this and makes them a bit more flexible in use. There's not much available depth, though, so anything other than shallow plugs won't go all the way home.
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Old 24th Mar 2017, 10:45 pm   #6
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Default Re: Megger 500v needs intructions, leads and fixing

Easy enough to test and use it.

To fit test leads, usually there is a push button on each side of the dial near to the dial which you depress then poke the bare end of an insulated test lead with an aligator clip on the other end, into the hole in the side near the button, then let go of the button, whereby the test lead will be held in place.

Then to test if it's working, just clip a series of resistors of various values in the test clips and crank the handle to check that the reading corresponds with the value of the resistor. EG 1 meg, 100k etc. It's not of course an Ohmmeter as such, but if you test with resistors it will confirm that it would give an accurate indication of insulation leakage when used for its intended purpose.

You can use it for testing caps for leakage (or not), remembering of course that you'll be poking 500V into them.

If you clip the test leads together, when cranked, the meter will read a dead short - needle right to the end of the scale.

You don't have to crank the handle furiously - just two or three rotations as you would if using a hand-drill will give you the reading, and the speed of cranking has no bearing on the reading. To state the obvious, keep fingers away from the test leads when cranking and discharge any caps!

Hope that helps.
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Old 24th Mar 2017, 11:12 pm   #7
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Default Re: Megger 500v needs intructions, leads and fixing

Thank you one and all, that's a very full catch of answers in an amazingly short time. I searched for a handbook on the web, obviously not well enough.
I'll find the number when I can, no doubt someone is collecting them.
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Old 14th Apr 2017, 4:52 pm   #8
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Default Re: Megger 500v needs intructions, leads and fixing

Well, it cranks and makes volts, reads dead short but will not read infinite when O/C.
So it seems to be duff, anyone know how to repair these?
Inside there seems to be just 2 coils, presumably as resistances, the generator and movement, which is free and looks undamaged.
Help!
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Old 14th Apr 2017, 5:19 pm   #9
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Default Re: Megger 500v needs intructions, leads and fixing

This is a very pertinent thread - I bought one very recently, also with no test leads, and the drifting needle also worried me. I'd better try it out!

Many thanks,

Andy
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Old 14th Apr 2017, 6:26 pm   #10
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Default Re: Megger 500v needs intructions, leads and fixing

Sam how close does it get to infinity when testing o/c?

To repeat what John said, www.richardsradios.co.uk has a repair 'case history', a picture of the internals and I think there is a download of one of the manuals. I have never taken one apart, I notice it all looks rather tight inside though.

The meter compares the ratio of the current in the two coils.

Last edited by The Philpott; 14th Apr 2017 at 6:32 pm.
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Old 14th Apr 2017, 6:51 pm   #11
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Default Re: Megger 500v needs intructions, leads and fixing

Unfortunately that E & V generator is nothing like the Megger.
The needle stays around mid scale with an open circuit when cranked, but does go full over to short circuit with a link across the terminals.

Is this a bridge circuit with the 2 coils and 2 windings on the movement?
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Old 14th Apr 2017, 9:21 pm   #12
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Default Re: Megger 500v needs intructions, leads and fixing

Wee-Megger.
There are a variety of models with different ranges and test voltages. All much the same.
I attach some notes. I have various manuals, but they are too large to post, and do not offer much more than you will learn by taking the case off and looking hard.

The clever thing is the two coil meter movement, with no control spring. One coil operates from the test voltage applied and the other from the current through the resistance/insulation being tested. So the reading is almost independant of the voltage and the speed at which you turn the handle. The original Evershed and Vignoles patent.

