Megger Circuit Testing Ohmmeter.

[noble kiwi]

Megger Circuit Testing Ohmmeter.

I have just acquired an old Megger Circuit Testing Ohmmeter, as appears in early Megger literature, to add to my collection of Megger items. Serial number 267928, I understand that it is over 70 yrs. old. It came in a very grubby but intact leather case, in which it must have lived for a very long time unused. There is no evidence showing that any form of alternative battery has been fitted and the old 4.5 volt style has not been available down this way, post World War II. Unfortunately the matching connecting leads and prods are missing.

It has cleaned up good as new, all components check out as having logical resistance and the meter movement works freely. I have fitted 3 AA cells and get over scale meter movement when temporary connecting leads are shorted together.

The instructions inside the case lid, describe a two terminal type with a white battery button, as well as a three terminal type, which is what I have. A twin conductor lead is referred to which is connected to two adjacent terminals, one smaller than the other with a precaution to match them correctly. The other lead I gather is a normal single conductor, possibly of a certain length and resistance.

The movement has typical Megger dual windings, with no return spring and it would appear that the two terminal model operates as do Megger generator units which combine a low ohms function, and as a result zero adjustment is not called for. I have some Megger data but nothing provides any detail regarding the twin conductor set up, which I would guess involves a means of countering test lead resistance.

Google has not been able to help. I have traced out the circuit, but only so far, as I am unable to properly differentiate between the meter windings without disconnecting things. I would sincerely appreciate it someone could throw some light on the subject, so that I can make up some leads and get things working. Probably silly, but I am only interested in collecting properly working items.

Thanks in anticipation,

Trevor.

[Herald1360]

What style of 4.5V battery is that?

The curiously named "flat" battery perhaps?

[WME_bill]

I posted some comments about this curious two coil meter on 18 May 2009, under title Re: Megger series 3 insulation tester. That may give you some ideas. Billn

https://www.vintage-radio.net/forum/...ad.php?t=40687

[noble kiwi]

Herald1360,

Yes, down this way the battery was referred to as a lantern or flashlight battery and the similar style larger three volt, as a bike lamp battery.


Special thanks WME bill,

However as I pointed out, I understand how the typical Megger system, as used in the series, 3 operates. It is the twin test lead arrangement which has me confused.

I gather that this was not a popular Megger model, but hope someone will know details.

Trevor.

[noble kiwi]

A similar tester is illustrated here. ---

http://www.ebay.com/itm/VINTAGE-MEGG...5#ht_500wt_717

[QQVO6/40]

Trevor. Look up details of a "Null balance Earth tester" I guess this is not your instrument however it will give you an idea of how the 4 wire or 3 wire resistance measurements are made. i have a couple of Megger earth balance testers and I use them for low ohms testing which they are quite good at. As long as there is little inductance in the circuit I am testing as the tester develops current at about 400Hz. Do not hesitate to contact me if you need to or we can keep it here for the benefit of other people. Just another thought. Look up details of a "Kelvin Bridge" as it will have the same sort of information

[noble kiwi]

Thank you Australia,

I know what you have in mind, as I have in my collection a cable fault localiser which incorporates a Kelvin bridge type set up. The confusing aspect is that there is apparently two conductors running to a single probe, to be used in conjunction with another probe connected with a single conductor. There is only three connections to the instrument, only two points make contact with the circuit under test and there is no provision for any form of control for adjustment within the instrument.

I can also see no relationship with the “guard” terminal used on Megger insulation testers, as here again three test connection points are called for.

Do the twin conductors comprise some form of loop within a bridge to adjust for lead resistance? Is the idea to prevent the use of leads other than those supplied with the equipment which have a calibrated resistance? The plot thickens.

Trevor.

[noble kiwi]

Having delved deeply and received clues from a member of a US antique radio forum who has a meter and leads in perfect condition, I have been able to sort out the functions and circuitry. After isolating the meter coils, applying voltage and making a slight mechanical adjustment to restore the exact zero setting, the metre is amazingly accurate on all ranges, at all points on the two scales. Relatively low resistance leads are a must. It will prove a useful piece of kit, as well as being a collectable representation of proven long lived, made in England quality.

It turns out that the dual test lead simply provides a means of placing the on/off push button on one probe and which is provided with a suitable handle. This rather than having the momentary switch integral with the unit, as per the alternative two terminal model. Otherwise it is not possible to use two hands when making connections when measuring. Evidently both models were designed to overcome the problem of users forgetting to switch off after use.

[Chaparal]

You can find the battery (Flat Battery) new on ebay

for example
PANASONIC SPECIAL POWER 3R12 FLACHBATTERIE 4,5V - 1.99 Euro
or
2 x 4,5 Volt Block VARTA Superlife Zink-Chlorid, 4,5V, 3R12, 3LR12
- 3.19 euro

peter

[noble kiwi]

Originally Posted by Chaparal

You can find the battery (Flat Battery) new on ebay

for example
PANASONIC SPECIAL POWER 3R12 FLACHBATTERIE 4,5V - 1.99 Euro
or
2 x 4,5 Volt Block VARTA Superlife Zink-Chlorid, 4,5V, 3R12, 3LR12
- 3.19 euro

peter

Special thanks Peter.

However it is much easier for me to purchase 3 AA batteries anywhere anytime and they do the job perfectly.

Whatever, the trouble you have gone to very much appreciated.

As it happens just today I acquired another very similar Ohmmeter, this one being a "Record" Serial number O.2384, with two ranges 0 -3 and 0 - 30 Ohms. It also incorporates a dual winding moving coil meter set up, much like the Megger. As far as I can make out, the back of the scale plate is scratch marked, "3000-82, O.2384 and 26/11/48. Nothing internal appears to have been disturbed.

The scale plate carries a note, "Deduct resistance of connecting leads." All in all at this point I am unable to understand any advantage in this form of instrument, except that the scale is more linear. I have found that one of the current carrying hair springs has become detached, but hope to repair same and it will be interesting to check its accuracy.

The battery compartment is embossed "STD No 8# BATTERY with outlines showing that two are required. This is a type I have never run across and appears would measure 70 X 20 mm. In this instance it will be very easy to shift the contact end clips for shorter AA cells.

The meter carries no makers name, but in every way appears to be an Evershed and Vignoles Ltd product. Any light which can be thrown on aspects of its origin will be greatfully received.

Thanks in anticipation, Trevor.