'Megger' Circuit Testing Ohmmeter (early 1960s)

[Phil G4SPZ]

I'd been looking for a decent low-range analogue ohmmeter for a while, and this little meter appeared at a recent amateur radio club surplus sale. No-one else was interested, so for a mere £2 it became mine.

The green Bakelite case was undamaged and in relatively good condition. The main problem was that the old 1289/3R12 4.5 volt 'flashlight' battery had been left inside and had leaked, corroding the positive terminal to the extent that it snapped right off as I was cleaning it. I cleaned up the remainder of the 4BA stub with a rotary wire brush, then using a 60 watt iron I soldered a 4BA solder tag onto it. A couple of short leads and a 3 x AA cell holder with batteries returned the meter to working order.

This is the three terminal model which has two ranges, 0-3 ohms and 0-30 ohms, and was intended for use with special test leads, including a dual positive lead having an in-line battery switch. I searched the Forum, found this old thread from 2012, and with some advice kindly provided by Forum member Trevor ('Noble Kiwi') I made up some new test leads. Rather than using a switched battery probe, I made up three leads with crocodile clips. I discovered that the resistance of the common negative lead is critical, which is not surprising since it appears in series with the resistance under test. I tried several lengths of cable and probes before settling on one which measured less than 0.1 ohm, and was delighted to find that the meter is very accurate when testing several resistors ranging from 0.1 ohm up to 30 ohms.

This instrument has already proved itself very useful!

Phil

[jmedley]

Good job Phil, lovely small instrument.
Jim

[Phil G4SPZ]

Thanks, Jim! Hope you're well.

I do like these small Meggers. The two you kindly passed on to me a while ago both get regular use at the Museum and the Repair Cafe. Kindest regards,

Phil

[Lloyd 1985]

I’ve got 2 of those, 1 still in its original case. Never actually tried either of them out! I bought them purely for the lovely green mottled Bakelite! I also have one of the 500v ‘Wee Megger’s’ which unfortunately has an OC generator.

Regards
Lloyd

[Phil G4SPZ]

I see that Meggers start to accumulate, just like Avometers... A second Circuit Testing Ohmmeter has just smuggled its way into the collection. A different model, with two ranges: 0 to 500 ohms, and 100 ohms to 50,000 ohms. This one came with an original set of test leads and a nice 1943 Megger "hand-book on continuity and polarity testing". It even had a good battery inside it.

A small amount of corrosion was removed from the battery terminals with a rotary wire brush and a spray of Servisol. The meter worked, but the high range switch was intermittent and the readings were inaccurate, so the lid came off for a look inside.

Servisol dealt with the intermittent range switch contacts. The meter indicated below zero on the low range with the probes shorted, and above infinity on the high range with the probes apart, so I was looking for a mechanical zero adjustment. There wasn't one, nor any electrical adjustment. On closer inspection, the pointer looked slightly bent to the left near where it's attached to the moving coil assembly, so with great care and some trial and error I made some gentle minor adjustments. This has resulted in the meter now properly indicating zero, infinity and all points in between.

The test current on the low ohms range is around 30-35mA and the probe without the pushbutton is positive.

Phil

[noble kiwi]

Kia ora Phyl,

Great that you have acquired a meter complete with test leads.

The probe is exactly as I had anticipated it would be and why I substituted a simple jumper by way of a compromise. I also now recall that I adjusted the movement on mine as you have done. Not surprising considering the need for static acuracy.

During my discussion with you I ventured the opinion that this Meggar offered very little of advantage when compered with the functions combined within an "AVO". Further thought has changed my mind on this. In fact I now realise that way back I went to the considerable trouble of making mine functional with the prospect of using it, but somehow time elapsed and I forgot about it being on hand.

This AVO and Megger stuff is certainly collectable on the basis of the absolute "spare no cost" approach in respect of production. I have a another still useable little gem in the form of a small light meter, (not a photo exposure meter) styled exactly in the form of full size AVO. Not that rare, but a cute and cuddly miniature. LOL The small Wee Meggers have the same sort appeal and I have several.

Regards, Trevor.

[The Philpott]

If you liked that, you might appreciate this! :

Duplex (crank) 500v megger with 1 ohm and 1 Meg at centre scale. Screw in type terminals (top and bottom on photo 1) one of which has crumbled a bit. This effectively came as a pair with the LR Ohmmeter Avominor 'Special' and were both used by an aircraft fitter in the RN. The age of the megger would be interesting but i imagine it would be around 1944. Still works. Thank God it hasn't got batteries in it!

[Phil G4SPZ]

I have one of those Duplex Meggers too, Dave, kindly donated to me by my good friend and fellow Forum member Jim Medley. Mine has push-in terminals and works well, apart from the movement having a tendency to stick at the LH end. A gentle tap on the case before winding the handle always frees it off. I think that these were designed for electricians testing electrical installations for compliance with the then IEE Regulations, perhaps the edition from around 1937 which called for an insulation resistance > 1 Megohm and an earth continuity of < 1 ohm. Pity the low resistance range isn't properly divided up and scaled. Mine lives in my tool box and gets a regular outing once a month at the Repair Cafe.