If you get it to go to zero when terminals are shorted, then the meter is likely to be OK. But a low test reading suggests the generator, or the diodes or the half bridge is failing. Check the voltage from the generator.
There are models which use a battery powered transistor generator, so if the cranking generator is failed, make up a battery powered one! or get another wee megger. They keep turning up on ebay.
I would endorse what David G4EBT says.
The test leads are quite conventional. My Megger Major and my Avo BM7 use 4mm banana plugs. My Metrohm (1969) uses spring loaded terminals which accept 1/8inch or smaller.
BoaterSam comments about the makers. The Evershed and Vignoles Megger was very much the market leader.
Competitors like Edgcumbe with Metrohm and Avo with BM7 & 8 use conventional moving coil meters, but with some form of voltage regulation and diodes shunted across the meter coil to give a more open scale shape.
Then Thorn/Avo bought up E & V, and now trade as Megger. Our Avo 8 is no longer made, nor the VCM.
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File Type: pdf Wee-megger description-wm.pdf (1.14 MB, 146 views)

Last edited by WME_bill; 14th Apr 2017 at 9:31 pm.
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Old 14th Apr 2017, 9:41 pm   #13
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Default Re: Megger 500v needs intructions, leads and fixing

My Megger Major didn't quite reach infinity at one time. When I opened it up, the end of the hairspring had a slight twist in it, and 'pinged' back when I stroked it with a tiny screwdriver to good effect.
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Old 14th Apr 2017, 9:45 pm   #14
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Default Re: Megger 500v needs intructions, leads and fixing

Wow, thanks Bill, that should get me started. First off I'll find which model.
I have wound it up on a DMM and only get 350 v or so.

Russel W B, hair spring, with these eyes? I'll have a look, I've got 3 magnifying glasses.

Thanks all,
Sam.

Last edited by Boater Sam; 14th Apr 2017 at 9:48 pm. Reason: addition
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Old 14th Apr 2017, 11:02 pm   #15
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Default Re: Megger 500v needs intructions, leads and fixing

These days i need 3 magnifying glasses- often in series...

x10 ought to do it, with good light!
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Old 15th Apr 2017, 8:13 am   #16
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Default Re: Megger 500v needs intructions, leads and fixing

Sounds like the control coil circuit is open, that's why it flops about open and goes to the end when shorted.
 
Old 15th Apr 2017, 9:51 am   #17
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Default Re: Megger 500v needs intructions, leads and fixing

Quote:
Originally Posted by Boater Sam View Post

Russel W B, hair spring, with these eyes? I'll have a look, I've got 3 magnifying glasses.
I've just had a delve back through my posts. It wasn't a hair-spring, but a spring connector. And I likely used a zip tie to rectify it. I didn't need as strong a pair of specs back then!

http://www.vintage-radio.net/forum/s...t=Megger+Major
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Old 16th Apr 2017, 12:02 pm   #18
M0FYA Andy
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Default Re: Megger 500v needs intructions, leads and fixing

I've just tried mine, but it's not looking very optimistic!
I bought it recently, to be fair there was no claim that it worked, but with my interest in Air Ministry equipment I liked the fact that it has AM reference numbers both on the meter scale and on the leather case.

I shorted the terminals, but no movement from the pointer on winding the handle.

Looking inside, somebody has clearly been there before me! For whatever reason the wire connecting the generator to the meter had been disconnected, however replacing the tag back under the screw and trying again shows no change.

It might be relegated to 'interesting relic' status.

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Old 19th Apr 2017, 3:53 pm   #19
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Default Re: Megger 500v needs intructions, leads and fixing

I too have a 500 volt wee Megger with a mottle case, out of interest I connected my Fluke multimeter to the Megger output I expected 500 volts, but only got 375. I tried the Megger on a second meter thinking perhaps the meter was loading the Megger but the reading was the same as before. Otherwise its a good meter, a slight crack on the case where it must have been dropped sometime. Any suggestion will be well received. Ted
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Old 19th Apr 2017, 7:19 pm   #20
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Default Re: Megger 500v needs intructions, leads and fixing

For lower than expected output voltage, try disconnecting the snubber capacitor fitted to all but very early types and measuring the output voltage again- the typically 100nF/few tens of kilohm snubber isn't fundamental to operation, presumably damping any tendency to needle quiver. If it immediately seems easier to wind, there's a clue... The capacitor may test OK at typically low ohmmeter voltage, but leak in a manner that avalanches as voltage goes up- as it is across the generator output before the actual meter circuit, the movement doesn't "know" about capacitor leakage and it doesn't register. A decent quality 1kV film type should be fine.

If the megger is going to be used with any seriousness as an insulation checker, it's worth making sure it really is giving out 500 (or whatever) V!
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