I absolutely agree, Trevor. These small Meggers are positively beautifully built inside, and the Bakelite case is so thick and solid that it looks like it should survive a hammer attack, or at least a severe drop test. That said, we have a 500 volt Wee Megger at the Black Country Museum with a bosted case. Oh yes, I've got an Avo light meter too, which I actively use for measuring the candlepower output of vintage paraffin lamps... but I digress.

Going back to the Circuit Testing Ohmmeter, it appears that there were four versions made, with different scale ranges, of which I now have two, and a growing desire to obtain examples of the other two, which I'm trying hard to resist It's the 0-3 ohm range that is proving the most useful, as this covers the very low regions of resistance that most conventional Avometers struggle to reach.

[Phil G4SPZ]

Here's a picture of my Duplex Megger, which judging from its serial number 903211 is a later model than yours, Dave.

I am wondering whether it would be worthwhile starting a "Megger" thread along the lines of our favourite and long-running Avometer thread, not for any specific research purpose but to illustrate the wide range of instruments of this type made by Evershed & Vignoles that are now in the hands of collectors, restorers and enthusiasts. Dave, Trevor and I appear to have modest Megger collections. What do people think?

Phil

[Herald1360]

Slightly OT, but for low ohms, my old Russian multimeter used a technique which put the unknown resistor in parallel with an internal reference resistor rather than the series arrangement normally found. This reversed the scale in a similar fashion to the duplex Meggers.

[turretslug]

Not the Ц 4324 by any chance? I had one, almost certainly from Z + I, that used that principle and very good for low ohms readings. It got pinched from my car long ago, occasionally nostalgia makes me want another but, being pragmatic, the likes of the Maplin £10 20k/V job probably makes more sense.

I've a lovely mottled green Bakelite low ohms Megger as per the OP in the rountuit queue, TBH it shouldn't take too much fettling. It's good to see that this sort of kit has a few fans.

[The Philpott]

Chris, interestingly the AVO High Sensitivity model (mine's 1948) with the low ohm press button range, measures the voltage across the unknown resistance.

Phil, your Duplex megger must be around 1951-52 using Richard's rough guide. Hasn't been left in the sun unlike mine. Once i found that it worked - and was a 500v job- i didn't/don't plan to get another one (but can i stick to this..?)

The terminals are quite rough looking bits of brass thread, over which bakelite knobs with female brass inserts fit. When they are removed they have a nasty habit of rolling away and falling onto the floor, which might explain why one is damaged.

[TonyDuell]

There was an Italian pocket multimeter, something like a 'Supertester 680R' that used sockets to select the various ranges. It came with a hairpin link which you could put between the Ohms-common and Ohms*1 sockets and then zero the meter at the right hand end of the scale. You put the probes in 2 other sockets and it then formed a 'shunt' low-ohms meter. Worked very well. It's a pity the 2 I have both have faulty meter movements..

I have one of those Megger circuit testing ohmmeters somewhere too. I can't remember what ranges it has. I must dig it out.

[The Philpott]

Note that Phil paid £2 for one and i paid £2.40 for one- if someone is asking silly money you just walk away.

Postage is a bit more obviously..

[philthespark]

I have one of those meters, I bought it brand new in the box for a fiver, it had no leads, apparently they were sold separately! It did however come with the instruction manual, and a piece of paper stating that should I require a copy of the current wiring regulations, then they could be purchased from them for around £2. Now given that when I bought a copy of the 17th edition regs it cost me over £80, it started me thinking, I wonder what they'd say if you contacted them and asked them to honor their price on a copy of the current regs? lol.

[Phil G4SPZ]

Originally Posted by The Philpott

Note that Phil paid £2 for one...

I paid £25 for the second one though, mainly because I was keen to acquire a set of original leads, and they were in good condition.

Interesting point about the Regs, Phil, though I doubt that your request for a modern copy for £2 would elicit a positive response! Wages for an electrician in 1939 would have been around £3 a week, so today's Regs are probably cheaper in real terms.

[philthespark]

Very true, though I bet they didn't change as often as they do today, lol. I remember when I first started out, you had a megger if you were lucky, nowadays I have so many meters it's untrue. back in the day you proved earth with a test lamp, stick one side on live and the other on the thing you wanted to know was earthed, if it lit it was earthed! Now it's loop testing and measuring down to 0.0? of an ohm, personally I was much happier with the test lamp, life was certainly easier, and you had less gear to get pinched!

[Phil G4SPZ]

Going back to the Circuit Testing Ohmmeter, according to the Evershed and Vignoles product catalogue from 1951 (with acknowledgements to the excellent Richard's Radios website) four models were available:

0 - 3 ohms and 0 - 30 ohms
0 - 30 ohms and 0 - 300 ohms
0 - 500 ohms and 100 - 50,000 ohms (upper range reversed)
0 - 1,000 ohms and 100 - 200,000 ohms (upper range reversed)

With the exception of the first model listed, these instruments don't offer any resistance measuring capability that isn't present in any self-respecting Avometer!

[Herald1360]

Originally Posted by turretslug

Not the Ц 4324 by any chance?

No, its big brother, the Ц 4317.

This one in fact:

https://www.vintage-radio.net/forum/...85&postcount=8